Microsoft KB Archive/39372

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DOCERR: MOV mem,accum and MOV accum,mem Formats Documented Wrong

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.

Sample Code

;Assemble options needed: none

             .model small

myoffset     dw      23


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