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Apple II Computer Info

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In article <[email protected]>,

John Edward Flanders <[email protected]> wrote:

> Could anyone please explain the locations and functions of the memory bank

> switches on an Apple //e with 128k memory?

I assume you are referring to the IIe-specific bank switches relating

to the 64K auxiliary memory, and not the $D000-$FFFF soft-switches,

which are the same as the ][/][+ with a language card installed. (I

have described these anyway.)

Before mentioning the soft-switches, I should first cover the firmware

routines that are normally used to access auxiliary memory. These

routines are generally easier to use than trying to control the

soft-switches directly, as it avoids the problem of your memory

switching out from underneath you while you are trying to execute code!

Two special routines are provided in the IIe firmware. They are

dependent on the built-in 80-column firmware, so they might not be

available if a card with firmware on it is installed in slot 3.

AUXMOVE ($C311) is used to copy data either way between main and

auxiliary memory (in the $0200-$BFFF area).

Before calling AUXMOVE, the following zero page locations must be set up:

$3C - $3D Source starting address (low byte, high byte)

$3E - $3F Source ending address (low byte, high byte)

$42 - $43 Destination startion address (low byte, high byte)

In addition, the carry flag must be set to indicate the direction of

the move: C=1 to move from main to auxiliary memory, C=0 to move from

auxiliary to main memory.

XFER ($C314) is used to transfer execution between code running in main

and auxiliary memory.

Locations $03ED and $03EE must be set to the address of the destination

routine in the other bank. The carry and overflow flags must be set

as follows: C=1 to transfer execution from main to auxiliary memory,

C=0 for auxiliary to main memory; V=1 to use auxiliary zero page,

stack and language card, V=0 to use main zero page, stack and language

card.

When using this routine, it is also necessary to save and restore the

main and auxiliary stack pointers, which are stored at locations $0100

and $0101 respectively (both in auxiliary memory).

Since these routines execute from the I/O space, they are able to

access both banks without causing problems. For code executing from

main memory, it is possible to write to auxiliary memory, but reading

from auxiliary memory is not directly possible (your code is switched

out).

Auxiliary Memory Soft-switches

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Apple II Computer Technical Information : Apple II Family Hardware Info

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