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Apple II
Technical Notes
_____________________________________________________________________________
                                                  Developer Technical Support


Apple IIe
#3:    Double High-Resolution Graphics

Revised by:    Matt Deatherage, Glenn A. Baxter & Cameron Birse November 1988
Written by:    Peter Baum                                      September 1983

This Technical Note is a tutorial on double high-resolution (hi-res) graphics, 
a feature available on 128K Apple IIe, IIc, and IIGS computers.
_____________________________________________________________________________


Introduction

This Note was originally written in the early days of double high-resolution 
graphics.  At that time, there was no Apple IIc or IIGS, therefore, some of 
the things originally said may seem a little strange today, five years later.

For example, this Note talks a fair amount about being sure that you have a 
Revision B Apple IIe with the jumper installed.  All Apple IIe computers 
shipped since about mid-1983 have a Revision B motherboard, so this is not 
that big a concern anymore; furthermore, nearly every IIe out there has the 
aforementioned jumper already installed (it is not even an option on some 
third-party 80-column cards for the IIe).

Also, the IIc and IIGS are functionally equivalent (for the purposes of this 
article) to a Revision B IIe with the properly-jumpered 80-column card 
installed, and most of the references made to the Apple IIe apply equally to 
the IIc and IIGS.  We have tried to update most of the references to avoid 
confusion.

Considering the myriad of programming utilities, games, graphics programs, and 
other software that now uses double high-resolution graphics, it is probable 
that this Note will not be as vital as it once was.  If you are writing in 
AppleSoft BASIC, you will probably find it easier to purchase a commercial 
double hi-res BASIC utility package to add double hi-res commands to 
AppleSoft, rather than writing your own routines.  Similarly, those who want 
double hi-res art will find a double hi-res art application much easier than 
trying to draw it from the monitor or machine language.

However, if you have the insatiable curiosity about these things that Apple II 
owners and developers so often are blessed (cursed?) with, this Note will show 
you how double high-resolution works, as well as giving a few type-along 
examples in the monitor to get your feet wet.

This article describes the double high-resolution display mode which is 
available in the Apple IIc, IIGS, and the Apple IIe (with an extended 80-
column card).  Double hi-res graphics provides twice the horizontal resolution 
and more colors than the standard high-resolution mode.  On a monochrome 
monitor, double hi-res displays 560 horizontal by 192 vertical pixels, while 
on a color monitor, it allows the use of 16 colors.


Double High-Resolution on the Apple II Series

What is It?

The double high-resolution display mode that is available for the Apple IIe 
provides twice the horizontal resolution of the standard high-resolution mode.  
On a standard black-and-white video monitor, standard hi-res displays 280 
columns and 192 rows of picture elements (pixels); the double hi-res mode 
displays 560 x 192 pixels.  On a color monitor, the standard hi-res mode 
displays up to 140 columns of colors, each color being selected from the group 
of six colors available, with certain limitations.  Double hi-res displays 140 
columns of color, for which all 16 of the low-resolution colors are available.

                               Black/White          Color
                 Standard    280 x 192 pixels    140 columns
                 Hi-Res                          6 colors

                 Double      560 x 192 pixels    140 columns
                 Hi-Res                          16 colors

         Table 1-Comparison of Standard and Double Hi-Res Graphics

How Do I Install It?

Installation of the double hi-res mode on your Apple IIe depends on the 
following three conditions, discussed in detail below:

1.    Presence of a Revision B motherboard
2.    Installation of an extended 80-column text card with jumper
3.    A video monitor with a bandwidth of at least 14 MHz

First, your Apple IIe must have a Revision B (Rev-B) motherboard.  To find out 
whether your computer's motherboard is a Rev-B, check the part number on the 
edge of the board nearest the back panel, above the slots.  If the board is a 
Rev-B, the part number will be 820-0064-B.  (Double hi-res does not work on 
systems containing a Rev-A motherboard.)  If your computer's motherboard is 
not a Rev-B, and if you want to obtain one, contact your local Apple dealer.

The second condition for installing double hi-res on your IIe is that it must 
have an extended 80-column text card installed.  This card must be installed 
with a jumper connecting the two Molex-type pins on the board.

Warning:    If your IIe has a Rev-A motherboard, do not use an extended 
80-column card with the jumper connection mentioned above; 
the system will not work at all if you do.

The last requirement for operation in double hi-res mode is that your video 
monitor must have a bandwidth of at least 14 MHz.  This bandwidth is necessary 
because a television set that requires a modulator will not reproduce some 
characters or graphic elements clearly, due to the high speed at which the 
computer sends out dots in this mode.  Because most of the video monitors 
having a bandwidth of up to 14 MHz are black-and-white, the working examples 
in this article do not apply to color monitors.  If you have a video monitor, 
please use it--instead of a television set--to display the following examples.  
The AppleColor composite monitors will work just fine.


Your Turn to be Creative (Volunteers, Anyone?)

The tutorial that occupies the rest of this Note assumes you are working at 
your Apple II as you read.  The second part of the lesson demonstrates the 
double hi-res mode; therefore, before embarking on the second part, you should 
install a jumpered extended 80-column card in your Rev-B Apple IIe (or use any 
Apple IIc or IIGS).


Hands-On Practice with Standard Hi-Res

The Apple II hi-res graphics display is bit-mapped.  In other words, each dot 
on the screen corresponds to a bit in the computer's memory.  For a real-life 
example of bit-mapping, perform the following procedure, according to the 
instructions given below.  (The symbol  indicates a carriage return.)

1.    Boot the system.
2.    Engage the Caps Lock key, and type HGR.  (This instruction 
      should clear the top of the screen.)
3.    Type CALL -151 .  (The system is now in the monitor mode, and 
      the prompt should appear as an asterisk (*).)
4.    Type 2100:1 .  One single dot should appear in the upper left-
      hand corner of the screen.

Congratulations!  You have just plotted your first hi-res pixel.  (Not an 
astonishing feat, but you have to start somewhere.)

With a black-and-white monitor, the bits in memory have a simple 
correspondence with the dots (pixels) on the screen.  A dot of light appears 
if the corresponding bit is set (has a value of 1), but remains invisible if 
the bit is off (has a value of zero).  (The dot appears white on a black-and-
white monitor, and green on a green-screen monitor, such as Apple's Monitor 
/// or Monitor II.  For simplicity, we shall refer to an invisible dot as a 
black dot or pixel.)  Two visible dots located next to each other appear as a 
single wide dot, and many adjacent dots appear as a line.  To obtain a display 
of another dot and a line, follow the steps listed below:

1.    Type 2080:40 .  A dot should appear above and to the right of 
      the dot you produced in the last exercise.
2.    Type 2180:7F .  A small horizontal line should appear below 
      the first dot you produced.

