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Fixed handling of redundant address/data override prefixes

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This commit is contained in:
Jeff Parsons 2015-07-17 10:14:17 -07:00
commit b6e8eb8877
4 changed files with 68 additions and 19 deletions
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43
modules/pcjs/lib/x86ops.js
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@ -219,7 +219,7 @@ X86.opPUSHCS = function PUSHCS()
};
/**
* op=0x0F (POP CS) (undocumented on 8086/8088; replaced with opInvalid on 80186/80188, and op0F on 80286 and up)
* op=0x0F (POP CS) (undocumented on 8086/8088; replaced with opInvalid() on 80186/80188, and op0F() on 80286 and up)
*
* @this {X86CPU}
*/
@ -1375,10 +1375,18 @@ X86.opGS = function GS()
X86.opOS = function OS()
{
if (I386) {
/*
* See opAS() for a discussion of multiple prefixes, which applies equally to both
* operand-size and address-size prefixes.
*
* The simple fix here is to skip the bulk of the operation if the prefix is redundant.
*/
this.opFlags |= X86.OPFLAG.DATASIZE;
this.dataSize ^= 0x6; // that which is 2 shall become 4, and vice versa
this.dataMask ^= (0xffff0000|0); // that which is 0x0000ffff shall become 0xffffffff, and vice versa
this.updateDataSize();
if (!(this.opPrefixes & X86.OPFLAG.DATASIZE)) {
this.dataSize ^= 0x6; // that which is 2 shall become 4, and vice versa
this.dataMask ^= (0xffff0000|0); // that which is 0x0000ffff shall become 0xffffffff, and vice versa
this.updateDataSize();
}
this.nStepCycles -= this.cycleCounts.nOpCyclesPrefix;
}
};
@ -1393,10 +1401,31 @@ X86.opOS = function OS()
X86.opAS = function AS()
{
if (I386) {
/*
* Live and learn: multiple address-size prefixes can and do occur on a single instruction,
* and contrary to my original assumption that the prefixes act independently, they do not.
* During Windows 95 SETUP, the following instruction is executed:
*
* 06AF:1B4D 67672E CS:
* 06AF:1B50 FFA25A1B JMP [BP+SI+1B5A]
*
* which is in fact:
*
* 06AF:1B4D 67672E CS:
* 06AF:1B50 FFA25A1B0000 JMP [EDX+00001B5A]
*
* The other interesting question is: why/how did this instruction get encoded that way?
* All I can say is, there were no explicit prefixes in the source (BSG.ASM), so we'll chalk
* it up to a glitch in MASM.
*
* The simple fix here is to skip the bulk of the operation if the prefix is redundant.
*/
this.opFlags |= X86.OPFLAG.ADDRSIZE;
this.addrSize ^= 0x06; // that which is 2 shall become 4, and vice versa
this.addrMask ^= (0xffff0000|0); // that which is 0x0000ffff shall become 0xffffffff, and vice versa
this.updateAddrSize();
if (!(this.opPrefixes & X86.OPFLAG.ADDRSIZE)) {
this.addrSize ^= 0x06; // that which is 2 shall become 4, and vice versa
this.addrMask ^= (0xffff0000|0); // that which is 0x0000ffff shall become 0xffffffff, and vice versa
this.updateAddrSize();
}
this.nStepCycles -= this.cycleCounts.nOpCyclesPrefix;
}
};