Compaq 21264 - Manual (Page 109)

Alpha 21264/EV67 Hardware Reference Manual Cache and External Interfaces 4–21
System Port
System designers can minimize pin count for systems with a small memory by config-
uring both the bank interleave on cache block boundary mode and the page hit mode
formats into a short bus format. The pin SysAddOut_L[1] and/or SysAddOut_L[0]
are not used (selected by Cbox CSR SYS_BUS_SIZE[1:0]). Table 4–13 lists the values
for SYSBUS_FORMAT and SYS_BUS_SIZE[1:0] and shows the maximum physical
memory size.
Because addresses above the maximum PA are not visible to the external system, any
memory transaction generated to addresses above the maximum PA are detected and
converted to transactions to NXM (nonexistent memory) and processed internally by
the 21264/EV67.
4.7.4 21264/EV67-to-System Commands Descriptions
Table 4–14 describes the 21264/EV67-to-system commands.
Table 4–13 Maximum Physical Address for Short Bus Format
SYSBUS_
FORMAT SYSBUS_
SIZE[1:0]
Maximum PA Comment
0 00 42 Bank interleave + full address
0 01 36 Bank interleave + SysAddOut_L[0] unused
0 10 Illegal Illegal combination
0 11 34 Bank interleave + both SysAddOut_L[1:0] are used for I/O
1 00 38 Page hit mode + full address
1 01 36 Page hit mode + SysAddOut_L[0] unused
1 10 Illegal Illegal combination
1 11 34 Page hit mode + both SysAddOut_L[1:0] are unused
Table 4–14 21264/EV67-to-System Commands Descriptions
Command
Command
[4:0]
Function
NOP 00000 The 21264/EV67 drives this command on idle cycles during reset. After
the clock forward reset period, the first NZNOP is generated and this
command is no longer generated.
ProbeResponse 00001 Returns probe status and ID number of the VDB entry holding the
requested cache block.
NZNOP 00010 This nonzero NOP helps to parse the command packet.
VDBFlushRequest 00011 VDB flush request. The 21264/EV67 sends this command to the system
when an internally generated transaction Bcache index matches a Bcache
victim or probe in the VDB. The system should flush VDB entries
associated with all probe and WrVictimBlk transactions that occurred
before this command.
MB
1
00111 Indicates an MB was issued, optional when Cbox CSR
SYSBUS_MB_ENA[0] is set.
ReadBlk 10000 Memory read.