How to implement SignExtend() function in C ?

[kestaz]

I looking how to implement SignExtend function in C:
EIP = EIP + SignExtend(Destination);
I read it in intel manuals. Intel manuals has this function explanation but still don't know .. So how ?

[Combuster]

What about the explanation don't you get?

Since you're probably talking about emulating some x86 hardware, you basically add a signed byte to an int. Sign extension happens automatically in C once you convert between the correct types.

[kestaz]

Just SET most significant byte ?
a = (a & 0x8000) ?

One man do to implement such function like this

unsigned char c ;

signed extended = -c ; ? Does this correct ?

[kestaz]

i was not right like this: a |= 0x8000 ;

[bewing]

Code: Select all

void foobar()
{
	uint16 i;
	int64 s;
	i = 45000;
	s = (int64) i;		// sign extension happens automatically -- s is negative
}

[kestaz]

Ok i wrote this:

void emulator::jcc8(unsigned char rel8)
{
short offset = rel8 ; // sign extended
ip = (ip + offset - 1) & 0xFFFF;
}