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Apple II
Technical Notes
                                                  Developer Technical Support

Apple IIe
#5:    /INH Line

Revised by:    Glenn A. Baxter                                  November 1988
Written by:    Peter Baum                                            May 1984

This Technical Note describes how a peripheral card on the Apple IIe should 
use the inhibit (/INH) line.  This information is true for the Apple IIe only.

Using the /INH Line on the Apple IIe


One of the new features of the Apple IIe is the ability to add more memory or 
override existing memory from a peripheral card.  This feature, which uses the 
/INH line on the peripheral slots, has been expanded from its original purpose 
on the Apple ][+ of disabling the on-board ROM and allowing the language card 
(RAM) to reside in the same address space.  The Apple IIe allows any part of 
memory to be replaced by memory on a peripheral card.  This Note explains how 
a peripheral card should use the /INH line.


Presently, only a few peripheral devices use the /INH line in the IIe for 
memory expansion.  One type of card uses /INH for RAM expansion by switching 
in extra language cards, while another class of cards uses it to extend the 
built-in 80-column ROM code by replacing it with their own ROM code.  Other 
cards use /INH so that they can have more than one stack and zero page.  
Future peripheral cards can take advantage of the /INH line to do even fancier 
memory expansion, such as keeping multiple programs running in memory at the 
same time.

More memory, either ROM or RAM, can be added by mapping the memory into the 
same address space as existing memory.  The processor can then select which 
memory, the on-board or the additional, it wants to use by setting a register 
(or soft switch).  This technique of switching different blocks of memory into 
the same address space is called bank switching.  An example of this technique 
for extending memory is found in the Apple ][+ language card and in the bank 
switched memory on the IIe.

How It Works

When the /INH line, pin 32 in slots 1-7, is pulled low, all memory on the 
motherboard and in the auxiliary slot is disabled (including memory on the 80-
column and extended 80-column cards).  This action allows a peripheral card in 
slots 1-7 to enable its memory onto the bus.

When the 6502 reads a byte from memory, the data typically comes from one of 
three places:  motherboard ROM, motherboard RAM, or RAM on one of the 80-
column cards in the auxiliary slot.  When the /INH line is pulled low, all of 
the above mentioned ROM and RAM is disabled and will not drive the data bus.  
This disabling allows the peripheral slots to drive the bus by enabling data 
onto it.  The 6502 will then read data from the peripheral card instead of a 
location on the motherboard or auxiliary slot.

During a 6502 write cycle, if the /INH line is pulled low, then motherboard 
and auxiliary card RAM are both disabled.  A peripheral card can then read a 
byte off the data bus and store it.


Because pulling the /INH line low disables all of memory, the peripheral card 
must be very careful when it does this.  If only a small piece of memory is to 
be banked into a specific address space, then the /INH line should only be 
pulled on memory references to that address space.  Otherwise the motherboard 
memory will be disabled and the processor will read or write to the wrong 
memory and the program will not work properly.  For example, if a peripheral 
card wants to replace the zero page with memory on the card, then the /INH 
line should be pulled low only on references to the address space between $0 
and $FF.  If the /INH line is pulled during a processor instruction fetch from 
the monitor ROM at $F800, the 6502 will read the wrong instruction (or a 
floating bus) and probably crash the program.

Pulling the /INH line at specific addresses is called select decoding.  The 
hardware on the peripheral card does this by checking the address bus of the 
6502, and if the address falls in the correct range, the card pulls the /INH 
line low.  In the earlier example of a new zero page, if the address bus was 
in the range $0-$FF the card would pull /INH low.

Differences:  IIe vs. ][+

On the Apple ][+, select decoding was not necessarily needed because the /INH 
line only affected the ROM and not the RAM.  If the Apple ][+ peripheral card 
wanted to bank in extra language cards at the addresses $D000-$FFFF, then it 
could pull the /INH line and keep it low during any memory access.  This 
action would disable the on-board ROM and not any other memory accesses such 
as zero page or stack.  This same card would not work in the IIe, since the 
next instruction fetch to RAM after pulling /INH low would read a floating bus 
because all the memory would be disabled.

