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Apple II
Technical Notes
                                                  Developer Technical Support

Apple IIe
#2:    Hardware Protocol for Doing DMA

Revised by:    Glenn A. Baxter & Rob Moore                      November 1988
Written by:    Peter Baum                                        January 1983

This Technical Note explains the hardware protocol for doing direct memory 
access (DMA) on the Apple IIe and Apple ][ and is meant as a guideline for 
developing peripherals which do DMA on these machines, not as a specification 
for future Apple products or revisions.

This Note covers the timing differences between the Apple ][ and IIe and also 
gives tips on how to design a peripheral card that will work in both systems.  
The reader should be very familiar with either the Apple ][+ or the Apple IIe, 
especially the timing on the data and address buses in relation to the 6502.

DMA is used by peripheral cards in the Apple II family to transfer data 
directly into memory without benefit of the processor.  Transfer of data from 
a peripheral device into RAM can normally be handled one byte at a time under 
control of the processor.  By using DMA, you can achieve greater data transfer 
rates than the 6502 can handle in software.  This transfer rate can approach 
the full-cycle time of the memory.  This technique can also be used to 
transfer single data bytes into memory without requiring the CPU to process an 
interrupt, which can be very time consuming.

The DMA process entails five steps:  turn the processor off, gain access to 
the R/W* line and both address and data buses, complete the data transfer, 
release the data and address buses, and finally, allow the microprocessor to 
restart.  This Note covers each of these steps in detail.

At this point, I should caution the prospective developer that DMA on an Apple 
][+ or Apple IIe can only be done under certain circumstances.  Because DMA 
turns off the processor, any program with a software timing loop will not work 
properly.  These programs assume that each instruction will take a fixed 
amount of time, which is not true when the processor stops in the middle of an 
instruction.  This assumption means that the Apple II disk drives will not 
work since they require a timing loop to read a disk.  (Co-processor cards 
work with DMA because they initiate the disk access and know that DMA cannot 
be used until the disk is finished).

Another problem is that because of the mapping scheme used on the Apple IIe 
extended 80-column (64K) card, a peripheral card cannot tell which memory bank 
is being used without a complicated detection scheme.  This problem means that 
if a DMA device writes to a certain memory space, it might not be able to read 
the same data back.

 _______                   INTERNAL DATA BUS (MD 0-7)
|       |<---------+-----+-----------+---------+------------+---------+
|       |          ^     ^           ^         ^            |         |
|       |          |  ___|______     |         |       _____v______   |
|       |          | | Keyboard |  __v__   ____v____  | DATA LATCH |  |
|       |          | |__________| | 64K | | 80 COL. | |____________|  |
|       |       ___|___      ^    | RAM | | AUX RAM |          |      |
|       |      | EO-FF |     |KDB |_____| |_________|--------->+      |
| 6502A |      |  ROM  |     |         ^          ^  VIDEO DATA|      |
|       |      |_______|     |         |          |            |      |
|       |        ^    ^      |      +--+----------+-----+      |   ___v_____
|       |        |    |ROMEN |      |RAM ADDR BUS (0-7) |      |  |  DATA   |
|       |        |   _|______|______|__                 |      |  |  LATCH  |
|       |        |  |      MMU         |------------------------->| 74LS245 |
|       |        |->|__________________|  MD IN/OUT     |      |  |_________|
|       |        |       ^                              |      |      ^
|       |        |       |                            __|__    |      |
|       |        |       |                           | IOU |   |      |
|       |        +---------------------------------->|_____|<--+      |
|       |        |       |                            ^   |    |      |
|       |        |       |TIMING AND CONTROL SIGNALS  |   |    |      |
|       |<---------------+----------------------------+   |    |      |
|       |        |       |                              __v____v__    |
|       |   ___  |       +--------------+              |  VIDEO   |   |
|       |  | 7 | |       |              |              | CIRCUITS |   |
|       |  | 4 | |   ____|_____    _____v______        |__________|   |
|       |  | L | |  |  TIMING  |  | PERIPHERAL |                      |
|       |  | S | |  | CIRCUITS |  |   SLOTS    |<---------------------+
|       |  | 2 | |  |__________|  |____________|   EXTERNAL DATA BUS
|       |  | 4 | |                      ^ 
|       |  | 4 | |ADDRESS BUS (A0-A15)  | 

