/*When Terry Davis was a kid with a Commodore 64,
the 6502 chip had no multiply instruction
and this is how he had to do it, except,
he used more registers in this example.
*/
asm {
//Opcodes are slightly different to make writing the 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
};

//The 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;