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:
| 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.