From Bits and Bytes to Pixels

The seven low-order bits in each display byte control seven adjacent dots in a 
row.  A group of 40 consecutive bytes in memory controls a row of 280 dots (7 
dots per byte, multiplied by 40 bytes).  In the screen display, the least-
significant bit of each byte appears as the leftmost pixel in a group of 7 
pixels.  The second least-significant bit corresponds to the pixel directly to 
the right of the pixel previously displayed, and so on.  To watch this 
procedure in action, follow the steps listed below.  The dots will appear in 
the middle of your screen.


1.    Type 2028:1 .
2.    Type 2828:2 .
3.    Type 3028:4 .

The three bits you specified in this exercise correspond to three pixels that 
are displayed one after another, from left to right.

The most-significant bit in each byte does not correspond to a pixel.  
Instead, this bit is used to shift the positions of the other seven bits in 
the byte.  For a demonstration of this feature, follow the steps listed below:

1.    Type 2050:8 .
2.    Type 2850:8 .
3.    Type 3050:8 .

You will notice that the dots align themselves vertically.  Now do the 
following:

4.    Type 2450:88 .

The new dot (that is, the one that corresponds to the bit you just specified) 
does not line up with the dots you displayed earlier.  Instead, it appears to 
be shifted one "half-dot" to the right.

5.    To demonstrate that this dot really is a new dot, and not just the old 
      dot shifted by one dot position, type 2050:18 , 2850:18 .

You will notice that the dot mentioned under step 4 (the dot that was not 
aligned with the other seven dots) is straddled by the dots above and below 
it.  (The use of magnifying lenses is permitted.)

Shifting the pixel one half-dot, by setting the high, most-significant bit is 
most often used for color displays.  When the high bit of a byte is set to 
generate this shifted dot (which is also called the half-dot shift), then all 
the dots for that byte will be shifted one half dot.  The half-dot shift does 
not exist in the double hi-res mode.

The Figure 1 shows the memory map for the standard hi-res graphics mode.

                             Horizontal Offset
     Base | $00 | $01 | $02 | $03         $24 | $25 | $26 | $27 |
     _____|_____|_____|_____|_____       _____|_____|_____|_____|
     $2000|     |     |     |     |     |     |     |     |     |
     $2080|     |     |     |     |     |     |     |     |     |
     $2100|     |     |     |     |     |     |     |     |     |
     $2180|     |     |     |     |     |     |     |     |     |
     $2200|     |     |     |     |     |     |     |     |     |
     $2280|     |     |     |     |     |     |     |     |     |
     $2300|     |     |     |     |     |     |     |     |     |
     $2380|     |     |     |     |     |     |     |     |     |
     $2028|     |     |     |      \     \    |     |     |     |
     $20A8|     |     |     |       \     \   |     |     |     |
     $2128|     |     |     |       /     /   |     |     |     |
     $21A8|     |     |     |      /     /    |     |     |     |
     $2228|     |     |     |     /     /     |     |     |     |
     $22A8|     |     |     |    /     /      |     |     |     |
     $2328|     |     |     |   /     /       |     |     |     |
     $23A8|     |     |     |   \     \       |     |     |     |
     $2050|     |     |     |    \     \      |     |     |     |
     $20D0|     |     |     |     |     |     |     |     |     |
     $2150|     |     |     |     |     |     |     |     |     |
     $21D0|     |     |     |     |     |     |     |     |     |
     $2250|     |     |     |     |     |     |     |     |     |
     $22D0|     |     |     |     |     |     |     |     |     |
     $2350|     |     |     |     |     |     |     |     |     |
     $23D0|     |     |     |     |     |     |     |     |     |

                 Figure 1 - Standard Hi-Res Memory Map

Note: This memory map is in Chapter 2 of the Apple IIe Technical Reference,
      First Printing, January 1987.
 
Figure 2 shows the box subdivisions for the memory map in Figure 1.

               Offset              Bit
               from                            LSB
               base   | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
               _______|___|___|___|___|___|___|___|
               +$0000 |   |   |   |   |   |   |   |
               +$0400 |   |   |   |   |   |   |   |
               +$0800 |   |   |   |   |   |   |   |
               +$0C00 |   |   |   |   |   |   |   |
               +$1000 |   |   |   |   |   |   |   |
               +$1400 |   |   |   |   |   |   |   |
               +$1800 |   |   |   |   |   |   |   |
               +$1C00 |   |   |   |   |   |   |   |

        Figure 2 - Box Subdivisions of the Standard Memory Map
        
Note: This figure is the inset of the hi-res graphics display map in
      Chapter 2 of the Apple IIe Technical Reference, First Printing, January 1987.

For example, the first memory address of each screen line for the first few 
lines is as follows:

$2000, $2400, $2800, $2C00, $3000, $3400, $3800, $3C00, $2080, $2480, etc.

Each of the 24 boxes contains 8 screen lines for a total of 192 vertical lines 
per screen.  Each of the  40 boxes per line contains 7 pixels for a total of 
280 pixels horizontally across each line.


The Intricacies of Double Hi-Res

Because the double high-resolution graphics mode provides twice the horizontal 
dot density as standard hi-res graphics does, double hi-res requires twice as 
much memory as does standard hi-res.  If you spent many hours committing the 
standard hi-res memory map to memory, don't despair; double hi-res still uses 
the hi-res graphics page (but only to represent half the picture, so to 
speak).  In the double hi-res mode, the hi-res graphics page is compressed to 
fit into half of the display.  The other half of the display is stored in 
memory (called the auxiliary (aux) memory) on the extended 80-column card.  
(This article refers to the standard hi-res graphics page, which resides in 
main memory, as the motherboard (main) memory.)

The auxiliary memory uses the same addresses used by the standard hi-res 
graphics page (page 1, $2000 through $3FFF).  The hi-res graphics page stored 
in auxiliary memory is known as hi-res page 1X.  The graphics pages in 
auxiliary memory are bank-switched memory, which you can switch in by 
activating some of the soft switches.  (Adventurous readers may want to skip 
ahead to Using the Auxiliary Memory, which appears later in this Note.)

