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The Commodore PET 8096 and 8296 (also known as CBM 8096 and CBM 9296) computers have at least 96 kByte of RAM and can remap the ROM area with RAM. This makes them a nice base for OS/A65. Although they don't have many I/O devices at least they provide an 80 columns screen.
In the CBM 8x96 one can remap the upper 32k of CPU address space by
writing certain values to $fff0. The blocks $8000-$bfff and
$c000-$ffff can be remapped to two different banks of RAM
each. The screen memory ($8000-$9000) and I/O
($e800-$efff) can be set to `peek-through' separately.
In the OS/A65 memory configuration the lower 32k and one bank of the upper
32k of RAM are used for the OS image (the `ROM') and the lib6502 managed
application memory. The screen memory and I/O area are not accessible for
lib6502 applications. The second upper 32k RAM bank is used (with screen
and I/O mapped through) as system memory area, together with some RAM
from $300-$b00. Also there are the mirrors of the other
virtual console screen areas - when switching screens they are copied around.
Addr RAM 0 RAM 1
$ffff ----------------------
$fff0 configuration register
$ff00 ------- -------
system
| memory
$f000 -------
| I/O
$e800 -------
lib6502
application
memory
$b000 -------
| virtual
screens
$9000 | -------
screen
$8000 | -------
$6800 -------
OS/A65 |
`ROM'
image |
$0b00 ------
system no
RAM memory!
$0200 ------
Stack |
$0100 ------
Zerop.
$0000 ----------------------
Building the 8x96 ROM image is simple. Just go to the arch/cbm8x96/boot directory and type "make". This should build all files. "make osa.d64" should make a d64 VC1541 disk image with all necessary files. You have to have my new XA version in the path, though.
To customize the ROM, change edit rom.a65 in arch/cbm8x96. You can gain some
memory when you remove the shell/monitor from the ROM image.
(You could replace it with the lib6502 monitor "mon" to be loaded like
"lsh".)
When running "make" then the command "file65" should print the addresses
used for the different segments. They must not overlap. In addition the
data segment must be below $8000, as it is needed by lib6502 that can
be called from both RAM banks.
To move the lib6502 RAM area, edit the values for Memstart, Memend
and Zerostart at the end of rom.a65.
To move the segments, edit the Makefile and change the addresses for the
segments in the XA calls. The text segment address must be two below
the actual start as it also counts the two byte starting address used
for loading. In addition you have to edit the value for the definition
of ROMSTART in rom.a65.
Put the files "loader", "lsh" and "rom" on a disk accessible by the PET 8x96. load "loader" into BASIC memory and start it (probably edit it for the right load device number). This boots OS/A65.
Build a version for this computer is a bit more difficult because of the restricted memory. I have now tried to build such a version. The device does not (yet) check for 40 columns but can now use only 80 cols.
The memory map is straightforward. First zeropage, stack, then system memory (all of it), then OS image, then lib6502 memory. At the end ($7800) a second virtual screen, and at $8000 the normal screen area. At $90-$93 in the zeropage the PET ROM IRQ and BRK vectors are located and have to be used.
Basically the same applies as for the 8x96.