AMD AMD-K6-2/500AFX Data Sheet - Page 178

AMD-K6®-2 Processor Data Sheet Preliminary Information 21850J/0-February 2000 Locked Cycles Basic Locked Operation The processor asserts LOCK# during a sequence of bus cycles to ensure the cycles are completed without allowing other bus masters to intervene. Locked operations can consist of two to five cycles. LOCK# is asserted during the following operations: s An interrupt acknowledge sequence s Descriptor Table accesses s Page Directory and Page Table accesses s XCHG instruction s An instruction with an allowable LOCK prefix In order to ensure that locked operations appear on the bus and are visible to the entire system, any data operands addressed during a locked cycle that reside in the processor's cache are flushed and invalidated from the cache prior to the locked operation. If the cache line is in the modified state, it is written back and invalidated prior to the locked operation. Likewise, any data read during a locked operation is not cached. The processor negates LOCK# for at least one clock between consecutive sequences of locked operations to allow the system logic to arbitrate for the bus. The processor asserts SCYC during misaligned locked transfers on the D[63:0] data bus. The processor generates additional bus cycles to complete the transfer of misaligned data. Figure 69 shows a pair of read-write bus cycles. It represents a typical read-modify-write locked operation. The processor asserts LOCK# off the same clock edge that it asserts ADS# of the first bus cycle in the locked operation and holds it asserted until the last expected BRDY# of the last bus cycle in the locked operation is sampled asserted. (The processor negates LOCK# off the same clock edge.) 158 Bus Cycles Chapter 5