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Q66170: Repeat Prefix (REP) Documentation Error |
/pubs/pc/reference/microsoft/kb/Q66170/ |
Article: Q66170
Version(s): 5.00 | 5.10
Operating System: MS-DOS | OS/2
Flags: ENDUSER | docerr
Last Modified: 12-NOV-1990
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.
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