Microsoft KB Archive/39372
Article ID: 39372
Article Last Modified on 10/29/2003
- 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
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.
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.
;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