AMD-K6-2E Datasheet, PDF (156/332 Pages) Advanced Micro Devices – AMD-K6™-2E Embedded Processor

AMD-K6™-2E Processor Data Sheet
Preliminary Information
22529B/0—January 2000
6.3
Memory Reads and Writes
Single-Transfer
Memory Read and
Write
The AMD-K6-2E processor performs single or burst memory bus
cycles.
s The single-transfer memory bus cycle transfers 1, 2, 4, or 8
bytes and requires a minimum of two clocks.
s Misaligned instructions or operands result in a split cycle,
which requires multiple transactions on the bus.
s A burst cycle consists of four back-to-back 8-byte (64-bit)
transfers on the data bus.
Figure 52 on page 139 shows a single-transfer read from memory,
followed by two single-transfer writes to memory. For the
memory read cycle, the processor asserts ADS# for one clock to
validate the bus cycle and also drives A[31:3], BE[7:0]#, D/C#,
W/R#, and M/IO# to the bus. The processor then waits for the
system logic to return the data on D[63:0] (with DP[7:0] for
parity checking) and assert BRDY#. The processor samples
BRDY# on every clock edge starting with the clock edge after
the clock edge that negates ADS#. See “BRDY# (Burst Ready)”
on page 95.
During the read cycle, the processor drives PCD, PWT, and
CACHE# to indicate its caching and cache-coherency intent for
the access. The system logic returns KEN# and WB/WT# to
either confirm or change this intent. If the processor asserts
PCD and negates CACHE#, the accesses are noncacheable, even
though the system logic asserts KEN# during the BRDY# to
indicate its support for cacheability. The processor (which
drives CACHE#) and the system logic (which drives KEN#) must
agree in order for an access to be cacheable.
The processor can drive another cycle (in this example, a write
cycle) by asserting ADS# off the next clock edge after BRDY# is
sampled asserted. Therefore, an idle clock is guaranteed
between any two bus cycles. The processor drives D[63:0] with
valid data one clock edge after the clock edge on which ADS# is
asserted. To minimize processor idle times, the system logic
stores the address and data in write buffers, returns BRDY#, and
performs the store to memory later. If the processor samples
EWBE# negated during a write cycle, it suspends certain
activities until EWBE# is sampled asserted. See “EWBE#
(External Write Buffer Empty)” on page 102. In Figure 52, the
138
Bus Cycles
Chapter 6