                Figure 1 - Apple IIe Functional Block Diagram

Though the differences between the Apple IIe and Apple ][+ architecture appear 
to be significant to a device which uses DMA, this should not affect the 
design in most cases.  A good rule of thumb is that if a device is designed to 
work on the Apple IIe, then it will be backward compatible and also run on the 
Apple ][+.  The converse is not true; cards that use DMA on the Apple ][+ 
might not work on the Apple IIe, hence, most of the descriptions in this Note 
refer to the Apple IIe with occasional references to the Apple ][+.  For 
example, the timing specifications listed are calculated from the Apple IIe 
timing paths unless otherwise noted.

Occasionally the descriptions refer to a chip on the motherboard of the Apple 
IIe, so a set of Apple IIe schematics should be nearby.  The corresponding 
parts on the Apple ][+ will be specified when applicable.

The following paragraphs describe and define some of the terms that are used 
throughout this Note.  The Apple IIe block diagram on the previous page may be 
helpful when reading about the buses.

01 (phase one) time    The time when the 01 system timing signal is high.  
                       During this time the data bus, address bus, and RAM are
                       used for video refreshing.  This time is also called the
                       video cycle or video phase.
0o (phase zero) time   The time when 0o clock is high.  0o is the inverse of 
                       01.  During this time the microprocessor uses the data
                       and address buses.  This time is also known as the CPU
                       cycle or CPU phase.
IOU and MMU            Two MOS custom chips inside the Apple IIe.  See chapter
                       7 of the Apple IIe Technical Reference Manual for more
                       details on the custom chips.
Data bus               The microprocessor, ROM, and RAM are connected to this
                       bus.  On the IIe this bus generally has MOS components
                       connected to it rather than TTL and is sometimes called
                       the MOS data bus.  A 74LS245 bidirectional bus
                       transceiver (location B2 on the original motherboard)
                       connects this internal bus to an external bus that the
                       outside world sees through the peripheral slots.  The
                       data bus connected to the peripheral slots is called the
                       external data bus.  The Apple ][ does not have these two
                       data buses.  Instead, the peripheral slots are connected
                       to the ROM, CPU data buffers, and RAM data inputs.  The
                       RAM data outputs are multiplexed with the keyboard data
                       onto this bus.
Address bus            There are three different sections to the address bus on
                       the Apple IIe.  The first section consists of the
                       addresses from the 6502A into a pair of 74LS244s
                       (locations B1,B3).  Part two connects the other side of
                       the '244 to the address bus that the peripheral slots
                       see.  Also connected on this bus are the MMU, the ROM,
                       and the chips that decode I/O SELECT, DEVICE SELECT, and
                       I/O STROBE.  The third address bus is generated by the
                       custom chips and is only used to access the RAM.  The
                       MMU and IOU automatically multiplex this bus with the
                       high byte and low byte of an address during any RAM 
                       access, whether it be for video refresh or for a 
                       microprocessor instruction fetch.  This third bus is
                       called the RAM address bus.  The Apple ][ also has these
                       three buses, but uses 8T97s and discrete logic instead
                       of the 74LS244 and custom chips.
6502 microprocessor    In the Apple IIe a 6502A, a 2 MHz part is used instead 
                       of the 1 MHz 6502 used in the Apple ][+.  Since the
                       custom chips in the Apple IIe are MOS and slower than
                       the TTL in the Apple ][+, the faster 6502A was used to
                       guarantee better margins.  For example, the 6502A sets
                       up the address bus faster on the Apple IIe than the 6502
                       does in the Apple ][+.

On the IIe, all the timing signals are generated by the PAL timing chip, 
except for the 7 M signal which is generated from an 74S109 or 74109 (early 
versions of the IIe).  Although both the PAL and the 74S019 use the 14 M 
signal for a clock, there will be some skew between edges of the 7 M clock and 
the timing signals from the PAL, such as the edges of 0o or 01.  This skew 
means the 7 M clock edge may rise as much as 20 ns before, or 5 ns after, the 
0o falling edge.  The clock signals of the Apple ][+ should be tighter than 
this (probably within 5 ns of each other) since 7 M, 0o, and 01 are all 
generated from the same chip, a 74S175.  Take this skew into account whenever 
using the 7 M signal in a design.