Another Feature

For those of you who love to muck around in the guts of the Apple IIe, one 
more feature has been added to the /INH function.  The /INH line will also 
override DMA accesses to memory on the motherboard.  This override means that 
if a peripheral card uses DMA to read or write to memory, another peripheral 
card could pull the /INH line and process the DMA access.  An example of this 
would be a co-processor card using the memory on a RAM card in another slot.  
Rather than have the co-processor write to the memory on the motherboard then 
have the 6502 write to the RAM card, the co-processor can write to an address 
that the RAM card recognizes.  The RAM card could then pull the /INH line and 
it would be free to read or write the data bus.  This technique could also be 
used by a co-processor to write directly to a printer card in another slot.


The peripheral card must wait for the address bus to settle, which occurs a 
maximum of 190 ns after the falling edge of 0o, before pulling the /INH line.  
(The 6502A maximum address setup time is 140 ns from 02, with a worst case 
6502A skew of 50 ns from 0o to 02.)  To guarantee that the RAM is disabled and 
a write does not accidentally take place to the motherboard, the /INH line 
must be pulled low within 330 ns of 0o.

            ___________________________                             __________
___________|       Video phase         |___________________________|

___________                             ___________________________
           |___________________________|        CPU phase          |__________

            _______________   210 ns    _______________             __________
___________|    280 ns     |___________|               |___________|

___________________  _________________________________________________________
ADDR               \/

         | 190 ns   |
                    | 140 ns |
_________           _________  _________________________________
INH      \_________/         \/            valid                \_____________
_________/         \_________/\_________________________________/
              1         2

1. The INH line can be pulled high at this time.
2. The INH line can be pulled low (or high) after the addresses are valid at
   190 ns, but before 300 ns (from 0o).

                      Figure 1 - INH Line Timing Signals


Figure 2 illustrates a simple example of a circuit that can be used to 
implement the /INH function.

               |        |
        A15 ---|        |
               | Select |
        A14 ---| Decode |
               | Logic  |             _______
        __/\/--|        |            |       |
               |        |------------|D  7  Q|--- INH* J[1-7]32
        A0  ---|        |  PULLINH*  |   4   |
               |________|            |   L   |
                                     |   S   |
                |\                   |   3   |
        Q3  ----| >o-----------------|>  7   |
                |/                   |_  9   |
        0o  -------------------------|G      |

            Figure 2 - Circuit Implementing /INH Function

An Application

The circuit in Figure 3 can be used to replace the code in the monitor ROM, 
from location $FC00 to $FFFF, with custom code.  Anytime the address space 
between $FC00-$FFFF is accessed, the /INH line is pulled low, the motherboard 
memory is disabled, and the circuit's 1K RAM is enabled instead.  Part of this 
feature can be disabled and the motherboard memory can be read by keeping the 
switch connected to +5 volts (READDIS).  Whenever the system writes to any 
location in the address space $FC00-$FFFF, the circuit will disable any RAM on 
the motherboard and instead write into the 1K RAM.

Here is a series of commands that can be used with the circuit to replace the 
reset vector at $FFFC and $FFFD.  A new reset routine can be written that will 
print the screen or save the status of all the registers whenever the Reset 
key is pressed.

Start the system with the circuit's switch connected to +5 (READDIS).  Doing 
so will enable the system to read the monitor ROM during startup, before the 
1K RAM has been initialized.

Get into the monitor by typing CALL -151.  The system prompt should now be an 
asterisk (*).  Copy the monitor ROM into the 1K RAM with the command 
FC00 \ /  |              +--|_____/                  |       |
         O   |              |                           |       |
        _|___|_____         |            _____          |_      |
A9  ---| __        |        |           /     |--+------|G     Q|---+
A8  ---| OE  WE  CE|------------------O(   00 |  |      |_______|   |
A7  ---|           |--- D7  |           \_____|-----+               |
A6  ---|   2159    |--- D6  |                    |  +---------------+
A5  ---|  1K RAM   |--- D5  |                    |  
A4  ---|           |--- D4  |                    |    ^ +5V
A3  ---|           |--- D3  |                    |    |
A2  ---|           |--- D2  |                    |    o READDIS
A1  ---|           |--- D1  |                    |   /
A0  ---|___________|--- D0  +----------------------o/
PH0 ---------------------------------------------+    o READDEN

           Figure 3 - Circuit to Replace Monitor ROM Code

Further Reference
    o    Apple IIe Technical Reference Manual

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