Owner/Editor: Ved Shanbhogue

Zacas introduces three instructions to perform atomic compare and swap operations

• AMOCAS.W/D/Q - compare-and-swap word/doubleword/quadword

AMOCAS provides an easy and typically faster way to perform thread synchronization operations 

• For by lock-free and wait-free algorithms. 
• Scale better to highly parallel systems than LR/SC.

AMOCAS required to guard against the ABA problem in algorithms that manipulate pointers

• The algorithms associate a reference counter with the pointer variable
• Perform updates using a quadword compare and swap (of both the pointer and the counter)
• Double/Quadword CAS instructions support implementation of such algorithms