/*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;