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AltairZ80: i86: Don't clear segment override on REPE/REPNE

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MASM generates REP CS:MOVSW as:

REP
CS:
MOVSW

The Seattle Computer Products 8086 assembler allows the segment override
as a discrete opcode.  This means that:

CS:
REP
MOVSW

is also possible, and in fact used in the SCP Monitor v1.6.

The i86 emulator was clearing the override flags when REPE/REPNE,
is encountered, which causes the segment override to be lost.
Verifying the behavior of a real 80286 processor in an IBM PC/AT
(5170) using MS-DOS 6.22 DEBUG shows that both instruction sequences
yield the desired segment override.
This commit is contained in:
Howard M. Harte 2022-11-05 19:55:40 -07:00 • committed by Paul Koning
parent e1d27aa167
commit e91fbebe3d
1 changed files with 0 additions and 2 deletions
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2
AltairZ80/i86_ops.c
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@ -4321,14 +4321,12 @@ static void i86op_lock(PC_ENV *m)
static void i86op_repne(PC_ENV *m)
{
m->sysmode |= SYSMODE_PREFIX_REPNE;
DECODE_CLEAR_SEGOVR(m);
}
/*opcode=0xf3*/
static void i86op_repe(PC_ENV *m)
{
m->sysmode |= SYSMODE_PREFIX_REPE;
DECODE_CLEAR_SEGOVR(m);
}
/*opcode=0xf4*/