Hello there,
I am trying to self-learn some Assembly and just picked up The Shellcoder's Handbook. There is a short C program regarding a triangle in Ch. 1:
int triangle (int width, int height){
int array[5] = {0,1,2,3,4};
int area;
area = width * height/2;
return (area);
}In disassembling the code using gnu's gdb and using intel, I see the following instructions:
(gdb) disassemble
Dump of assembler code for function triangle:
0x080483d7 <triangle+0>: push ebp
0x080483d8 <triangle+1>: mov ebp,esp
0x080483da <triangle+3>: sub esp,0x20
0x080483dd <triangle+6>: mov DWORD PTR [ebp-0x18],0x0
0x080483e4 <triangle+13>: mov DWORD PTR [ebp-0x14],0x1
0x080483eb <triangle+20>: mov DWORD PTR [ebp-0x10],0x2
0x080483f2 <triangle+27>: mov DWORD PTR [ebp-0xc],0x3
0x080483f9 <triangle+34>: mov DWORD PTR [ebp-0x8],0x4
0x08048400 <triangle+41>: mov eax,DWORD PTR [ebp+0xc]
0x08048403 <triangle+44>: imul eax,DWORD PTR [ebp+0x8]
0x08048407 <triangle+48>: mov edx,eax
0x08048409 <triangle+50>: shr edx,0x1f
0x0804840c <triangle+53>: lea eax,[edx+eax*1]
0x0804840f <triangle+56>: sar eax,1
0x08048411 <triangle+58>: mov DWORD PTR [ebp-0x4],eax
0x08048414 <triangle+61>: mov eax,DWORD PTR [ebp-0x4]
0x08048417 <triangle+64>: leave
0x08048418 <triangle+65>: ret
End of assembler dump.Most of it makes perfect sense but I was confused a bit by what was going on with 0x08...09 (triangle+50) through 0x08...0f (triangle+56). From what I understand, the shr instruction is an algorithm for signed numbers to determine whether the number is positive or negative by getting rid of all but the H.O bit which would be 1 if negative. From what I understand, this would determine whether or not to assign eax value of edx + 1 in the lea instruction. If it's >= 0, you assign just edx value to eax (as here using positive numbers as I have). Then the sar instruction would effectively divide the result by two which would get returned. I am mostly looking for confirmation that I am understanding what is going on but I would like to see this mathematically shown if possible.
Also, is there any significance to multiplying eax by 1 in the lea instruction?
Thanks for any help in advance!
