jakethecake
Technical User
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:
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.
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 bottomThe 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.