// 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
TestAndSet: // Assume it is
called as TestAndSet(&lock).
// This code is gcc/linux/intel x86 specific.
ebx, which is about to be modified.
%ebx # &lock to ebx
# 1 (true) to eax
// Swap eax
and lock. Value 1 (true) is copied to lock, eax receives
// old lock
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
value (old value of lock) is already in register eax