IMC-2001 Firmware
Intro
The ROM consists of two ROM chips. The bigger chip labelled “ROM1” was broken (it got really hot) and could not be read (it returned all zeroes). It has no markings on it, so one can only guess it is something AT28C256 compatible.
The ROMs sit in the special “slot 0”. The board has two other unused ROM sockets and an arrow pointing to an additional ROM socket on the motherboard itself.
On the schematics the slot is not labelled “slot 0” but “ROM” and this gives away it’s special purpose for the machine.
ROM 2
ROM 2 (sticker with hand-written red 2 on top) is a Mitsubishi M5L2732K, 32k-bit, 4k-ROM, seems very ok during operation (the carvings say “M5L2732K, 83080L, JAPAN”).
lower-case patch
Comparing ROM2 to the standard Apple autostart ROM (https://6502disassembly.com/a2-rom/AutoF8ROM.html) we see only one difference at:
FD80 (F800+580)
AutoF8ROM
00000580 90 02 29 df 9d 00 02 c9 8d d0 b2 20 9c fc a9 8d |..)........ ....|
monitor.rom:
00000580 90 02 ea ea 9d 00 02 c9 8d d0 b2 20 9c fc a9 8d |........... ....|
fd80: 90 02 bcc ADDINP
fd82: 29 df and #$df ;shift to upper case
fd84: 9d 00 02 ADDINP sta IN,x
fd87: c9 8d cmp #$8d
fd89: d0 b2 bne NOTCR
fd8b: 20 9c fc CROUT1 jsr CLREOL
fd8e: a9 8d CROUT lda #$8d
fd90: d0 5b bne COUT
EA EA are 2 nops where the and #$df is. This seems to do away with uppercasing the byte coming from the keyboard, so that lower-case character work.
ROM 1
ROM 1 is hard to guess, what it originally contained (as mine got fried).
addressing mode
I put back the original Autostart ROM, the addressing on the PCB is a little bit weird. As the monitor was working I could write a test ROM with
#!/usr/bin/tcc -run -Werror
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main( int argc, char *argv[] )
{
FILE *f = fopen( "./pattern.rom", "w" );
char *buf = malloc( 1024 );
for( int i = 0; i < 32; i++ ) {
memset( buf, i, 1024 );
fwrite( buf, 1024, 1, f );
}
fclose( f );
free( buf );
return 0;
}
and get the following pattern in the monitor:
D000 1C
D400 1D
D800 1E
DC00 1F
E000 18
E400 19
E800 1A
EC00 1B
So the replacement AT28C256 is arranged as follows:
0x6000 18 must contain the second 4k of the apple rom starting with e000 (24k)
0x7000 1c must contain the first 4k of the apple rom d000
So I ended up doing this:
dd if=/dev/zero of=zero1 bs=1 count=24576
dd if=Applesoft of=apple1 bs=1 skip=4096 count=4096
dd if=Applesoft of=apple2 bs=1 count=4096
cat zero1 apple1 apple2 > newrom1.rom
Unkown difference
When I watched the series on the Apple clone from Adrian’s Digital Basement (he has a nice 3 part mini-series on the fixing of an Apple clone of unknown origin) I noticed the following difference:
ROMS he published ‘Apple\ II\ Clone\ ROM\ E0\ 300854cc.bin’ which has one single difference in the E000 ROM:
; Convert FAC to integer. Must be -32767 <= FAC <= 32767.
e10c: a5 9d AYINT lda FAC ;exponent of value in FAC
e10e: c9 90 cmp #$90 ;abs(value) < 32768?
e110: 90 09 bcc MI2 ;yes, okay for integer
e112: a9 fe lda #<NEG32768 ;no; next few lines are supposed
e114: a0 e0 ldy #>NEG32768 ;to allow -32768 ($8000), but do not!
e116: 20 b2 eb jsr FCOMP ;because compared to -32768.00049
; <<< BUG: A=-32768.00049:A%=A is accepted, but PRINT A,A% shows that A=-
; 32768.0005 (ok), A%=32767 (wrong!) >>>
instead of a0e0 it has a0e5
; <<< meant to be -32768, which would be 9080000000 >>>
; <<< 1 byte short, so picks up $20 from next instruction >>>
e0fe: 90 80 00 00 NEG32768 .bulk $90,$80,$00,$00 ;-32768.00049 in floating point
maybe the variable got moved to e5fe for some reasons? hard to tell, especially as the other ROMs have no differences. I kept mine on the original autostart ROM.
PIN Layout
Slot Rom1 Rom2 Slot Rom1 Rom2
Pin Pin
26 12 14-12 25 28 24
27 20,18 24
28 22 23
29 22
30 21
31 20
32 19
33 18
34 17
35 16
36 15
37 14 2
38 13 23
39 12 20
40 11 24
41 10 25 23
42 9 3 1
43 8 4 2
44 7 5 3
45 6 6 4
46 5 7 5
47 4 8 6
48 3 9 7
49 2 PIN10 8
50 1