pcjs/disks/pc/unix/microport/system-v/2.3/README.md

Debugging Notes

The boot sector contains the following code:

0000:7C3E B003 MOV AL,03 0000:7C40 B200 MOV DL,00 0000:7C42 B417 MOV AH,17 0000:7C44 CD13 INT 13 0000:7C46 72B8 JC 7C00

For BIOSes that don't support INT 0x13, AH function 0x17, patch the JC with two NOPs; eg:

e 0:7c46 90 90

After booting, the following code is executed:

12E8:01D3 52              PUSH     DX                   ;history=5
12E8:01D4 51              PUSH     CX                   ;history=4
12E8:01D5 2BC9            SUB      CX,CX                ;history=3
12E8:01D7 BA6400          MOV      DX,0064              ;history=2
12E8:01DA ED              IN       AX,DX                ;history=1

For virtual machines that don't support word I/O to port 0x64, patch IN AX,DX with IN AL,DX; eg:

e 12e8:1da ec