The memory mapping for the hi-res graphics display is analogous to the 
technique used for the 80-column display.  The double hi-res display 
interleaves bytes from the two different memory pages (auxiliary and 
motherboard).  Seven bits from a byte in the auxiliary memory bank are 
displayed first, followed by seven bits from the corresponding byte on the 
motherboard.  The bits are shifted out the same way as in standard hi-res 
(least-significant bit first).  In double hi-res, the most significant bit of 
each byte is ignored; thus, no half-dot shift can occur.  (This feature is 
important, as you will see when we examine double hi-res in color.)

The memory map for double hi-res appears in Figure 3.

                        Horizontal Offset
       $00   $01   $02   $03         $24   $25   $26   $27
         M     M     M     M           M     M     M     M 
      A  a  A  a  A  a  A  a        A  a  A  a  A  a  A  a
     |u  i |u  i |u  i |u  i        u  i |u  i |u  i |u  i |
Base |x |n |x |n |x |n |x |n        x |n |x |n |x |n |x |n |
_____|__|__|__|__|__|__|__|__       __|__|__|__|__|__|__|__|
$2000|     |     |     |     |     |     |     |     |     |
$2080|     |     |     |     |     |     |     |     |     |
$2100|     |     |     |     |     |     |     |     |     |
$2180|     |     |     |     |     |     |     |     |     |
$2200|     |     |     |     |     |     |     |     |     |
$2280|     |     |     |     |     |     |     |     |     |
$2300|     |     |     |     |     |     |     |     |     |
$2380|     |     |     |     |     |     |     |     |     |
$2028|     |     |     |      \     \    |     |     |     |
$20A8|     |     |     |       \     \   |     |     |     |
$2128|     |     |     |       /     /   |     |     |     |
$21A8|     |     |     |      /     /    |     |     |     |
$2228|     |     |     |     /     /     |     |     |     |
$22A8|     |     |     |    /     /      |     |     |     |
$2328|     |     |     |   /     /       |     |     |     |
$23A8|     |     |     |   \     \       |     |     |     |
$2050|     |     |     |    \     \      |     |     |     |
$20D0|     |     |     |     |     |     |     |     |     |
$2150|     |     |     |     |     |     |     |     |     |
$21D0|     |     |     |     |     |     |     |     |     |
$2250|     |     |     |     |     |     |     |     |     |
$22D0|     |     |     |     |     |     |     |     |     |
$2350|     |     |     |     |     |     |     |     |     |
$23D0|     |     |     |     |     |     |     |     |     |

            Figure 3 - Double Hi-Res Memory Map

Note: This memory map is in Chapter 2 of the Apple IIe Technical Reference,
      First Printing, January 1987.

Each box is subdivided exactly the same way it is in the standard hi-res mode.


Obtaining a Double-Hi-Res Display

To display the double hi-res mode, set the following soft switches:

                       In the monitor    In AppleSoft
                       Read              PEEK
             HI-RES    $C057             49239
             GR        $C050             49232
             AN3       $C05E             49246
             MIXED     $C053             49235

                       In the monitor    In AppleSoft
                       Write             POKE
             80COL     $C00D             49165,0

Annunciator 3 (AN3) must be turned off to get into double hi-res mode.  You 
turn it off by reading location 49246 ($C05E).  Note that whenever you press 
Control-Reset, AN3 is turned on; therefore, each time you press Control-Reset, 
you must turn AN3 off again.

If you are using MIXED mode, then the bottom four lines on the screen will 
display text.  If you have not turned on the 80-column card, then every second 
character in the bottom four lines of text will be a random character.  (The 
reason is that although the hardware displays 80 columns of characters, the 
firmware only updates the 40-column screen, which consists of the characters 
in the odd-numbered columns.  The characters in even-numbered columns then 
consist of random characters taken from text page 1X in the auxiliary memory.)

To remove the even characters from the bottom four lines on the screen, type 
PR#3 from AppleSoft (type 3^P in the monitor).  This procedure clears the 
memory locations on page 1X.


Using the Auxiliary Memory

The auxiliary memory consists of several different sections, which you can 
select by using the soft switches listed below.  A pair of memory locations is 
dedicated to each switch.  (One location turns the switch on; the other turns 
it off.)  You activate a switch by writing to the appropriate memory location.  
The write instruction itself is what activates the switch; therefore, it does 
not matter what data you write to the memory location.  The soft switches are 
as follows:

                            In the monitor    In AppleSoft
                            Write             POKE
          80STORE    off    $C000             49152,0
                     on     $C001             49153,0
          RAMRD      off    $C002             49154,0
                     on     $C003             49155,0
          RAMWRT     off    $C004             49156,0
                     on     $C005             49157,0
          PAGE2      off    $C054             49236,0
                     on     $C055             49237,0
          HIRES      off    $C056             49238,0
                     on     $C057             49239,0

A routine called AUXMOVE ($C311), located in the 80-column firmware, is also 
very handy, as we will see below.

Accessing memory on the auxiliary card with the soft switches has the 
following characteristics.  Memory maps, which help clarify the descriptions, 
are in Figures 4, 5, and 6.

1.    To activate the PAGE2 and HIRES switches, you need only read 
      (PEEK) from the corresponding memory locations (instead of writing 
      to them, as you do for the other three switches).
2.    The PAGE2 switch normally selects the display page, in either 
      graphics or text mode, from either page 1 or page 2 of the 
      motherboard memory.  However, it does so only when the 80STORE 
      switch is off.
3.    If the 80STORE switch is on, then the function of the PAGE2 switch 
      changes.  When 80STORE is on, then PAGE2 switches in the text 
      page, locations $400-7FF, from auxiliary memory (text page 1X), 
      instead of switching the display screen to the alternate video 
      page (page 2 on the motherboard).  When 80STORE is on, the PAGE2 
      switch determines which memory bank (auxiliary or motherboard) is 
      used during any access to addresses $400 through 7FF.  When the 
      80STORE switch is on, it has priority over all other switches.
4.    If the 80STORE switch is on, then the PAGE2 switch only switches 
      in the graphics page 1X from the auxiliary memory if the HIRES 
      switch is also on.  (Note that this circumstance is slightly 
      different from that described in item 3.)  When 80STORE is on, and 
      if the HIRES switch is also on, then the PAGE2 switch selects the 
      memory bank (auxiliary or motherboard) for accesses to a memory 
      location within the range $2000 through 3FFF.  If the HIRES switch 
      is off, then any access to an memory location within the range 
      $2000 through 3FFF uses the motherboard memory, regardless of the 
      state of the PAGE2 switch.
5.    If the 80STORE switch is off, and if the RAMRD and RAMWRT switches 
      are on, then any reading from or writing to address space $200-
      $BFFF gains access to the auxiliary memory.  If only one of the 
      switches, RAMRD, for example, is set, then only the appropriate 
      operation (in this case a read) will be performed on the auxiliary 
      memory.  If only RAMWRT is set, then all write operations access 
      the auxiliary memory.  When the 80STORE switch is on, it has 
      higher priority than the RAMRD and RAMWRT switches.