Getting on the Bus (Exact Change Only)

1.    Pull DMA low during 01 time.

      On the Apple IIe, the DMA line controls the direction of the 
      74LS245, which enables the internal data bus outwards to the 
      peripheral slots or enables external data onto the internal bus.  
      Changing the state of the DMA line during 0o could cause the '245 
      to change directions, forcing the internal data bus to go tri-
      state during a microprocessor read.  The 6502 would read garbage 
      and the computer might go belly up by jumping to a random memory 

      On the Apple ][, pulling the DMA line always forces the CPU data 
      bus buffer to point inward and drive toward the 6502.  Pulling the 
      DMA line low during 0o of a write cycle would result in garbage 
      being written to memory, since the data bus to the RAM would 
      suddenly go tri-state.

2.    Wait 30 ns, then assert address bus and R/W* line.

      Before driving the address bus and R/W* line, the system must 
      process the transition on the DMA line and release the bus.  This 

        25 ns    'LS244 output disable from low level
        +5 ns    'S02 low to high level output transition
        30 ns    delay from DMA negative edge before driving address 

    The 30 ns wait will also work on the Apple ][, since it only needs 
    27 ns ('LS04 and 8T97).

    Ph1         _______|
                               213 ns (#3)
                ___________________  ___________  ______
    Address Bus    High Impedence  \/           \/ Valid
    from slots  ___________________/\___________/\______
                      ->|30 ns (#2)|<-
    DMA         ________

                       Figure 2 - Getting On The Bus

3.    Address and R/W* line must be valid within 213 ns of 01 positive 

      This constraint is needed to meet the setup requirements of the 
      IOU, MMU, and RAM.  This time can be derived from the 6502A (2 
      MHz) setup requirements.  The Apple ][ can wait for 300 ns before 
      data must be valid, because it uses the 1 MHz 6502 which has a 
      longer setup time.

Warning:    This specification (the address setup time) is the major 
cause of failure for cards which use DMA in the Apple IIe.  
Many DMA cards which were originally designed for the Apple 
][+ do not meet this specification.

During DMA (Keep Your Hands Inside the Bus at all Times)

1.    Don't drive the data bus during 01 time.

      On an Apple ][+, it is safe to drive the data bus 35 ns after 
      asserting the R/W* line low, regardless of the point in the timing 
      cycle.  When the R/W* line goes low, the 74LS257s at locations B6 
      and B7 tri-state the data bus, even in the middle of 0o or 01.  
      This action prevents a bus fight from occurring between a DMA 
      device and the system.

      At first glance of the Apple IIe logic schematics, it appears that 
      a bus fight cannot occur on the data bus.  During the 01 half of a 
      write cycle, the 74LS245 tri-states the data bus within 30 ns of 
      the R/W* line being pulled low.  While this does preclude a fight 
      from occurring on the data bus during 01, it doesn't prevent a bus 
      crash from occurring at the beginning of 0o.  At the beginning of 
      0o, the 74LS245 is enabled and will drive the external data bus.  
      If the peripheral card also drives the data bus, there could be a 
      horrendous bus fight, since the 74LS245 can source 15 ma and sink 
      24 ma per line.  This might cause a spike on the ground plane, 
      which could cause a processor to reset on a co-processor card.

      Let us take a look at the problem by stepping thru Figure 3, a 
      timing diagram.

      The diagram starts with the video cycle of a read operation.  
      During the video cycle, the video refresh data is read from the 
      RAM and put on the data bus.  This video data will appear on the 
      peripheral slot (external) data bus because the 74LS245, as can be 
      seen from Table 1, drives outward during 01 of a read cycle.

      Typically, the address bus and R/W* line would be setup by the 
      6502A during 01 for the next CPU cycle, but instead, a peripheral 
      card pulls the DMA line low.  As explained earlier, the peripheral 
      device should wait at least 30 ns before driving the address bus 
      and R/W* line.  In this first DMA cycle, the peripheral card wants 
      to read a byte from RAM, so it keeps the R/W* line high.

