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Q39372: DOCERR: MOV mem,accum and MOV accum,mem Formats Documented Wrong

Article: Q39372
Product(s): Microsoft Macro Assembler
Version(s): 5.0,5.1,5.1a,6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 5.0, 5.1, 5.1a, 6.0, 6.0a, 6.0b 
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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

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Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM500 kbMASM600a kbMASM510a kbMASM600b
Version           : :5.0,5.1,5.1a,6.0,6.0a,6.0b

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