       Main Memory                     Auxiliary Memory
       ___________  _________________  ___________ 
$FFFF |///////////|                   |           |
      |///Bank////|                   |   Bank    |
      |/Switched//|                   | Switched  |
      |//Memory///|                   |  Memory   |
      |///////////|                   |           |
      |___________|___________  _____ |___________|___________ 
$DFFF |///////////|///////////|       |           |           |
      |//Bank 1///|//Bank 2///|       |  Bank 1   |  Bank 2   |
$D000 |///////////|///////////|       |           |           |
      |___________|___________| _____ |___________|___________|
       ___________  _________________  ___________ 
$BFFF |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$5FFF |///////////|                   |           |
      |//Hi-Res///|                   |  Hi-Res   |
      |/Graphics//|                   | Graphics  |
      |//Page 2///|                   |  Page 2X  |
$4000 |///////////|                   |           |
      |___________| _________________ |___________|
$3FFF |///////////|                   |           |
      |//Hi-Res///|                   |  Hi-Res   |
      |/Graphics//|                   | Graphics  |
      |//Page 1///|                   |  Page 1X  |
$2000 |///////////|                   |           |
      |___________| _________________ |___________|
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$0BFF |///////////|                   |           |
      |///Text////|                   |   Text    |
      |//Page 2///|                   |  Page 2X  |
$0800 |///////////|                   |           |
      |___________| _________________ |___________|
$07FF |///////////|                   |           |
      |///Text////|                   |   Text    |
      |//Page 1///|                   |  Page 1X  |
$0400 |///////////|                   |           |
      |___________| _________________ |___________|
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$01FF |///////////|                   |           |
      |/Stack and/|                   | Alternate |
      |/Zero Page/|                   | Stack and |
      |///////////|                   | Zero Page |
$0000 |///////////|                   |           |
      |___________| _________________ |___________|

 __________________________    ______
| 80STORE      | OFF | ON  |  |//////|
|______________|_____|_____|  |//////|
| PAGE2        |  X  | OFF |  |//////|
|______________|_____|_____|  |______|
| HIRES        |  X  |  X  |   Active
|______________|_____|_____|   Memory
| RAMRD/RAMWRT | OFF | OFF |
|______________|_____|_____|

                    Figure 4A - Memory Map One-A

       Main Memory                     Auxiliary Memory
       ___________  _________________  ___________ 
$FFFF |///////////|                   |           |
      |///Bank////|                   |   Bank    |
      |/Switched//|                   | Switched  |
      |//Memory///|                   |  Memory   |
      |///////////|                   |           |
      |___________|___________  _____ |___________|___________ 
$DFFF |///////////|///////////|       |           |           |
      |//Bank 1///|//Bank 2///|       |  Bank 1   |  Bank 2   |
$D000 |///////////|///////////|       |           |           |
      |___________|___________| _____ |___________|___________|
       ___________  _________________  ___________ 
$BFFF |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$5FFF |           |                   |///////////|
      |  Hi-Res   |                   |//Hi-Res///|
      | Graphics  |                   |/Graphics//|
      |  Page 2   |                   |//Page 2X//|
$4000 |           |                   |///////////|
      |___________| _________________ |___________|
$3FFF |           |                   |///////////|
      |  Hi-Res   |                   |//Hi-Res///|
      | Graphics  |                   |/Graphics//|
      |  Page 1   |                   |//Page 1X//|
$2000 |           |                   |///////////|
      |___________| _________________ |___________|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$0BFF |           |                   |///////////|
      |   Text    |                   |///Text////|
      |  Page 2   |                   |//Page 2X//|
$0800 |           |                   |///////////|
      |___________| _________________ |___________|
$07FF |           |                   |///////////|
      |   Text    |                   |///Text////|
      |  Page 1   |                   |//Page 1X//|
$0400 |           |                   |///////////|
      |___________| _________________ |___________|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$01FF |///////////|                   |           |
      |/Stack and/|                   | Alternate |
      |/Zero Page/|                   | Stack and |
      |///////////|                   | Zero Page |
$0000 |///////////|                   |           |
      |___________| _________________ |___________|

 __________________________    ______
| 80STORE      | OFF | ON  |  |//////|
|______________|_____|_____|  |//////|
| PAGE2        |  X  | ON  |  |//////|
|______________|_____|_____|  |______|
| HIRES        |  X  |  X  |   Active
|______________|_____|_____|   Memory
| RAMRD/RAMWRT | ON  | ON  |
|______________|_____|_____|

                    Figure 4B - Memory Map One-B

       Main Memory                     Auxiliary Memory
       ___________  _________________  ___________ 
$FFFF |///////////|                   |           |
      |///Bank////|                   |   Bank    |
      |/Switched//|                   | Switched  |
      |//Memory///|                   |  Memory   |
      |///////////|                   |           |
      |___________|___________  _____ |___________|___________ 
$DFFF |///////////|///////////|       |           |           |
      |//Bank 1///|//Bank 2///|       |  Bank 1   |  Bank 2   |
$D000 |///////////|///////////|       |           |           |
      |___________|___________| _____ |___________|___________|
       ___________  _________________  ___________ 
$BFFF |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$5FFF |///////////|                   |           |
      |//Hi-Res///|                   |  Hi-Res   |
      |/Graphics//|                   | Graphics  |
      |//Page 2///|                   |  Page 2X  |
$4000 |///////////|                   |           |
      |___________| _________________ |___________|
$3FFF |///////////|                   |           |
      |//Hi-Res///|                   |  Hi-Res   |
      |/Graphics//|                   | Graphics  |
      |//Page 1///|                   |  Page 1X  |
$2000 |///////////|                   |           |
      |___________| _________________ |___________|
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$0BFF |///////////|                   |           |
      |///Text////|                   |   Text    |
      |//Page 2///|                   |  Page 2X  |
$0800 |///////////|                   |           |
      |___________| _________________ |___________|
$07FF |           |                   |///////////|
      |   Text    |                   |///Text////|
      |  Page 1   |                   |//Page 1X//|
$0400 |           |                   |///////////|
      |___________| _________________ |___________|
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$01FF |///////////|                   |           |
      |/Stack and/|                   | Alternate |
      |/Zero Page/|                   | Stack and |
      |///////////|                   | Zero Page |
$0000 |///////////|                   |           |
      |___________| _________________ |___________|

