66 Datasheet
Land Listing and Signal Descriptions
4.2 Alphabetical Signals Reference
Table 4-3. Signal Description (Sheet 1 of 8)
Name Type Description
A[35:3]#
Input/
Output
A[35:3]# (Address) define a 2
36
-byte physical memory address space. In sub-
phase 1 of the address phase, these signals transmit the address of a
transaction. In sub-phase 2, these signals transmit transaction type information.
These signals must connect the appropriate pins/lands of all agents on the
processor FSB. A[35:3]# are protected by parity signals AP[1:0]#. A[35:3]# are
source synchronous signals and are latched into the receiving buffers by
ADSTB[1:0]#.
On the active-to-inactive transition of RESET#, the processor samples a subset
of the A[35:3]# signals to determine power-on configuration. See Section 6.1 for
more details.
A20M# Input
If A20M# (Address-20 Mask) is asserted, the processor masks physical
address bit 20 (A20#) before looking up a line in any internal cache and before
driving a read/write transaction on the bus. Asserting A20M# emulates the 8086
processor's address wrap-around at the 1-MB boundary. Assertion of A20M# is
only supported in real mode.
A20M# is an asynchronous signal. However, to ensure recognition of this signal
following an Input/Output write instruction, it must be valid along with the
TRDY# assertion of the corresponding Input/Output Write bus transaction.
ADS#
Input/
Output
ADS# (Address Strobe) is asserted to indicate the validity of the transaction
address on the A[35:3]# and REQ[4:0]# signals. All bus agents observe the
ADS# activation to begin parity checking, protocol checking, address decode,
internal snoop, or deferred reply ID match operations associated with the new
transaction.
ADSTB[1:0]#
Input/
Output
Address strobes are used to latch A[35:3]# and REQ[4:0]# on their rising and
falling edges. Strobes are associated with signals as shown below.
AP[1:0]#
Input/
Output
AP[1:0]# (Address Parity) are driven by the request initiator along with ADS#,
A[35:3]#, and the transaction type on the REQ[4:0]#. A correct parity signal is
high if an even number of covered signals are low and low if an odd number of
covered signals are low. This allows parity to be high when all the covered
signals are high. AP[1:0]# should connect the appropriate pins/lands of all
processor FSB agents. The following table defines the coverage model of these
signals.
BCLK[1:0] Input
The differential pair BCLK (Bus Clock) determines the FSB frequency. All
processor FSB agents must receive these signals to drive their outputs and
latch their inputs.
All external timing parameters are specified with respect to the rising edge of
BCLK0 crossing V
CROSS
.
Signals Associated Strobe
REQ[4:0]#, A[16:3]# ADSTB0#
A[35:17]# ADSTB1#
Request Signals Subphase 1 Subphase 2
A[35:24]# AP0# AP1#
A[23:3]# AP1# AP0#
REQ[4:0]# AP1# AP0#
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