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BUS TRANSACTIONS AND OPERATIONS
5.2.1.4.
RESERVED MEMORY WRITE TRANSACTION
Reserved Memory Write
This transaction is reserved, and must not be issued by any bus agents. Future bus agents may
use this encoding. Current memory agents and snooping agents must treat this transaction as a
Memory Write Transaction.
5.2.2.
I/O Transactions
An agent issues an I/O transaction to read or write an I/O location. The addressed agent is the
agent primarily responsible for completion of the I/O transaction. I/O transaction may be de-
ferred in the snoop phase by any agent as described in the later subsection.
The I/O transactions are indicated using the following request encodings:
REQa[4:0]#
read
1
0
write
1
0
Ab[15:8]#
BE[7:0]#
I/O transactions have similar request fields to memory transactions. However, the address space
is always 64K+3 bytes 1 . Therefore, A[35:17]# will always be zero. A[16]# is zero except when
the first three bytes above the 64Kbyte space are accessed (I/O wraparound). BE[7:0]# will al-
ways indicate at most 4 bytes when issued by the Pentium Pro processor.
The LEN[1:0]# signals are identical to the memory transactions, and are used to indicate the
length of the I/O transaction. It indicates how much data will be transferred over the bus. Re-
sponse to reserved encodings should be the largest transfer size supported.
1
®
Pro processor is backwards compatible with previous implementations of the Intel Architecture I/O
T h e P en ti u m
space. A[16]# is active whenever an I/O access is made to 4 bytes from addresses 0FFFDH, 0FFFEH, or 0FFFFH.
A[16]# is also active when an I/O access is made to 2 bytes from address 0FFFFH.
5-6
REQa[2:0]#
0
0
0
W/R#=0
0
0
W/R#=1
SMMEM#
SPLCK#=0
1
REQb[4:0]#
DSZ[1:0]#
rsvd
Ab[7:3]#
rsvd
DEN#
1
LEN[1:0]
rsvd
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