 __________________________    ______
| 80STORE      | ON  |     |  |//////|
|______________|_____|_____|  |//////|
| PAGE2        | ON  |     |  |//////|
|______________|_____|_____|  |______|
| HIRES        | OFF |     |   Active
|______________|_____|_____|   Memory
| RAMRD/RAMWRT | OFF |     |
|______________|_____|_____|

                  Figure 5A - Memory Map Two-A
                    
       Main Memory                     Auxiliary Memory
       ___________  _________________  ___________ 
$FFFF |///////////|                   |           |
      |///Bank////|                   |   Bank    |
      |/Switched//|                   | Switched  |
      |//Memory///|                   |  Memory   |
      |///////////|                   |           |
      |___________|___________  _____ |___________|___________ 
$DFFF |///////////|///////////|       |           |           |
      |//Bank 1///|//Bank 2///|       |  Bank 1   |  Bank 2   |
$D000 |///////////|///////////|       |           |           |
      |___________|___________| _____ |___________|___________|
       ___________  _________________  ___________ 
$BFFF |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$5FFF |///////////|                   |           |
      |//Hi-Res///|                   |  Hi-Res   |
      |/Graphics//|                   | Graphics  |
      |//Page 2///|                   |  Page 2X  |
$4000 |///////////|                   |           |
      |___________| _________________ |___________|
$3FFF |           |                   |///////////|
      |  Hi-Res   |                   |//Hi-Res///|
      | Graphics  |                   |/Graphics//|
      |  Page 1   |                   |//Page 1X//|
$2000 |           |                   |///////////|
      |___________| _________________ |___________|
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$0BFF |///////////|                   |           |
      |///Text////|                   |   Text    |
      |//Page 2///|                   |  Page 2X  |
$0800 |///////////|                   |           |
      |___________| _________________ |___________|
$07FF |           |                   |///////////|
      |   Text    |                   |///Text////|
      |  Page 1   |                   |//Page 1X//|
$0400 |           |                   |///////////|
      |___________| _________________ |___________|
      |///////////|                   |           |
      |///////////|                   |           |
      |///////////|                   |           |
      |___________| _________________ |___________|
$01FF |///////////|                   |           |
      |/Stack and/|                   | Alternate |
      |/Zero Page/|                   | Stack and |
      |///////////|                   | Zero Page |
$0000 |///////////|                   |           |
      |___________| _________________ |___________|

 __________________________    ______
| 80STORE      | ON  |     |  |//////|
|______________|_____|_____|  |//////|
| PAGE2        | ON  |     |  |//////|
|______________|_____|_____|  |______|
| HIRES        | ON  |     |   Active
|______________|_____|_____|   Memory
| RAMRD/RAMWRT | OFF |     |
|______________|_____|_____|

                    Figure 5B - Memory Map Two-B

       Main Memory                     Auxiliary Memory
       ___________  _________________  ___________ 
$FFFF |///////////|                   |           |
      |///Bank////|                   |   Bank    |
      |/Switched//|                   | Switched  |
      |//Memory///|                   |  Memory   |
      |///////////|                   |           |
      |___________|___________  _____ |___________|___________ 
$DFFF |///////////|///////////|       |           |           |
      |//Bank 1///|//Bank 2///|       |  Bank 1   |  Bank 2   |
$D000 |///////////|///////////|       |           |           |
      |___________|___________| _____ |___________|___________|
       ___________  _________________  ___________ 
$BFFF |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$5FFF |           |                   |///////////|
      |  Hi-Res   |                   |//Hi-Res///|
      | Graphics  |                   |/Graphics//|
      |  Page 2   |                   |//Page 2X//|
$4000 |           |                   |///////////|
      |___________| _________________ |___________|
$3FFF |           |                   |///////////|
      |  Hi-Res   |                   |//Hi-Res///|
      | Graphics  |                   |/Graphics//|
      |  Page 1   |                   |//Page 1X//|
$2000 |           |                   |///////////|
      |___________| _________________ |___________|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$0BFF |           |                   |///////////|
      |   Text    |                   |///Text////|
      |  Page 2   |                   |//Page 2X//|
$0800 |           |                   |///////////|
      |___________| _________________ |___________|
$07FF |///////////|                   |           |
      |///Text////|                   |   Text    |
      |//Page 1///|                   |  Page 1X  |
$0400 |///////////|                   |           |
      |___________| _________________ |___________|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$01FF |///////////|                   |           |
      |/Stack and/|                   | Alternate |
      |/Zero Page/|                   | Stack and |
      |///////////|                   | Zero Page |
$0000 |///////////|                   |           |
      |___________| _________________ |___________|

 __________________________    ______
| 80STORE      | ON  |     |  |//////|
|______________|_____|_____|  |//////|
| PAGE2        | OFF |     |  |//////|
|______________|_____|_____|  |______|
| HIRES        | OFF |     |   Active
|______________|_____|_____|   Memory
| RAMRD/RAMWRT | ON  |     |
|______________|_____|_____|

                 Figure 6A - Memory Map Three-A

       Main Memory                     Auxiliary Memory
       ___________  _________________  ___________ 
$FFFF |///////////|                   |           |
      |///Bank////|                   |   Bank    |
      |/Switched//|                   | Switched  |
      |//Memory///|                   |  Memory   |
      |///////////|                   |           |
      |___________|___________  _____ |___________|___________ 
$DFFF |///////////|///////////|       |           |           |
      |//Bank 1///|//Bank 2///|       |  Bank 1   |  Bank 2   |
$D000 |///////////|///////////|       |           |           |
      |___________|___________| _____ |___________|___________|
       ___________  _________________  ___________ 
$BFFF |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$5FFF |           |                   |///////////|
      |  Hi-Res   |                   |//Hi-Res///|
      | Graphics  |                   |/Graphics//|
      |  Page 2   |                   |//Page 2X//|
$4000 |           |                   |///////////|
      |___________| _________________ |___________|
$3FFF |///////////|                   |           |
      |//Hi-Res///|                   |  Hi-Res   |
      |/Graphics//|                   | Graphics  |
      |//Page 1///|                   |  Page 1X  |
$2000 |///////////|                   |           |
      |___________| _________________ |___________|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$0BFF |           |                   |///////////|
      |   Text    |                   |///Text////|
      |  Page 2   |                   |//Page 2X//|
$0800 |           |                   |///////////|
      |___________| _________________ |___________|
$07FF |///////////|                   |           |
      |///Text////|                   |   Text    |
      |//Page 1///|                   |  Page 1X  |
$0400 |///////////|                   |           |
      |___________| _________________ |___________|
      |           |                   |///////////|
      |           |                   |///////////|
      |           |                   |///////////|
      |___________| _________________ |___________|
$01FF |///////////|                   |           |
      |/Stack and/|                   | Alternate |
      |/Zero Page/|                   | Stack and |
      |///////////|                   | Zero Page |
$0000 |///////////|                   |           |
      |___________| _________________ |___________|

