Understanding %eax, (%eax), Cmp, And Jumps In Assembly Language.pdf

Additional Notes - Brandon Lum

This note clarifies concepts related to `%eax` and jump instructions (`cmp`, `test`, and their associated jumps).

%eax vs (%eax):

- `%eax` refers to the register %eax.
- `(%eax)` refers to the value stored in the register %eax.

Example:

- If `%eax = 0xffffe008`, then `movw %eax, %ebx` will copy the value 0xffffe008 into %ebx.
- Conversely, `movw (%eax), %ebx` will load the value stored at the memory location pointed to by %eax into %ebx (which is 0xaabbccdd in this case).

Cmp and Test Instructions:

These instructions are used for conditional jumps. They compare values and set flags within the processor based on the result.

Jump Types: `je` (jump if equal), `jne` (jump if not equal), `jg` (jump if greater), `jge` (jump if greater or equal), etc.

How it Works:

- `cmp x, y`: Calculates `y - x` and sets flags based on the result.
- `test x, y`: Calculates `y & x` and sets flags based on the result.

Flag Explanation:

Flags within the processor are set based on the comparison:

Zero Flag (ZF): 1 if result is zero, 0 otherwise.
Sign Flag (SF): 1 if most significant bit of the result is set, 0 otherwise.
Other Flags: Carry (CF), Parity (PF), Overflow (OF) - see recitation slides for details.

Examples:

- `cmp 0x3, 0x1`: Sets ZF to 0 (not equal) and SF based on the sign of -2.
- `test %eax, %eax`: Sets ZF to 1 (zero) if `%eax` is zero, else 0.

Jumping Based on Flags:

Jumps (`jmp *Operand`, `je`, etc.) only execute if the corresponding flag is set.