Machine-Level Programming II: Arithmetic & Control - Study Notes
Lecture Topics:
- Complete Addressing Mode, leal Instruction
- Arithmetic Operations (2-operand, 1-operand)
- Control: Condition Codes
- Conditional Branches
- Loop Constructs
Address Computation:
- leal Src, Dest: Computes address without memory reference
- Useful for:
* p = &x[i]
* Arithmetic expressions like x + k*y (k=1,2,4,8)
Example:
int mul12(int x) { return x*12; }
leal (%eax,%eax,2), %eax // x*3
sall $2, %eax // x*12
Arithmetic Instructions:
- 2-Operand: addl, subl, imull, sall, sarl, shrl, xorl, andl, orl
- 1-Operand: incl, decl, negl, notl
- XOR is commonly used to zero a register: xorl %eax, %eax
Example: Complex Expression
int arith(int x, int y, int z)
{
int rval = (x+y+z) * (x+4+48*y);
return rval;
}
Assembly:
movl 8(%ebp), %ecx
movl 12(%ebp), %edx
leal (%edx,%edx,2), %eax
sall $4, %eax
leal 4(%ecx,%eax), %eax
addl %ecx, %edx
addl 16(%ebp), %edx
imull %edx, %eax
Condition Codes:
- Set implicitly by arithmetic: CF, ZF, SF, OF
- Set explicitly via:
* cmpl Src2, Src1 (a - b)
* testl Src2, Src1 (a & b)
Read via:
- setX (sete, setne, sets, setns, setg, setge, setl, setle, seta, setb)
- jX (jmp, je, jne, js, jns, jg, jge, jl, jle, ja, jb)
