Q66170: DOCERR: Repeat Prefix (REP) Documentation Error

Article: Q66170
Product(s): Microsoft Macro Assembler
Version(s): 5.0,5.1
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), version 5.0 
- Microsoft Macro Assembler (MASM), version 5.1 
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SUMMARY
=======

On Page 368 of the "Microsoft Macro Assembler Programmer's Guide," the last
sentence in the second paragraph from the bottom states the following:

  Segment overrides can be used safely when interrupts are turned off, when a
  string instruction is used without a segment override, or when a 80386
  processor is used.

It should read as follows:

  Segment overrides can be used safely when interrupts are turned off, when a
  string instruction is used without a repeat prefix, or when a 80386 processor
  is used.

MORE INFORMATION
================

The repeat prefix may be used safely with segment overrides if a CLI instruction
is performed before the REP instruction and a STI instruction is performed after
it. This process shuts off interrupts during the REP instruction that contains a
segment override. Clearing interrupts in this manner is only needed for
instructions with segment overrides that are preceded with REP.

Please note that the execution of a repeated instruction can take a considerable
amount of time; you may not want to have interrupts shut off for that much time.
It is the programmer's responsibility to ensure that interrupts are not missed
while interrupts are off.

A better alternative if you are moving a large amount of data is to change the DS
register for the duration of the repeated instruction (so that the segment
override is not needed) and restore it afterwards.

The following is an example of using the CLI instruction to disable interrupts
and using the STI instruction to re-enable them:

      ;   set SI and DI

      MOV CX, 20   ; relatively small number of bytes--won't take long
      CLI
      REP MOVS dest,ES:source
      STI

      ;   ...

The following is an example of temporarily using the DS register to avoid the
segment override:

      ;   set SI and DI

      MOV  CX, 4000h ; lots to move -- this will take some time
      PUSH DS        ; save for later
      PUSH ES        ; same as ES: override -- change for other overrides
      POP  DS        ; load into DS
      REP  MOVS dest, source
      POP  DS        ; restore DS

Additional query words: 5.00 5.10

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Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM500
Version           : :5.0,5.1

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