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 topofstack values to be exchanged that is it only affects the stack  The useraccessible instruction that is used for implementing locks is RDLK ReadLock It exchanges a topofstack 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  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