REMQUE

Purpose

remove entry from head or tail of queue

Format

opcode entry.ab, addr.wl

Operation

If (all memory accesses can be completed) then

begin

            ((entry+4)) ß (entry); forward link of predecessor

            ((entry)+4) ß (entry+4);          backward link of successor

            addr ß entry;

end;

else

begin

            {backup instruction};               

            {initiate fault}                           

end;

Condition codes

N ß (entry) LSS (entry+4);

Z ß (entry) EQL (entry+4);     removed last entry

V ß entry EQL (entry+4);       no entry to remove

C ß (entry) LSSU (entry+4);

Exceptions

None

Opcodes

OF       REMQUE                    Remove Entry from Queue

Description

The queue entry specified by the entry operand is removed from the queue. The address operand is replaced by the ad dress of the entry removed. If there was no entry in the queue to be removed, the condition code V bit is set; otherwise it is cleared, If the queue is empty at the end of this instruction, the condition code Z-bit is set; otherwise it is cleared. The removal is a non-interruptible operation. Before performing any part of the operation, the processor validates that the entire operation can be completed. This ensures that if a memory management exception occurs, the queue is left in a consistent state.

Notes

1. Because the removal is non-interruptible, processes running in kernel mode can share queues with interrupt ser vice routines.

2. The INSQUE and REMQUE instructions are implemented such that cooperating software processes in a single processor may access a shared list without additional synchronization if insertions and removals are only at the head or tail of the queue.

3. During access validation, any access which cannot be completed results in a memory management exception even though the queue removal is not started.