Date: Mon, 26 Sep 94 14:31:08 +0100
From: buers AT DG1 DOT CHEMIE DOT UNI-KONSTANZ DOT DE (Dieter Buerssner)
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject:  bug in movedata (or in gas), with fix


Hello,

In the file dosmem.s in libsrc/c/dos gas produces for the jcxz zero
instructions code for a jump with 16 bit displacements. This will mask
the high word of EIP, so (when cx is zero) it will jump from say
0x00012345 to 0x00002347 instead of the correct 0x00012347.  In the
gas manual, there is mentioned, that using jcxz may be risky, but I
really don't understand what they mean. The code should get assembled
correctly (with 8-bit displacement, "short" jump).  The following
patch seems to fix the problem (using jz instead of jcxz).

Note, that this breaks code, that uses the conio library (in libpc).

BTW, this was quite tricky to find, because it will only show when
dosmem is linked at a high enough address. If you want to write
a small program, to find the problem, it won't show anymore.

Dieter

*** dosmem.os	Sat Oct  9 18:42:28 1993
--- dosmem.s	Fri Sep  9 00:05:00 1994
***************
*** 100,112 ****
  	movl	24(%ebp),%ecx
  	pushl	%ecx
  	shrl	$2,%ecx
! 	jcxz	no_big_move
  	rep
  	movsl
  no_big_move:
  	popl	%ecx
  	andl	$3,%ecx
! 	jcxz	no_little_move
  	rep
  	movsb
  no_little_move:
--- 100,112 ----
  	movl	24(%ebp),%ecx
  	pushl	%ecx
  	shrl	$2,%ecx
! 	jz	no_big_move
  	rep
  	movsl
  no_big_move:
  	popl	%ecx
  	andl	$3,%ecx
! 	jz	no_little_move
  	rep
  	movsb
  no_little_move: