AMD-K6 Datasheet, PDF (168/346 Pages) Advanced Micro Devices – AMD-K6 Processor

AMD-K6^® Processor Data Sheet
Preliminary Information
20695H/0—March 1998
Bus Backoff (BOFF#)
BOFF# provides the fastest response among bus-hold inputs.
Either the system logic or another bus master can assert BOFF#
to gain control of the bus immediately. BOFF# is also used to
resolve potential deadlock problems that arise as a result of
inquire cycles. The processor samples BOFF# on every clock
edge. If BOFF# is sampled asserted, the processor
unconditionally aborts any cycles in progress and transitions to
a bus hold state. (See “BOFF# (Backoff)” on page 87.) Figure 58
on page 151 shows a read cycle that is aborted when the
processor samples BOFF# asserted even though BRDY# is
sampled asserted on the same clock edge. The read cycle is
restarted after BOFF# is sampled negated (KEN# must be in
the same state during the restarted cycle as its state during the
aborted cycle).
During a BOFF#-initiated inquire cycle that hits a shared or
exclusive line, the processor samples BOFF# negated and
restarts any bus cycle that was aborted when BOFF# was
asserted. If a BOFF#-initiated inquire cycle hits a modified line,
the processor performs a writeback cycle before it restarts the
aborted cycle.
If the processor samples BOFF# asserted on the same clock
edge that it asserts ADS#, ADS# is floated but the system logic
may erroneously interpret ADS# as asserted. In this case, the
system logic must properly interpret the state of ADS# when
BOFF# is negated.
150
Bus Cycles
Chapter 6