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Home > Forums > Programmers > Languages > Assembly Forum

x86 Assembly, multiplication of 2 64 bit integers on 32 bit processor

thread272-1602954
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jakethecake (TechnicalUser)
6 May 10 9:51
Hi I want to write an x86 assembler function that multiplicates two 64 bit integers on a 32 bit processor. It has the following C signature:

void llmultiply(unsigned long long int l1, unsigned long long int l2, unsigned char *result);

The result of l1 * l2 are to be but in an array pointed to by *result.

After:

push ebp
mov ebp, esp

the stack looks like this:

CODE

| least significant byte of ebp                             | 0x3fffffe4  <---- ebp and stack pointer point here
|                 byte 1 of ebp                             | 0x3fffffe5
|                 byte 2 of ebp                             | 0x3fffffe6
|  most significant byte of ebp                             | 0x3fffffe7
| least significant byte of return address (byte 0)         | 0x3fffffe8  <--- the stack pointer points here after entering the function
|             byte 1 of return address                      | 0x3fffffe9
|             byte 2 of return address                      | 0x3fffffea
| most significant byte of return address (byte 3)          | 0x3fffffeb
| least significant byte of parameter 1 (byte 0 of l1)      | 0x3fffffec  ---
|                  byte 1 of l1                             | 0x3fffffed    |   here is A low
|                  byte 2 of l1                             | 0x3fffffee    |
|                  byte 3 of l1                             | 0x3fffffef  ---
|                  byte 4 of l1                             | 0x3ffffff0  ---
|                  byte 5 of l1                             | 0x3ffffff1    |   here is A high
|                  byte 6 of l1                             | 0x3ffffff2    |
| most significant byte of parameter 1 (byte 7 of l1)       | 0x3ffffff3  ---
| least significant byte of parameter 2 (byte 0 of l2)      | 0x3ffffff4  ---
|                  byte 1 of l2                             | 0x3ffffff5    |   here is B low
|                  byte 2 of l2                             | 0x3ffffff6    |
|                  byte 3 of l2                             | 0x3ffffff7  ---
|                  byte 4 of l2                             | 0x3ffffff8  ---   <---- [ebp + 20] points here
|                  byte 5 of l2                             | 0x3ffffff9    |   here is B high
|                  byte 6 of l2                             | 0x3ffffffa    |
| most significant byte of parameter 2 (byte 7 of l2)       | 0x3ffffffb  ---
| least significant byte of the address of the result array | 0x3ffffffc
|                 byte 1 of the address of the result array | 0x3ffffffd
|                 byte 2 of the address of the result array | 0x3ffffffe
|  most significant byte of the address of the result array | 0x3fffffff  <--- stack bottom

The multiplication can be done like this:
a * b = AH * BH * 2^64 + (AH * BL + AL * BH) * 2^32 + AL * BL.

Where H and L stands for high and low bits of a and b. The multiplication part is however not my biggest problem, I could solve that myself. What I'm having trouble with is putting the product in the result array so that it will be pointed to by the result pointer in:

void llmultiply(unsigned long long int l1, unsigned long long int l2, unsigned char *result);

So if someone could help me with that part atleast, some help on the multiplication part would also be welcome but not as important.
jakethecake (TechnicalUser)
10 May 10 8:53
So this is what I've got now:

CODE

    SECTION .data

    SECTION .text
    ALIGN    16
    BITS    32

AL_OFF    EQU     8    ; Offset from EBP to low  bits of a (AL)
AH_OFF    EQU     12    ; Offset from EBP to high bits of a (AH)
BL_OFF    EQU     16    ; Offset from EBP to low  bits of b (BL)
BH_OFF    EQU     20    ; Offset from EBP to high bits of b (BH)
RES_OFF    EQU     24    ; Offset from EBP to result array pointer
        
    GLOBAL llmultiply

llmultiply:
    PUSH EBP
    MOV EBP, ESP
        
    PUSH EAX
    PUSH EBX
    PUSH ECX
    PUSH EDX    
    PUSH EDI
    PUSH ESI

    MOV EAX, [EBP + AL_OFF]
    MOV EDX, [EBP + BL_OFF]
    MUL EDX             ; AL * BL, EAX = low(AL * BL), EDX = high(AL * BL)

    MOV EDI, EAX             ; Save low(AL * BL) in EDI
    MOV ESI, EDX                ; Save high(AL * BL) in ESI

    MOV EAX, [EBP + AH_OFF]
    MOV EDX, [EBP + BL_OFF]
    MUL EDX               ; AH * BL, EAX = low(AH * BL), EDX = high(AH * BL)

    ADD ESI, EAX               ; Add low(AH * BL) to high(AL * BL)
    ADC ECX, EDX                   ; Add carry from the previous addition to high(AH * BL) and save in ECX

    MOV EAX, [EBP + AL_OFF]
    MOV EDX, [EBP + BH_OFF]
    MUL EDX                     ; AL * BH, EAX = low(AL * BH), EDX = high(AL * BH)

    ADD ESI, EAX                 ; Add low(AL * BH) to high(AL * BL) and low(AH * BL)
    ADC ECX, EDX                 ; Add carry from the previous addition and high(AL * BH) to high(AH * BL) and earlier carry, save in ECX

    MOV EAX, [EBP + AH_OFF]
    MOV EDX, [EBP + BH_OFF]
    MUL EDX                     ; AH * BH, EAX = low(AH * BH), EDX = high(AH * BH)

    ADD EAX, ECX                 ; Add high(AL * BH), high(AH * BL) and carry to low(AH * BH)
    ADC EDX, 0                   ; Add carry from the previous addition to high(AH * BH)

    MOV EBX, [EBP + RES_OFF]
    MOV [EBX], EDI        ; Put low 32 bit of result in array
    MOV [EBX + 4], ESI        ; Put next 32 bit of result in array
    MOV [EBX + 8], EAX        ; Put next 32 bit of result in array
    MOV [EBX + 12], EDX        ; Put high 32 bit of result in array

    POP ESI
    POP EDI
    POP EDX
    POP ECX
    POP EBX
    POP EAX
        
    POP EBP            ; restore EBP reg
    RET                ; return

It's almost working but there's a carry missing somewhere, for an example:

FFFEFFFFFFFFFFFF * FFFF0001FFFFFFFF should be: FFFE0002FFFDFFFE0001FFFE00000001

but I get:
FFFE0001FFFDFFFE0001FFFE00000001

So there should be another carry added to EDX.
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