xref: /gem5/src/arch/x86/emulenv.cc

/*
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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 * Authors: Gabe Black
 */

#include "arch/x86/emulenv.hh"
#include "base/misc.hh"

using namespace X86ISA;

void EmulEnv::doModRM(const ExtMachInst & machInst)
{
    assert(machInst.modRM.mod != 3);
    //Use the SIB byte for addressing if the modrm byte calls for it.
    if (machInst.modRM.rm == 4 && machInst.addrSize != 2) {
        scale = 1 << machInst.sib.scale;
        index = machInst.sib.index | (machInst.rex.x << 3);
        base = machInst.sib.base | (machInst.rex.b << 3);
        //In this special case, we don't use a base. The displacement also
        //changes, but that's managed by the predecoder.
        if (machInst.sib.base == INTREG_RBP && machInst.modRM.mod == 0)
            base = NUM_INTREGS;
        //In -this- special case, we don't use an index.
        if (index == INTREG_RSP)
            index = NUM_INTREGS;
    } else {
        if (machInst.addrSize == 2) {
            warn("I'm not really using 16 bit MODRM like I'm supposed to!\n");
        } else {
            scale = 0;
            base = machInst.modRM.rm | (machInst.rex.b << 3);
            if (machInst.modRM.mod == 0 && machInst.modRM.rm == 5) {
                //Since we need to use a different encoding of this
                //instruction anyway, just ignore the base in those cases
                base = NUM_INTREGS;
            }
        }
    }
    //Figure out what segment to use. This won't be entirely accurate since
    //the presence of a displacement is supposed to make the instruction
    //default to the data segment.
    if (base != INTREG_RBP && base != INTREG_RSP ||
            0/*Has an immediate offset*/) {
        seg = SEGMENT_REG_DS;
        //Handle any segment override that might have been in the instruction
        int segFromInst = machInst.legacy.seg;
        if (segFromInst)
            seg = (SegmentRegIndex)(segFromInst - 1);
    } else {
        seg = SEGMENT_REG_SS;
    }
}