 __________________________    ______
| 80STORE      | ON  |     |  |//////|
|______________|_____|_____|  |//////|
| PAGE2        | OFF |     |  |//////|
|______________|_____|_____|  |______|
| HIRES        | ON  |     |   Active
|______________|_____|_____|   Memory
| RAMRD/RAMWRT | ON  |     |
|______________|_____|_____|

                        Figure 6B - Memory Map Three-B


Shortcuts:  Writing to Auxiliary Memory from the Keyboard

Press Control-Reset, then type CALL -151  (to enter the monitor).  Now 
type the following hexadecimal addresses to turn on the double hi-res mode:

     C057      (for hi-res)
     C050      (for graphics)
     C053      (for mixed mode)
     C05E      Turns off AN3 for double hi-res
     C00D:0    Turns on the 80COL switch

This procedure usually causes the display of a random dot pattern at the top 
of the screen, while the bottom four lines on the screen contain text.  To 
clear the screen, follow the steps listed below:

1.    Type 3D0G  to return to BASIC.
2.    Type HGR  to clear half of the screen.  (The characters you 
      type will probably appear in alternating columns.  This is not a 
      cause for alarm; as noted above, the firmware simply thinks you 
      are working with a 40-column display.)  Remember that hi-res 
      graphics commands do not know about the half of the screen stored 
      on page 1X in the auxiliary memory.  Therefore, only page 1 (that 
      is, the first half) of the graphics page on the motherboard is 
      cleared.  As a result, in the the screen display, only alternate 
      7-bit columns appear cleared.

      On the other hand, if all of the screen columns were cleared after 
      the HGR command, then chances are good that you are not in double 
      hi-res mode.  If your screen was cleared then to determine which 
      mode you are in, type the following instructions:

           CALL -151
           2000:FF
           2001<2000.2027M

      If a solid line appears across the top of the screen, you are not 
      in double hi-res mode.  (The line that appears should be a dashed 
      or intermittent line:  - - - - - across the screen.)  If you are 
      not in double hi-res mode, then make sure that you do have a Rev. 
      B motherboard, and that the two Molex-type pins on the extended 
      80-column card are shorted together with the jumper block.  Then 
      re-enter the instructions listed above.

If you are staring at a half-cleared screen, you can clear the non-blank 
columns by writing zeros to addresses $2000 through $3FFF on graphics page 1X 
of auxiliary memory.  To do so, simply turn on the 80STORE switch, turn on the 
PAGE2 switch, then write to locations $2000, $2001, $2002, and so on up 
through $3FFF.  However, this procedure will not work if you try it from the 
monitor.  The reason is that each time you invoke a monitor routine, the 
routine sets the PAGE2 switch back to page 1 so that it can display the most 
recent command that you entered.  When you try to write to $2000, etc. on the 
auxiliary card, instead it will write to the motherboard memory.

Another way to obtain the desired result is to use the monitor's USER command, 
which forces a jump to memory location $3F8.  You can place a JMP instruction 
starting at this memory location, so the program will jump to a routine that 
writes into hi-res page 1X.  Fortunately, the system already contains such a 
routine:  AUXMOVE.


Using AUXMOVE

You use the AUXMOVE routine to move data blocks between main and auxiliary 
memory, but the task still remains of setting up the routine so that it knows 
which data to write, and where to write it.  To use this routine, some byte 
pairs in the zero page must be setup with the data block addresses, and the 
carry bit must be fixed to indicate the direction of the move.  You may not be 
surprised to learn that the byte pairs in the zero page used by AUXMOVE are 
also the scratch-pad registers used by the monitor during instruction 
execution.  The result is that while you type the addresses for the monitor's 
move command, those addresses are being stored in the byte pairs used by 
AUXMOVE.  Thereafter, you can call the AUXMOVE command directly, using the 
USER (Control-Y) command.

In practice, then, enter the following instructions:

     C00A:0               (turns on the 80-column ROM, which 
                          contains the AUXMOVE routine)
     C000:0               (reason explained below)
     3F8: 4C 11 C3        (the jump to AUXMOVE)
     2000<2000.3FFF ^Y    (where ^Y indicates that you should type 
                          Control-Y)

The syntax for this USER (Control-Y) command is:

                    {AUXdest}<{MBstart}.{MBend}^Y

The command copies the values in the range MBstart to MBend in the motherboard 
memory into the auxiliary memory beginning at AUXdest.  This command is 
analogous to the MOVE command.

You can use this procedure to transfer any block of data from the motherboard 
memory to hi-res page 1X.  Working directly from the keyboard, you can use a 
data block transferred this way to fill in any part of a double hi-res screen 
image.  The image to be stored in hi-res page 1X (i.e., the image that will be 
displayed in the even-numbered columns of the double hi-res picture) must 
first be stored in the motherboard memory.  You can then use the Control-Y 
command to transfer the image to hi-res page 1X.

The AUXMOVE routine uses the RAMRD and RAMWRT switches to transfer the data 
blocks.  Because the 80STORE switch overrides the RAMRD and RAMWRT switches, 
the 80STORE switch must be turned off--otherwise it would keep the transfer 
from occurring properly (hence the write to $C000 above).

If the 80STORE and HIRES switches are on and PAGE2 is off, when you execute 
AUXMOVE, any access to an address located within the range from $2000 to $3FFF 
inclusive would use the motherboard memory, regardless of how RAMRD and RAMWRT 
are set.  Entering the command C000:0  turns off 80STORE, thus letting the 
RAMRD and RAMWRT switches control the memory banking.

