BUG: No Warning Moving Segment Register to Extended Reg.

• Microsoft Macro Assembler for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11
• Microsoft Macro Assembler for OS/2, versions 5.1, 5.1a, 6.0, 6.0a, and 6.0b

SYMPTOMS

With the Microsoft Macro Assembler (MASM), an application can move data between one of the 16-bit segment registers and a 32-bit extended register. The assembler does not provide any error or warning message when an application makes such a move even though the high-order 16 bits of the 32-bit extended register are filled with a random value or are discarded in the move operation.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 5.1, 5.1a, 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

Even though assembling the following code example should generate two assembler error or warning messages, it does not. The application runs; however, the EAX register is not restored to its original value as may be intended.

Sample Code

; Assembler options needed: /c /W3

.MODEL small .386 .STACK 4096

.CODE start:

    mov eax, 12345678h
    mov es, eax    ; this should generate a warning or an error
    mov eax, 87654321h
    mov eax, es    ; this should generate a warning or an error
    mov ax, 4C00h
    int 21h