Mutually Exclusive: Summary of the Hardware Approach

2.13.2 Mutual exclusion: the hardware approach

Interrupt disable (mask interrupt)

• For single CPU system
• Using the method of shielding interrupt, the process cannot be switched during shielding, achieving the purpose of mutual exclusion of processes

while (true ) {
    
    
        disable interrupt    //屏蔽中断
        critical section          //临界区
        enable interrupt       //启用中断
        remainder                //其余部分
}

Evaluation

• Obviously low efficiency
• Unable to respond to external requests during blocking
• Unable to switch process
• Cannot work in a multi-processor environment

Special machine instructions

• The two machine instructions specified by the processor designer are executed in one instruction cycle and will not be interrupted or interfered by other instructions
• Can also be used in a multi-processor environment

1. Compare instruction and exchange instruction compare_and_swap

Compare the value of a memory cell with the test value, exchange if they are equal

int compare_and_swap(int *word, int testval, int newval){
    
    
		int oldval; 
		oldval = *word;
		if(oldval == testval) *word = newval;
		return oldval;
}	

2. Exchange

Atomic exchange of register and memory values

procedure exchange(var r: register; var m: memory); 
	var temp; 
	begin 
	temp := m; 
	m := r; 
	r := temp; 
	end 

Evaluation

advantage

• Supports multiple processors
• Simple and easy to prove
• Support multiple critical sections

Disadvantage

• Busy waiting
• Hunger may appear
• Possible deadlock