
3-16
PRELIMINARY
Signal Descriptions
Advanci ng the S tandar ds
The Write-Back/Write-Through (WB/WT#)
input allows the system to define the write
policy of the on-chip cache on a line-by-line
basis. If WB/WT# is sampled high during a line
fill cycle and PWT is low, the line is defined as
write-back and is stored in the exclusive state. If
WB/WT# is sampled high during a write to a
write-through cache line (shared state) and
PWT is low, the line is transitioned to
write-back (exclusive state). If WB/WT# is
sampled low or PWT is high, the line is defined
as write-through and is stored in (line fill), or
remains in (write), the shared state. Table 3-8
(Page 3-16) lists the effects of WB/WT# on the
state of the cache line for various bus cycles.
3.2.11 Bus Arbitration
The bus arbitration signals (BOFF#, BREQ,
HOLD, and HLDA) allow the M II CPU to relin-
quish control of its local bus when requested by
another bus master device. Once the processor
Table 3-8. Effects of WB/WT# on
Cache Line State
BUS CYCLE
TYPE
PWT
WB/
WT#
WRITE
POLICY
MESI
STATE
Line Fill
0
Write-
through
Shared
Line Fill
0
1
Write-
back
Exclusive
Line Fill
1
x
Write-
through
Shared
Memory Write
(Note)
00
Write-
through
Shared
Memory Write
(Note)
01
Write-
back
Exclusive
Memory Write
(Note)
1x
Write-
through
Shared
Note: Only applies to memory writes to addresses that are currently
valid in the cache.
has released its bus, the bus master device can
then drive the local bus signals.
Back-Off (BOFF#) is an active low input that
forces the M II CPU to abort the current bus
cycle and relinquish control of the CPU's local
bus in the next clock. The M II CPU responds
to BOFF# by entering the bus hold state as
listed in Table 3-9 (Page 3-17). The M II CPU
remains in bus hold until BOFF# is negated.
Once BOFF# is negated, the M II CPU restarts
any aborted bus cycle in its entirety. Any data
returned to the M II CPU while BOFF# is
asserted is ignored. If BOFF# is asserted in the
same clock that ADS# is asserted, the M II CPU
may float ADS# while in the active low state.
Bus Request (BREQ) is an active high output
asserted by the M II CPU whenever a bus cycle
is pending internally. The M II CPU always
asserts BREQ in the first clock of a bus cycle
with ADS# as well as during bus hold and
address hold states if a bus cycle is pending. If
no additional bus cycles are pending, BREQ is
negated prior to termination of the current
cycle.
Bus Hold Request (HOLD) is an active high
input used to indicate that another bus master
requests control of the CPU's local bus. After
recognizing the HOLD request and completing
the current bus cycle or sequence of locked bus
cycles, the M II CPU responds by floating the
local bus and asserting the hold acknowledge
(HLDA) output. The bus remains granted to the
requesting bus master until HOLD is negated.
Once HOLD is sampled negated, the M II CPU
simultaneously drives the local bus and negates
HLDA.