Article ID: 39372
Article Last Modified on 10/29/2003
APPLIES TO
- Microsoft Macro Assembler 5.0
- Microsoft Macro Assembler 5.1 Standard Edition
- Microsoft Macro Assembler 5.1a
- Microsoft Macro Assembler 6.0 Standard Edition
- Microsoft Macro Assembler 6.0a
- Microsoft Macro Assembler 6.0b
This article was previously published under Q39372
SUMMARY
There is a documentation error on page 77 of the "Microsoft Macro Assembler 5.1 Reference" and page 112 of the "Microsoft Macro Assember Reference" that comes with MASM 6.0, 6.0a, and 6.0b. The encodings given for the following forms of the MOV instruction are incorrectly documented
MOV mem, accum
MOV accum, mem
The manual describes the format of these instructions as 101000dw on those pages. The section describing the encodings (p. 27 for MASM 5.1 and p. 59 for MASM 6.0, 6.0a, 6.0b) says that a value of 1 for d (the direction flag) means that the move goes from memory to register and a 0 for d means to move from the register to memory. This results in an incorrect encoding for these forms of the MOV instruction.
MORE INFORMATION
Page 3-118 of "Intel's iAPX 86/88, 186/188 User's Manual (Programmer's Reference)" provides the correct information.
MOV mem, accum should be 1010001w
MOV accum, mem should be 1010000w
The assember encodes these instructions using the general case
MOV mem, reg
MOV accum, mem
rather than specifically using the special forms for the accumulator. The following is an example of how to encode the forms that use the accumulator by hand.
Sample Code
;Assemble options needed: none
.model small
.data
myoffset dw 23
.code
start: MOV ax, @data
MOV ds, ax
memaccum db 0a1h
dw offset myoffset
accummem db 0a3h
dw offset myoffset
MOV ah, 4ch
INT 21h
END start
Additional query words: 5.00 5.10 5.10a 6.00 6.00a 6.00b
Keywords: KB39372
