Difference between revisions of "Patching OS/2 for fast machines"
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| − | This was just posted today on [http://www.os2museum.com OS/2 Museum]... | + | This was just posted today on [http://www.os2museum.com OS/2 Museum]... This is needed for all 1.x series versions of [[OS/2]] for fast machines, regardless if they are being emulated or not. |
Revision as of 03:13, 21 May 2011
This was just posted today on OS/2 Museum... This is needed for all 1.x series versions of OS/2 for fast machines, regardless if they are being emulated or not.
The crash is a division by zero (trap 0) on a DIV CX instruction where the CX register contains zero. The way around this problem is patching OS/2 and getting rid of the bad loop. The original code sequence looks like this:
MOV DX, 3
L1:
XOR CX, CX
L2:
LOOP L2 ; loop 65,536 times
DEC DX ; run the loop 3 times
JNE L1
CLI
... ; read current tick
STI
SUB CX, BX
MOV AX, 1F4h ; 500
MOV BX, 0C8h ; 200
MUL BX ; 500 * 200 = 100,000
DIV CX ; traps if CX = 0
SHR CX, 1
This obviously became a problem as far back as 1991, because OS/2 1.30.1 contains slightly different code:
MOV BX, 0C8h ; 200
MUL BX
CMP CX, 0
JNE L3
MOV CX, 1Fh ; 31
L3:
DIV CX
In other words, if CX is zero, it is forced to a non-zero value to avoid the division by zero trap.
As it turns out, the MOV/MOV/MUL/DIV instruction sequence is quite unique and occurs only in the BASEDD0x.SYS drivers. To avoid the crash, it is sufficient to remove the DIV instruction, replacing it by NOPs.
It's simple enough to do in HEX.
