FIX: Upper Half of 32-bit Registers, GS, and FS Corrupt

MS-DOS         | WINDOWS

The information in this article applies to:

• Microsoft CodeView for MS-DOS, versions 4.0, 4.01, and 4.05
• Microsoft CodeView for Windows, versions 4.0, 4.01

SYMPTOMS

When single-stepping or tracing through code in Microsoft CodeView versions 4.0, 4.01, and 4.05, the lower half of the 32-bit registers (eax, ebx, edi, etc.) is always preserved, but the upper half may be corrupted. Other 386- specific registers, such as the gs and fs registers, may also be corrupted. This problem also occurs when animating. This problem does not occur if the instructions are not executed one at a time.

STATUS

Microsoft has confirmed this to be a problem in CodeView version 4.0, 4.01, and 4.05. This problem was corrected in CodeView version 4.1.

MORE INFORMATION

The following sample code can be used to demonstrate this problem. The ebx register will get the value FFFFFFFF, but single-stepping on the NOP will change the value to 0000FFFF.

Sample Code

; Assemble options needed: /Zi

_text SEGMENT para public 'code'

   ASSUME cs:_text, ds:_text

   .386

   MOV ebx, 0ffffffffh
   NOP

   MOV ah, 4ch
   INT 21h

stack SEGMENT para stack 'stack'

   DW 64 dup( ? )

   END main