Understanding the "Lock" Instruction in x86 Assembly

The "lock" instruction in x86 assembly is a prefix that enforces exclusive ownership of the bus for the subsequent instruction. This ensures that the CPU has complete control over the cache line for that instruction's duration.

Deactivation of Bus Lock

Contrary to common understanding, the "lock" prefix does not cause the CPU to lock the bus indefinitely. The lock is released after the execution of the subsequent instruction. This allows the CPU to maintain optimal performance by only locking the bus when absolutely necessary.

Implementation of Addition using "Lock"

The code snippet you provided implements an atomic increment of a long word at a memory location specified by the value in the ecx register:

1. movl 4(%esp), �x: Copies the address of the variable to be incremented into the ecx register.
2. incl (�x): Atomically increments the long word at the address stored in the ecx register.
3. mov $0,�x: Sets the eax register to 0.
4. setne %al: Sets the value of the al register to 1 if the new value of the variable is not equal to 0 (indicating that the increment operation was successful).

The result is that the variable is atomically incremented by 1, and the eax register is set to 0 if the new value is 0, or 1 otherwise. Note that this operation is an increment, not an addition.