The Control-Y trick described above only works for transferring data blocks 
from the main (motherboard) memory to auxiliary memory (because the monitor 
always enters the AUXMOVE routine with the carry bit set).  To move data 
blocks from the auxiliary memory to the main memory, you must enter AUXMOVE 
with the carry bit clear.  You can use the following routine to transfer data 
blocks in either direction:

     301:AD 0 3      (loads the contents of address $300 into the 
                     accumulator)
     304:2A          (rotates the most-significant bit into the carry 
                     flag)
     305:4C 11 C3    (jump to $C311 (AUXMOVE))
     3F8:4C 1 3      (sets the Control-Y command to jump to address 
                     $301)

Before using this routine, you must modify memory location $300, depending on 
the direction in which you want to transfer the data blocks.  If the transfer 
is from the auxiliary memory to the motherboard, you must clear location $300 
to zero.  If the transfer is from the motherboard to the auxiliary memory, you 
must set location $300 to $FF.


Two Double Hi-Res Pages

So far, we have only discussed using graphics pages 1 and 1X to display double 
hi-res pictures.  But--analogous to the standard hi-res pages 1 and 2--two 
double hi-res pages exist:  pages 1 and 1X, at locations $2000 through 3FFF, 
and pages 2 and 2X, at locations $4000 through 5FFF.  The only trick in 
displaying the second double hi-res page is that you must turn off the 80STORE 
switch.  If the 80STORE switch is on, then only the first page (1 and 1X) is 
displayed.  Go ahead and try it:

     C000:0    to turn off the 80STORE switch
     C055      to turn on the PAGE2 switch

The screen will fill with another display of random bits.  Clear the screen 
using the instructions listed above (in the Using AUXMOVE section ).  However, 
this time, use addresses $4000 through 5FFF instead.  (Don't be alarmed by the 
fact that the figures you are typing are not displayed on the screen.  They 
are being "displayed" on text page 1.)

     4000:0
     4001<4000.5FFFM
     4000<4000.5FFF ^Y

You will be delighted to learn that you can also use this trick to display two 
80-column text screens.  The only problem here is that the 80-column firmware 
continually turns on the 80STORE switch, which prevents the display of the 
second 80-column screen.  However, if you write your own 80-column display 
driver, then you can use both of the 80-column screens.


Color Madness

It should come as no surprise that color-display techniques in double hires 
are different from color-display techniques in standard hi-res.  This 
difference is because the half-dot shift does not exist in double hi-res mode.

Instead of going into a dissertation on how a television set decodes and 
displays a color signal, I'll simply explain how to generate color in double 
hi-res mode.  In the following examples, the term color monitor refers to 
either an NTSC monitor or a color television set.  Both work; however, the 
displays will be much harder to see on the color television.  The generation 
of color in double hi-res demands sacrifices.  A 560 x 192 dot display is not 
possible in color.  Instead, the horizontal resolution decreases by a factor 
of four (140 dots across the screen).  Just as with a black-and-white monitor, 
a simple correspondence exists between memory and the pixels on the screen.  
The difference is that four bits are required to determine each color pixel.  
These four bits represent 16 different combinations:  one for each of the 
colors available in double hi-res.  (These are the same colors that are 
available in the low-resolution mode.)

Let's start by exploring the pattern that must be stored in memory to draw a 
single colored line across the screen.  Use a color demonstration program 
(such as COLOR.TEST from older DOS 3.3 System Master disks) to adjust the 
colors displayed by your monitor.  After you have adjusted the colors, exit 
from the color demonstration program.

The instructions that appear below are divided into groups separated by blank 
lines.  Because it is very difficult (and, on a television set, almost 
impossible) to read the characters you are typing as they appear on the 
screen, you will probably make typing errors.  If the instructions appear not 
to work, then start again from the beginning of a group of instructions.

     CALL -151           (to get into the monitor routine/program)
     C050                (This set of instructions puts the computer
     CO57                into double hi-res model.
     C05E
     C00D:0
     2000:0              (This set of instructions clears first one half
     2001<2000.3FFFM     of the screen, and then the other half of
     3F8: 4C 11 C3       the screen.)
     2000<2000.3FFF^Y

     2100:11 4           (Two red dots appear on top left of 
                         screen)
     2102<2100.2126M     (A dashed red line appears across screen)

     2150:8 22           (Two green dots appear near bottom left)
     2152<2150.2175M     (Dashed green line appears across screen)

     2100<2150.2177^Y    (Fills in the red line)

In contrast to conditions in standard hi-res, no half-dot shift occurs, and 
the most-significant bit of each byte is not used.

As noted above, four bits determine a color.  You can paint a one-color line 
across the screen simply by repeating a four-bit pattern across the screen, 
but it is much easier to write a whole byte rather than just change four bits 
at a time.  Since only seven bits of each byte are displayed (as noted earlier 
in our discussion of black-and-white double hi-res) and the pattern is four 
bits wide, it repeats itself every 28 bits or four bytes.  Use the 
instructions listed below to draw a line of any color across the screen by 
repeating a four byte pattern for the color as shown in Table 2.

     2200: main1 main2   (Colored dots appear at the left edge)
     2202<2200.2226M     (A dashed, colored line appears)

     2250: aux1 aux2
     2250<2250.2276M

     2200<2250.2276^Y    (Fills in line, using the selected color)

                                                Repeated
                                                Binary
          Color         aux1  main1 aux2  main2 Pattern
          Black          00    00    00    00    0000
          Magenta        08    11    22    44    0001
          Brown          44    08    11    22    0010
          Orange         4C    19    33    66    0011
          Dark Green     22    44    08    11    0100
          Grey1          2A    55    2A    55    0101
          Green          66    4C    19    33    0110
          Yellow         6E    5D    3B    77    0111
          Dark Blue      11    22    44    08    1000
          Violet         19    33    66    4C    1001
          Grey2          55    2A    55    2A    1010
          Pink           5D    3B    77    6E    1011
          Medium Blue    33    66    4C    19    1100
          Light Blue     3B    77    6E    5D    1101
          Aqua           77    6E    5D    3B    1110
          White          7F    7F    7F    7F    1111

                   Table 2-The Sixteen Colors

In Table 2, the heading aux1 indicates the first, fifth, ninth, thirteenth, 
etc. byte of each line (i.e., every fourth byte, starting with the first 
byte).  The heading main1 indicates the second, sixth, tenth, fourteenth, etc. 
byte of each line (i.e., every fourth byte, starting with the second byte).  
The aux2 and main2 headings indicate every fourth byte, starting with the 
third and fourth bytes of each line, respectively.  Aux1 and aux2 are always 
stored in auxiliary memory, while main1 and main2 are always stored in the 
motherboard memory.

