Test_and_Set is a special assembly language instruction that does two operations autonomously. That is, the instruction can not be interrupted in the middle and it is not necessary to disable interrupts. Test_and_Set (TS) is a privileged instruction requiring supervisory mode permissions. (See CPU Execution Mode ). The action of Test_and_Set is described with the following pseudo-code:
boolean Test_and_Set( boolean memory[m] ) [ if( memory[m] ) // lock denied return True; else // lock granted memory[m] = True; return False; > ] >
TS(m): [Reg_i = memory[m]; memory[m] = TRUE;]
The Autonomous Two Step TS Instruction
struct semaphore int value = initial value>; boolean mutex = FALSE; boolean hold = TRUE; queue m, h; >; shared struct semaphore s; /* P() */ acquire_semaphore(struct semaphore s) while(TS(s.mutex)) WAIT(s.m); /* wait for intertal mutex */ s.value--; if(s.value 0) s.mutex = FALSE; SIGNAL(s.m); while(TS(s.hold)) WAIT(s.h); /* wait - too many out */ > else s.mutex = FALSE; SIGNAL(s.m); > /* V() */ release_semaphore(struct semaphore s) while(TS(s.mutex)) WAIT(s.m); /* wait for intertal mutex */ s.value++; if(s.value >= 0) s.hold = FALSE; SIGNAL(s.h); > s.mutex = FALSE; SIGNAL(s.m); >
