Manipulation of time-ordered lists and instructions therefor

Claim 1

In a multitasking data processing system which includes a plurality of users each of which may require shared access to the same data in a memory, a method of removing an element from a queue stored in the memory, wherein the queue is associated with a queue tail marking the newest element in the queue and a queue head marking the oldest element in the queue and each queue element contains a pointer to the next element in the queue, comprising the steps of: a) placing the value of the queue head into a first operand, b) placing the address of the queue head into a second operand, c) placing the address of the next element pointer of the first element into a fourth operand, d) executing a first prescribed computer instruction which performs the steps of e) locking access to the memory location specified by the second operand, f) comparing the first operand with the contents of memory specified by the second operand, g) if the comparison is equal, loading into a third operand the next queue element pointer specified by the fourth operand, h) releasing the lock on the memory location specified by the second operand, and i) setting a return code indicating success or failure of loading the next queue element pointer specified by the fourth operand, and thereafter j) if the return code indicates that the first prescribed instruction was not successful, repeating the above steps, k) if the return code indicates that the first prescribed instruction was successful, removing the oldest element from the queue using the next element pointer obtained in step g).