G53OPS - Operating Systems

This course is run at the The University of Nottingham within the School of Computer Science & IT. The course is run by Graham Kendall (EMAIL : gxk@cs.nott.ac.uk)

Test and Set Lock

If we are given assistance by the instruction set of the processor we can implement a solution to the mutual exclusion problem. The instruction we require is called test and set lock (TSL). This instructions reads the contents of a memory location, stores it in a register and then stores a non-zero value at the address. This operation is guaranteed to be indivisible. That is, no other process can access that memory location until the TSL instruction has finished.

This assembly (like) code shows how we can make use of the TSL instruction to solve the mutual exclusion problem.

enter_region:
tsl register, flag ; copy flag to register and set flag to 1
cmp register, #0 ;was flag zero?
jnz enter_region ;if flag was non zero, lock was set , so loop
ret ;return (and enter critical region)

leave_region:
mov flag, #0 ; store zero in flag
ret ;return

Assume, again, two processes.

Process 0 calls enter_region. The tsl instruction copies the flag to a register and sets it to a non-zero value. The flag is now compared to zero (cmp - compare) and if found to be non-zero (jnz - jump if non-zero) the routine loops back to the top. Only when process 1 has set the flag to zero (or under initial conditions), by calling leave_region, will process 0 be allowed to continue.

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Last Updated : 14/01/2002