Microsoft KB Archive/39372

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

Article ID: 39372

Article Last Modified on 10/29/2003

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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

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This article was previously published under Q39372



SUMMARY
There is a documentation error on page 77 of the &quot;Microsoft Macro Assembler 5.1 Reference&quot; and page 112 of the &quot;Microsoft Macro Assember Reference&quot; 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 &quot;Intel's iAPX 86/88, 186/188 User's Manual (Programmer's Reference)&quot; 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

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