What does cmp %eax, 0x8(%rbp) mean and how is it different from cmp 0x8(%rbp), %eax?

cmp %eax, 0x8(%rbp) computes MEM[rbp+8] - eax; swapping the operands flips the subtraction, which matters for ordered jumps (jl/jg) though not for je/jne.

# Computes: MEM[rbp+8] - eax, sets flags
cmp %eax, 0x8(%rbp)
je  label

Reads as: "jump if MEM[rbp+8] == eax"

# Computes: eax - MEM[rbp+8], sets flags
cmp 0x8(%rbp), %eax
je  label

Reads as: "jump if eax == MEM[rbp+8]"

For je/jne it doesn't matter — equality is symmetric. But for jl/jg/etc., order matters:

Remember: cmp src, dst → condition is about dst relative to src.

Go deeper:

• Intel SDM (felixcloutier): CMP subtracts the second operand from the first (temp := SRC1 - SRC2) and sets flags — swapping operands flips the subtraction direction.
• Wikipedia — x86 assembly language: AT&T source-first vs Intel destination-first operand order, the root of the reading confusion.