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21264/EV68A Hardware Reference Manual
Cache and External Interfaces 4–37
System Port
point is the QW pointed to by the 21264/EV68A; however, some systems may find it
more beneficial to begin the transfer elsewhere. The system must always indicate the
starting point to the 21264/EV68A. The wrap order for subsequent QWs is interleaved.
Table 4–29 defines the method for systems to specify wrap and deliver data.
Note 1: Transfers to and from the 21264/EV68A have eight data cycles for a total
of eight quadwords. The starting point is defined by the system. The pre-
ferred starting point is the one pointed to by SysAddOut_L[5:4]. Systems
can insert the SysAddOut_L[5:4] into the SysDc[1:0] field of the com-
mand. See Table 4–30 for the wrap order.
Note 2: LW and byte/word read transfers differ from all other transfers. The system
unloads only four QWs of data into eight data cycles by sending each QW
twice (referred to as double-pumped data transfer). The first QW returned
is determined by SysAddOut_L[4:3]. The system again may elect to
choose its own starting point for the transfer and insert that value into
SysDc[1:0]. See Table 4–31 for the wrap order.
Table 4–30 defines the interleaved scheme for the wrap order.
Table 4–29 System Wrap and Deliver Data
Source/
Destination SysDc[4:2] SysDc[1:0] Size Rules
Memory 100 (ReadData) SysAddOut_L[5:4] Block (64 Bytes) See Note 1
Memory 101(ReadDataDirty) SysAddOut_L[5:4] Block (64 Bytes) See Note 1
Memory 110 (ReadDataShared) SysAddOut_L[5:4] Block (64 Bytes) See Note 1
Memory 111(Read DataShared/Dirty) SysAddOut_L[5:4] Block (64 Bytes) See Note 1
Memory 010 (WriteData) SysAddOut_L[5:4] Block (64 Bytes) See Note 1
I/O 100 (ReadData) SysAddOut_L[5:4] QW (8-64 Bytes) See Note 1
I/O 100 (ReadData) SysAddOut_L[4:3] LW(4-32 Bytes) See Note 2
I/O 100 (ReadData) SysAddOut_L[4:3] Byte/Word See Note 2
I/O 010 (WriteData) SysAddOut_L[5:4] QW (8-64 Bytes) See Note 1
I/O 010 (WriteData) SysAddOut_L[5:4] LW(4-32 Bytes) See Note 1
I/O 010 (WriteData) SysAddOut_L[5:4] Byte/Word See Note 1
Table 4–30 Wrap Interleave Order
PA Bits [5:3] of Transferred QW
First quadword 000 010 100 110
Second quadword 001 011 101 111
Third quadword 010 000 110 100
Fourth quadword 011 001 111 101
Fifth quadword 100 110 000 010