Multiplying 32 bit two numbers on 8086 microprocessor

Question

I have code example for multiplying two 16 bit numbers on 8086 and trying to update it for two 32 bit numbers multiplying.

start:
 MOV AX,0002h ; 16 bit multipli         


        

Answer 1

Solution n. 2 seems that not work if the product is large more then 32 Bit. Furthermore the shift instructions are wrong. This solution work correctly:

Procedure _PosLongIMul2; Assembler;

{INPUT:

 DX:AX-> First factor (destroyed).
 BX:CX-> Second factor (destroyed).

 OUTPUT:

 BX:CX:DX:AX-> Multiplication result.

 TEMP:

 BP, Di, Si}

Asm

     Jmp   @Go

 @VR:DD    0      {COPY of RESULT     (LOW)}
     DD    0      {COPY of RESULT    (HIGH)}

 @Go:Push  BP

     Mov   BP,20H {32 Bit Op.}

     XOr   DI,DI  {COPY of first op.  (LOW)}
     XOr   SI,SI  {COPY of first op. (HIGH)}

     Mov   [CS:OffSet @VR  ],Word(0)
     Mov   [CS:OffSet @VR+2],Word(0)
     Mov   [CS:OffSet @VR+4],Word(0)
     Mov   [CS:OffSet @VR+6],Word(0)

 @01:ShR   BX,1
     RCR   CX,1

     JAE   @00

     Add   [CS:OffSet @VR  ],AX
     AdC   [CS:OffSet @VR+2],DX
     AdC   [CS:OffSet @VR+4],DI
     AdC   [CS:OffSet @VR+6],SI

 @00:ShL   AX,1
     RCL   DX,1
     RCL   DI,1
     RCL   SI,1

     Dec   BP
     JNE   @01

     Mov   AX,[CS:OffSet @VR]
     Mov   DX,[CS:OffSet @VR+2]
     Mov   CX,[CS:OffSet @VR+4]
     Mov   BX,[CS:OffSet @VR+6]

     Pop   BP

End;

This works between two unsigned integer.

If you want to multiply a 32 Bit unsigned integer for a 16 Bit unsigned integer, you can use the Mul instruction as follow:

Function Mul32Bit(M1:LongInt;M2:Word):LongInt; Assembler;

Asm
 LEA   SI,M1
 Mov   AX,[SS:SI]
 Mov   CX,[SS:SI+2]
{CX:AX contains number to multiply by}
 Mov   BX,M2
{BX contains number that multiply}
 Mul   BX
 XChG  AX,CX
 Mov   SI,DX
 Mul   BX
 Add   AX,SI
 AdC   DX,0
{DX:AX:CX contains the result of multiplication}
 Mov   DX,AX
 Mov   AX,CX
{DX:AX contains the partial result of m. and is the function's result}
End;