/*When I was a kid with a Commodore 64,
the 6502 chip had no multiply inst
and this is how we had to do it, except,
I used more regs in this example.
*/
asm {
//Opcodes are slightly different to make writing my x86_64 assembler easier.
//See /Compiler/OpCodes.DD.
//You can clobber RAX,RBX,RCX,RDX,R8,R9. The compiler expects that.
MUL_BY_HAND_U8_U8_TO_U16: //This is only for fun.
//8bit * 8bit-->16bit
//AL*BL-->AX
MOV CL,8
SHL AX,8
@@05: SHL1 AX
JNC @@10
ADD AL,BL
@@10: DEC CL
JNZ @@05
RET
_MUL_BY_HAND_U8_U8_TO_U16:: //C callable
PUSH RBP
MOV RBP,RSP
MOV AL,U8 SF_ARG1[RBP] //SF_ARG1
MOV BL,U8 SF_ARG2[RBP]
CALL MUL_BY_HAND_U8_U8_TO_U16
MOVZX RAX,AX
POP RBP
RET1 16
_MUL_U64_U64_TO_U128::
//64bit * 64bit-->128bit
PUSH RBP
MOV RBP,RSP
MOV RBX,U64 SF_ARG3[RBP]
MOV RAX,U64 SF_ARG1[RBP] //SF_ARG1
MUL U64 SF_ARG2[RBP] //Res RDX:RAX 128bit
MOV U64 [RBX],RAX
MOV U64 8[RBX],RDX
POP RBP
RET1 24
};
//My convention is to put an underscore
//on C callable asm routines.
_extern _MUL_BY_HAND_U8_U8_TO_U16 U16 MulU8(U8 n1,U8 n2);
class U128
{
U64 lo,hi;
};
_extern _MUL_U64_U64_TO_U128 U0 MulU64(I64 n1,I64 n2,U128 *_prod);
U0 MulByHand()
{
U128 p;
"2*7=0x%X\n",MulU8(2,7);
"100*10=0x%X\n",MulU8(100,10);
MulU64(0x0123456789ABCDEF,0x1000001,&p);
"0x0123466789ABCDEF*0x1000001=0x%016X%016X\n",p.hi,p.lo;
}
MulByHand;