Patching OS/2 for fast machines - Revision history

Jnc: +cat

2018-12-17T01:44:59Z

+cat

Neozeed: the hex codes. oops

2011-05-21T05:35:38Z

the hex codes. oops

Neozeed at 02:13, 21 May 2011

2011-05-21T02:13:17Z

Neozeed: Created page with "This was just posted today on [http://www.os2museum.com OS/2 Museum]...

 The crash is a division by zero (trap 0) on a DIV CX instruction where the CX register contains z..."

2011-05-21T02:02:13Z

Created page with "This was just posted today on [http://www.os2museum.com OS/2 Museum]... <pre> The crash is a division by zero (trap 0) on a DIV CX instruction where the CX register contains z..."

New page

This was just posted today on [http://www.os2museum.com OS/2 Museum]...

<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.

@@ Line 1: / Line 1: @@
 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>
@@ Line 22: / Line 20: @@
      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:
@@ Line 31: / Line 30: @@
 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.
@@ Line 42: / Line 42: @@
 into this:
 <pre>B8F401BBC800F7E39090</pre>

← Older revision		Revision as of 05:35, 21 May 2011
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	It's simple enough to do in HEX.		It's simple enough to do in HEX.
		+
		+	So basically change this:
		+	<pre>B8F401BBC800F7E3F7F1</pre>
		+	into this:
		+	<pre>B8F401BBC800F7E39090</pre>

← Older revision		Revision as of 02:13, 21 May 2011
Line 1:		Line 1:
−	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.