      At this point we must switch over and use the Apple ][+ to explain 
      the timing required to read the data from RAM.  The rule of thumb, 
      that designing a DMA card for the Apple IIe will be backward 
      compatible and run on the Apple ][+, will not hold here.  On the 
      Apple ][+ data is valid on the peripheral connector a minimum of 
      468 ns from the 0o rising edge and holds to at least the falling 
      edge of 0o at 490 ns.  The hold time is actually the minimum 
      propagation delay from the falling edge of 0o thru the following 
      chips:  74LS257 at J1, 74LS139 at F2, 74LS20 at D2, 74LS00 at A2, 
      and finally to the enable of the 74LS257s at B5 and B6.  On the 
      Apple IIe a byte from RAM becomes valid at most 345 ns after the 
      rising edge of 0o and stays valid until the 0o falling edge.
_   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _   _
 |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| 

___               _____________               _____________               ____
   |_____________|             |_____________|             |_____________|

    _____________               _____________               _____________
___|             |_____________|             |_____________|             |____

R/W                          ->||<-(40-213 ns)
________________________________                              ________________
                            (5 ns Max)   (5 ns Max)    (5 ns Max)
OE 74LS245                    ->||<-       ->||<-        ->||<-
                                  ____________               _
_________________________________|            |_____________| |_______________
                                                         (5 ns Max)
/DMA                                                       ->||<-

MD IN/OUT                                   ->|  |<-(130 ns Max)
_________________________________________________|            |_______________

DATA           ->|          |<-(468 ns*)   ->|   |<-(170 ns Max)
                            ____             __  ____
___________________________/    \___________/  \/    \________________________
                           \____/           \__/\____/
                            valid                valid
  ->||<-(30 ns Min)
ADDR_  _____________________________  _____________________________  _________
___/ \/          valid              \/          valid              \/    valid

 ->|  |<-(40-213 ns)                       * This is an Apple ][+ specification

                           Figure 3 - Timing Diagram

      In the second DMA cycle, the timing diagram shows the peripheral 
      card writing a byte to memory.  In the first phase of the cycle, 
      the video phase, the address bus and R/W* line are setup by the 
      peripheral card within the timing specifications described 
      earlier, 213 ns.  Though the direction of the 74LS245 still points 
      toward the slots, the '245 is disabled when the R/W* line is 
      pulled low by the peripheral device (see Table 1).  This will tri-
      state the external data bus.  All the signals stay unchanged 
      through the rest of the video phase, until the CPU cycle starts 
      with the rising edge of 0o.

      Most bus fights occur at the beginning of the CPU cycle.  The CPU 
      cycle begins with address bus and R/W* line setup already and the 
      data bus tri-stated.  The signal MD IN/OUT, which drives the 
      74LS245 direction control, is generated by the MMU and is always 
      low during 01, so the 74LS245 drives toward the slots.  MD IN/OUT 
      uses the 0o rising edge to clock itself high during a DMA write 
      cycle, but because the MMU is a MOS chip the delay before MD 
      IN/OUT finally rises can be as long as 130 ns from the 0o rising 
      edge.  Hence, at the beginning of 0o the 74LS245 is in tri-state 
      mode, but with the direction set to drive toward the peripheral 

          PHO    R/W                    Stable State of 74LS245
           1      0    (Write to RAM)   High impedance
           1      1    (Read from RAM)  Outward driving external data
                                        bus (slots)
           0      0    (Write to RAM)   Inward driving into RAM
           0      1    (Read from RAM)  Outward driving external data
                                        bus (slots)

                      Table 1-Stable State of 74LS245

      Within 5 ns after 0o goes high, the chip enable to the 74LS245 
      goes low, enabling data onto the external data bus.  The 74LS245 
      specification guarantees that the data will be valid within 40 ns 
      from the chip enable.  If the peripheral device was also driving 
      the bus, there would be a bus crash.  To prevent this bus crash, 
      the data bus cannot be driven during 01, unless the data is pulled 
      off the bus before 0o goes high.  This means that the rising edge 
      of 0o cannot be used to gate data on and off the bus.  The bus 
      fight will occur before the peripheral card can tri-state the data 

      Data can only be enabled onto the bus after the 74LS245 has 
      changed directions and is driving the internal data bus.  The DMA 
      device must allow 130 ns for the MD IN/OUT line to change, plus 
      the delay for the 74LS245 to change directions which takes 25 ns, 
      for a total of 155 ns.