As you will infer from Table 2, the absolute position of a byte also 
determines the color displayed.  If you write an 8 into the first byte at the 
far left side of the screen (i.e., in the aux1 column), then a red dot is 
displayed.  But if you write an 8 into the third byte at the left side of the 
screen (the aux2 column), then a dark green dot is displayed.  Remember, the 
color monitor decides which color to display based on the relative position of 
the bits on each line (i.e., on how far the bits are from the left edge of the 
screen).

So far, so good.  But suppose you want to display more than one color on a 
single line.  It's easy:  just change the four-bit pattern that is stored in 
memory.  For example, if you want the left half of the line to be red, and the 
right half to be purple, then store the red pattern (8, 11, 22, 44) in the 
first 40 bytes of the line, then store the purple pattern (19, 33, 66, 4C) in 
the second 40 bytes of the line.  Table 2 is a useful reference tool for 
switching from one color to another, provided you make the change on a byte 
boundary.  In other words, you must start a new color at the same point in the 
pattern at which the old color ended.  For example, if the old color stops 
after you write a byte from the main1 column, then you should start the new 
color by storing the next byte in memory with a byte from the aux2 column.  
This procedure is illustrated below:

2028:11 44 11 44 11 44 11 77 5D 77 5D 77 5D    (creates a dashed line
2128: 8 22 8 22 8 22 8 22 6E 3B 6E 3B 6E       that is red then yellow)

2028<2128.2134^Y                               (fills in the rest of 
                                               the colors)


Switching Colors in Mid-Byte

If you want a line to change color in the middle of a byte, you will have to 
recalculate the column, based on the information in Table 2.  Suppose you want 
to divide the screen into three vertical sections, each a different color.  
The leftmost third of the screen ends in the middle of the 27th character from 
the left edge-that is, in an aux2 column of the color table.  (Dividing 27 by 
4 gives a remainder of 3, which indicates the third column, or aux2.)  Your 
pattern should change from the first color to the second color after the 5th 
bit of the 27th byte.  You can change the color in the middle of a byte by 
selecting the appropriate bytes from the aux2 column of Table 2 and 
concatenating two bits for the second color with five bits for the first 
color.

However, because the bits from each byte are shifted out in order from least 
significant to most significant, the two most significant bits (in this case 
bits 5 and 6, because bit 7 is unused) for the second color are concatenated 
with the five least significant bits for the first color.  For instance, if 
you want the color to change from orange (the first color) to green (the 
second color), then you must append the two most significant bits (5 and 6) of 
green to the five least significant bits (0-4) of orange.  In Table 2, the 
aux2 column byte for green is 19, and the two most significant bits are both 
clear.  The aux2 column byte for orange is 33, and the five least significant 
bits are equal to 10011.  The new byte calculated from appending green (00) to 
orange (10011) yields 13 (0010011).  Therefore, the first 26 bytes of the line 
come from the table values for orange; the 27th byte is 13, and the next 26 
bytes come from the table values for green.

     2300: 19 66                      (puts an orange line on the 
                                      screen)
     2302<2300.2310M
     2350: 4C 33
     2352<2350.2360M
     2300<2350.2360^Y

     230D: 33 4C 33 4C 33 4C 33 4C    (puts a green line next to it)
     235D: 13 66 19 66 19 66 19 66    (note the first byte)
     230D<235D.2363^Y

There you have it:  a basic explanation of how double hi-res works--except for 
one or two anomalies.  The first anomaly is that NTSC monitors have a limited 
display range.  The second anomaly shows one of the features of double hi-res 
versus a limitation of standard hi-res.

An NTSC color monitor decides what color to display based on its view of four 
bit windows in each line, starting from the left edge of the screen.  The 
monitor looks at the first four bits, determines which color is called for, 
then shifts one bit to the right and determines the color for this new four-
bit window.  But remember, the color depends not only on the pattern, but also 
the position of the pattern.  To compensate for relative position from the 
left edge of the screen, the monitor keeps track of where on each line each of 
these windows start.  (For those of you of the technical persuasion, this is 
done through the use of the color burst signal, which is a 3.58 MHz. clock).

Try this example:

     2000:0              Clears the screen
     2001<2000.3FFFM
     2000<2000.3FFF^Y

     2001:66             Draws an orange box in the upper left
     2401:66
     2801:66
     2C01:66
     3001:66

     2050:33             Draws a blue box below and to the right
     3402<2050.2050^Y    of the orange box
     3802<2050.2050^Y
     3C02<2050.2050^Y

Notice that if the blue box was drawn at the top of the screen, next to the 
orange box, they would overlap.  Yet, the boxes were drawn on two different 
columns, orange on main2 and blue on aux1.  This can be explained by the 
previous paragraph, and the sliding windows.  The monitor will detect the 
pattern for orange slightly after the main2 column, while the pattern for blue 
shows up before column aux1.

The orange pattern is as follows:

     0000000 | 0110011 |0000000    look at four-bit windows and you will see
        aux2 |   main2 |   aux1    an orange pattern overlaps on both sides

If a pattern is repeated on a line, this overlap does not cause a problem, 
since the same color just overlaps itself.  But watch what happens when a new 
pattern is started next to a different pattern:

     3002<2050.2050^Y    Puts a blue pattern next to the orange one
     2C02<2050.2050^Y
     2802<2050.2050^Y

Where the blue overlaps the orange, you will see a white dot.  This effect is 
because one of the four-bit windows the monitor sees is all 1s.  If two colors 
are placed right next to each other, the monitor will sometimes display a 
third color, or fringe, at the boundary.  This fringe effect is especially 
noticeable when there are a lot of narrow columns of different colors next to 
each other.  (Next time you run COLOR TEST take a look at the boundaries 
between the colors).

The orange and blue patter is as follows:

     0000000 |0110011 | 11001100    note the four 1s in a row at the boundary
        aux2 |  main2 |     aux1    between orange and blue


Conclusion

Now you have the tools and the rules to the double hi-res mode.  As you can 
see, double hi-res has more color with higher resolution than standard hi-res.  
You can even develop games that do fancy animation or scroll orange objects 
across green backgrounds.  You can develop word processing programs which use 
different fonts or proportional character sets in black and white.  Have fun 
playing with his new mode.


Further Reference
    o    Apple IIe Technical Reference Manual
    o    Apple IIc Technical Reference Manual, Second Edition
    o    Apple IIGS Hardware Reference
   
All content Copyright ©1984-2003, Terence J. Boldt, unless noted otherwise.