
2-16
PowerPC Microprocessor Family: The Bus Interface for 32-Bit Microprocessors
2.4.9 Write-Through (WT)—Output
Following are state and timing descriptions for the write-through (WT) output signal.
State Meaning
Asserted—Generally indicates that a single-beat transaction is write-
through, reflecting the value of the W bit for the block or page that
contains the address of the current transaction.
Negated—Generally indicates a transaction is not write-through.
Section 4.8, “External WIM Bit Settings,” describes exceptions to
the above.
Timing Comments
Assertion/Negation/High Impedance—The same as A[0–31].
2.4.10 Global (GBL)—Output
Following are state and timing descriptions for global signal (GBL) as an output signal. For
the 604e, HID0[23] lets software control the behavior of GBL for instruction fetches
through the address-translation mechanism; refer to 604e user documentation.
State Meaning
Asserted—Generally indicates that a transaction is global, reflecting
the setting of the M bit for the block or page that contains the address
of the current transaction (except in the case of write-back
operations, which are nonglobal.)
Negated—Generally indicates that a transaction is not global. For
603 and 604, this signal is negated on instruction fetches.
For exceptions, see Section 4.8, “External WIM Bit Settings.”
Assertion/Negation/High Impedance—The same as A[0–31].
Timing Comments
2.4.11 Global (GBL)—Input
Following are state and timing descriptions for GBL as an input signal.
State Meaning
Asserted—A transaction can be snooped; however, the processor
will not snoop reserved transaction types, bus operations associated
with the
eieio
,
eciwx
, or
ecowx
instructions, or address-only bus
transactions associated with an
lwarx
reservation set. Note that the
603 snoops the reservation address register for global and nonglobal
address transfers. This snooping is required for the 603’s
implementation of the
lwarx
and
stwcx.
instructions, which require
snoops on castouts and snoop pushes (nonglobal). Snoops with
GBL = 1 do not affect the cache state.
Negated—A transaction is not snooped by the processor.
Timing Comments
Assertion/Negation—The same as A[0–31].