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Hardware Reference Manual 35
Intel
®
IXP2800 Network Processor
Technical Description
2.3.1 Microengine Bus Arrangement
The IXP2800 Network Processor supports a single D_Push/D_Pull bus, and both Microengine
clusters interface to the same bus. Also, it supports two command buses, and two sets of
S_Push/S_Pull buses connected as shown in Table 3, which also shows the next neighbor
relationship between the Microengine.
2.3.2 Control Store
The Control Store is a RAM that holds the program that is executed by the Microengine. It holds
8192 instructions, each of which is 40 bits wide. It is initialized by the Intel XScale
®
core, which
writes to USTORE_ADDR and USTORE_DATA Local CSRs.
The Control Store is protected by parity against soft errors. Parity checking is enabled by
CTX_ENABLE[CONTROL STORE PARITY ENABLE]. A parity error on an instruction read
will halt the Microengine and assert an interrupt to the Intel XScale
®
core.
2.3.3 Contexts
There are eight hardware Contexts available in the Microengine. To allow for efficient context
swapping, each Context has its own register set, Program Counter, and Context specific Local
registers. Having a copy per Context eliminates the need to move Context specific information to/
from shared memory and Microengine registers for each Context swap. Fast context swapping
allows a Context to do computation while other Contexts wait for I/O (typically external memory
accesses) to complete or for a signal from another Context or hardware unit. (A context swap is
similar to a taken branch in timing.)
Table 3. IXP2800 Network Processor Microengine Bus Arrangement
Microengine
Cluster
Microengine
Number
Next
Neighbor
Previous
Neighbor
Command
Bus
S_Push and
S_Pull Bus
0
0x00 0x01 NA
0 0
0x01 0x02 0x00
0x02 0x03 0x01
0x03 0x04 0x02
0x04 0x05 0x03
0x05 0x06 0x04
0x06 0x07 0x05
0x07 0x10 0x06
1
0x10 0x11 0x07
11
0x11 0x12 0x10
0x12 0x13 0x11
0x13 0x14 0x12
0x14 0x15 0x13
0x15 0x16 0x14
0x16 0x17 0x15
0x17 NA 0x16