// This is an implentation of TestAndSet for gcc/linux/x86.
// The caller defines int lock. Calling TestAndSet(&lock) sets lock
// to 1 and returns the old value of lock. So, if lock is zero, then
// TestAndSet(&lock) returns zero and sets lock to one. This means
// the lock has been succesfully acquired.  On the other hand, if the lock
// had already been set to one by another process or thread,  then 1
// would be returned. This would indicate to the caller that the lock is
// already being held by another process or thread. The caller keeps retrying
// TestAndSet(&lock) until it returns 0.
    .globl TestAndSet
TestAndSet: // Assume it is called as TestAndSet(&lock).
            // This code is gcc/linux/intel x86 specific.
    // Preserve ebx, which is about to be modified.
    pushl    %ebx
    movl    8(%esp), %ebx       # &lock to ebx
    movl    $1, %eax            # 1 (true) to eax
    // Swap eax and lock. Value 1 (true) is copied to lock, eax receives
    // old lock value    
    xchg    %eax, (%ebx)    # Atomically exchange eax with lock. The
                        # atomicity of xchg is what guarantees that
                        # at most one process or thread can be
                        # holding the lock at any point in time.

    // Restore ebx
    popl    %ebx

    // Return value (old value of lock) is already in register eax