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A[35:3]#

Input/

Output

A[35:3]# (Address) define a 2

36

-byte physical memory address space. In 

subphase 1 of the address phase, these pins transmit the address of a 
transaction. In subphase 2, these pins transmit transaction type information. 
These signals must connect the appropriate pins 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]# pins to determine power-on configuration. See 

 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]# pins. 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 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

.

REQ[4:0]#, A[16:3]# 

ADSTB0#

A[35:17]#

ADSTB1#

A[35:24]#

AP0#

AP1#

A[23:3]#

AP1#

AP0#

REQ[4:0]#

AP1#

AP0#