View source for Patching OS/2 for fast machines

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.



<pre>
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.
</pre>

It's simple enough to do in HEX.