      After this 155 ns, the data must be valid on the bus within 55 ns, 
      because the RAM requires data be setup at the CAS falling edge, 
      which occurs 210 ns into 0o.  This does not leave any time to 
      spare, since, for example, a 74LS245 has a 40 ns enable time.  
      This timing criteria will also work for the Apple ][+ since the 
      setup time for 16K RAM is the same as the 64K RAM, and CAS also 
      falls at 210 ns.  The data hold time of 55 ns after CAS falls is 
      also the same for both the Apple IIe and the Apple ][+.

      Here is a scenario for a DMA write.  Set up the address bus and 
      R/W* line within the required 213 ns, then wait for the first 7 M 
      (pin 36 on slot) falling edge after 0o goes high before enabling 
      your buffer onto the data bus.  This edge will occur at 140 ns 
      into 0o, and when the gate delay is added, should guarantee the 
      buffer will not be driving the bus in the first 155 ns.  I don't 
      advocate depending on a minimum gate delay as standard design 
      practice (my college professor thinks public whipping would be a 
      justifiable punishment) but this is the real world (I'm not 
      getting graded anyway).  The data bus is valid by the time CAS 
      falls, and should be stable for at least another 55 ns or until 0o 

2.    The processor can be held off for a total of 10 microseconds.
      (10 0o clock cycles).

      This is true if a Rockwell 6502 is being used.  (A Synertek part 
      can be held off for as long as 40 Ásec.)  This time is the maximum 
      cycle time of the 6502 and if there are no clock transitions 
      within this time, it could result in internal registers (A,X,Y) 
      losing data.  This maximum time varies from manufacturer to 
      manufacturer of the 6502.

3.    MMU and IOU multiplex address bus

      The custom chips automatically handle the multiplexing required of 
      the RAM address bus.  The external device doing DMA must set up 
      the address bus and R/W* line within 213 ns of the rising edge of 
      01 just like the 6502A does.  The custom chips will automatically 
      generate the addresses to the RAM for the video refresh cycle 
      during 01, but use the addresses from the address bus to set up 
      for the next instruction cycle.  Hence, the only consideration on 
      the address bus during DMA is to meet the 213 ns setup time 

      The 213 ns setup time will also work with the Apple ][ since it 
      can take as long as 300 ns to set up the address bus and R/W* 

Getting Off the Bus

1.    Don't release DMA during 0o.

      This is analogous to step 1 of Getting on the Bus.  If the DMA 
      line is released during 0o the microprocessor will try to execute 
      a cycle during this time without the data or address bus set up 
      properly.  This random instruction fetch will probably cause the 
      system to crash.

2.    Tri-state address bus drivers on peripheral slots

      The DMA line is holding off the addresses from the 6502 onto the 
      internal address bus by tri-stating the two 74LS244s on the Apple 
      IIe bus and the 8T97s on the Apple ][+ bus.  The address bus and 
      R/W* line from the external device in the peripheral slots should 
      be tri-stated before releasing the DMA line or a bus fight will 
      occur between the internal bus buffers and the peripheral slot 

3.    Release DMA line

      These last two steps are the opposite of the first two steps 
      required to get on the bus.  Both of these steps, releasing the 
      address and R/W* lines then the DMA line, should be done within 
      178 ns of 01 going high.  This allows time for the 6502A to set up 
      the address and R/W* lines properly for the next instruction 

        213 ns    address set up requirements
          5 ns    'S02 output high-to-low transition
        -30 ns    'LS244 out enable time
        178 ns    to release DMA line and allow 6502 to set up address bus

    Again, the Apple ][ can take longer, up to 260 ns, before 
    releasing the DMA line.

Further Reference
    o    Apple IIe Technical Reference Manual

All content Copyright ©1984-2003, Terence J. Boldt, unless noted otherwise.