private void EmitModRM(byte subOpcode, ref AddrMode addrMode)
{
byte modRM = (byte)((subOpcode & 0x07) << 3);
if (addrMode.BaseReg > Register.None)
{
Debug.Assert(addrMode.BaseReg >= Register.RegDirect);
Register reg = (Register)(addrMode.BaseReg - Register.RegDirect);
Builder.EmitByte((byte)(0xC0 | modRM | ((int)reg & 0x07)));
}
else
{
byte lowOrderBitsOfBaseReg = (byte)((int)addrMode.BaseReg & 0x07);
modRM |= lowOrderBitsOfBaseReg;
int offsetSize = 0;
if (addrMode.Offset == 0 && (lowOrderBitsOfBaseReg != (byte)Register.EBP))
{
offsetSize = 0;
}
else if (InSignedByteRange(addrMode.Offset))
{
offsetSize = 1;
modRM |= 0x40;
}
else
{
offsetSize = 4;
modRM |= 0x80;
}
bool emitSibByte = false;
Register sibByteBaseRegister = addrMode.BaseReg;
if (addrMode.BaseReg == Register.None)
{
emitSibByte = (addrMode.IndexReg != Register.NoIndex);
modRM &= 0x38; // set Mod bits to 00 and clear out base reg
offsetSize = 4; // this forces 32-bit displacement
if (emitSibByte)
{
// EBP in SIB byte means no base
// ModRM base register forced to ESP in SIB code below
sibByteBaseRegister = Register.EBP;
}
else
{
// EBP in ModRM means no base
modRM |= (byte)(Register.EBP);
}
}
else if (lowOrderBitsOfBaseReg == (byte)Register.ESP || addrMode.IndexReg.HasValue)
{
emitSibByte = true;
}
if (!emitSibByte)
{
Builder.EmitByte(modRM);
}
else
{
modRM = (byte)((modRM & 0xF8) | (int)Register.ESP);
Builder.EmitByte(modRM);
int indexRegAsInt = (int)(addrMode.IndexReg.HasValue ? addrMode.IndexReg.Value : Register.ESP);
Builder.EmitByte((byte)((addrMode.Scale << 6) + ((indexRegAsInt & 0x07) << 3) + ((int)sibByteBaseRegister & 0x07)));
}
EmitImmediate(addrMode.Offset, offsetSize);
}
}
