AMD AMD-K6-2/500AFX Data Sheet - Page 168

AMD-K6®-2 Processor Data Sheet Preliminary Information 21850J/0-February 2000 AHOLD-Initiated Inquire Miss AHOLD can be asserted by the system to initiate one or more inquire cycles. To allow the system to drive the address bus during an inquire cycle, the processor floats A[31:3] and AP off the clock edge on which AHOLD is sampled asserted. The data bus and all other control and status signals remain under the control of the processor and are not floated. This functionality allows a bus cycle in progress when AHOLD is sampled asserted to continue to completion. The processor resumes driving the address bus off the clock edge on which AHOLD is sampled negated. In Figure 64, the processor samples AHOLD asserted during the memory burst read cycle, and it floats the address bus off the same clock edge on which it samples AHOLD asserted. While the processor still controls the bus, it completes the current cycle until the last expected BRDY# is sampled asserted. The system logic drives EADS# with an inquire address on A[31:5] during an inquire cycle. The processor samples EADS# asserted and compares the inquire address to its tag address in both the instruction and data caches. In Figure 64, the inquire address misses the tag address in the processor (both HIT# and HITM# are negated). Therefore, the processor proceeds to the next cycle when it samples AHOLD negated. (The processor can drive a new cycle by asserting ADS# off the same clock edge that it samples AHOLD negated.) For an AHOLD-initiated inquire cycle to be recognized, the processor must sample AHOLD asserted for at least two consecutive clocks before it samples EADS# asserted. If the processor detects an address parity error during an inquire cycle, APCHK# is asserted for one clock. The system logic must respond appropriately to the assertion of this signal. 148 Bus Cycles Chapter 5