`  == ``Primitive atomic instructions`` as relates to Burroughs mainframes ==  The section ``Primitive atomic instructions`` has a statement in it that reads: ``Atomic swap as used in some Burroughs mainframes, also called XCHG.``  The XCHG operator is emitted by the compiler to implement its needs in stack manipulation, but is not accessible to the user directly. [It was, in ESPOL, but ESPOL is no longer used.] It causes the two top-of-stack values to be exchanged; that is, it only affects the stack.  The user-accessible instruction that is used for implementing locks is RDLK (ReadLock). It exchanges a top-of-stack value with an address in memory, in one uininterruptible swell foop.  EXCH & RDLK are both atomic, in that they are not interruptible. Other ``atomic`` operators are RSUP & RSDN, which manipulate the top 3 stack cells. But only RDLK is accessible to programmers (in Algol & NEWP) & thus used with implementing locks.  Thus, I believe the line should read, ``Atomic swap as used in some Burroughs mainframes, also called RDLK.`` Or, it should stated another way, but involve the use of RDLK, not EXCH.`