
PRELIMINARY
3-11
3
Signal Descriptions
PCHK# is valid only during the second clock
immediately after read data is returned to the M
II CPU (BRDY# asserted). At other times
PCHK# is not active. Parity errors signaled by
the assertion of PCHK# have no effect on
processor execution.
3.2.7
Bus Cycle Definition
Each bus cycle is assigned a bus cycle type. The
bus cycle types are defined by six three-state
outputs: CACHE#, D/C#, LOCK#, M/IO#,
SCYC, and W/R# as listed in Table 3-7 (Page
3-12).
These bus cycle definition signals are driven
valid while ADS# is active. D/C#, M/IO#,
W/R#, SCYC and CACHE# remain valid until
the clock following the earliest of two signals:
NA# asserted, or the last BRDY# for the cycle.
LOCK# continues asserted until after BRDY# is
returned for the last locked bus cycle. The bus
cycle definition signals float during bus hold
states.
Cache Cycle Indicator (CACHE#) is an
output that indicates that the current bus cycle
is a potentially cacheable cycle (for a read), or
indicates that the current bus cycle is a cache
line write-back or line replacement burst cycle
(for a write). If CACHE# is asserted for a read
cycle and the KEN# input is returned active by
the system, the read cycle becomes a cache line
fill burst cycle.
Data/Control (D/C#) distinguishes between
data and control operations. When high, this
signal indicates that the current bus cycle is a
data transfer to or from memory or I/O. When
low, D/C# indicates that the current bus cycle
involves a control function such as a halt, inter-
rupt acknowledge or code fetch.
Bus Lock (LOCK#) is an active low output
which, when asserted, indicates that other
system bus masters are denied access to control
of the CPU bus. The LOCK# signal may be
explicitly activated during bus operations by
including the LOCK prefix on certain instruc-
tions. LOCK# is also asserted during descriptor
updates, page table accesses, interrupt
acknowledge sequences and when executing
the XCHG instruction. However, if the
NO_LOCK bit in CCR1 is set, LOCK# is
asserted only during page table accesses and
interrupt acknowledge sequences. The M II
CPU does not enter the bus hold state in
response to HOLD while the LOCK# output is
active.
Memory/IO (M/IO#) distinguishes between
memory and I/O operations. When high, this
signal indicates that the current bus cycle is a
memory read or memory write. When low,
M/IO# indicates that the current bus cycle is an
I/O read, I/O write, interrupt acknowledge
cycle or special bus cycle.
Split Cycle (SCYC) is an active high output
that indicates that the current bus cycle is part
of a misaligned locked transfer. SCYC is defined
for locked cycles only. A misaligned transfer is
defined as any transfer that crosses an 8-byte
boundary.
Write/Read (W/R#) distinguishes between
write and read operations. When high, this
signal indicates that the current bus cycle is a
memory write, I/O write or a special bus cycle.
When low, this signal indicates that the current
cycle is a memory read, I/O read or interrupt
acknowledge cycle.