Intel® IXP42X Product Line of Network Processors and IXC1100 Control Plane Processor
2.2.3
Instruction Memory Management Unit (IMMU)
For instruction pre-fetches, the IMMU controls logical-to-physical address translation, memory
access permissions, memory-domain identifications, and attributes (governing operation of the
instruction cache). The IMMU contains a 32-entry, fully associative instruction-translation, look-
aside buffer (ITLB) that has a round-robin replacement policy. ITLB entries zero through 30 can be
locked.
When an instruction pre-fetch misses in the ITLB, the IMMU invokes an automatic table-walk
mechanism that fetches an associated descriptor from memory and loads it into the ITLB. The
descriptor contains information for logical-to-physical address translation, memory-access
permissions, memory-domain identifications, and attributes governing operation of the I-cache.
The IMMU then continues the instruction pre-fetch by using the address translation just entered
into the ITLB. When an instruction pre-fetch hits in the ITLB, the IMMU continues the pre-fetch
using the address translation already resident in the ITLB.
Access permissions for each of up to 16 memory domains can be programmed. When an
instruction pre-fetch is attempted to an area of memory in violation of access permissions, the
attempt is aborted and a pre-fetch abort is sent to the core for exception processing. The IMMU and
DMMU can be enabled or disabled together.
2.2.4
Data Memory Management Unit (DMMU)
For data fetches, the DMMU controls logical-to-physical address translation, memory-access
permissions, memory-domain identifications, and attributes (governing operation of the data cache
or mini-data cache and write buffer). The DMMU contains a 32-entry, fully associative data-
translation, look-aside buffer (DTLB) that has a round-robin replacement policy. DTLB entries 0
through 30 can be locked.
When a data fetch misses in the DTLB, the DMMU invokes an automatic table-walk mechanism
that fetches an associated descriptor from memory and loads it into the DTLB. The descriptor
contains information for logical-to-physical address translation, memory-access permissions,
memory-domain identifications, and attributes (governing operation of the D-cache or mini-data
cache and write buffer).
The DMMU continues the data fetch by using the address translation just entered into the DTLB.
When a data fetch hits in the DTLB, the DMMU continues the fetch using the address translation
already resident in the DTLB.
Access permissions for each of up to 16 memory domains can be programmed. When a data fetch
is attempted to an area of memory in violation of access permissions, the attempt is aborted and a
data abort is sent to the core for exception processing.
The IMMU and DMMU can be enabled or disabled together.
2.2.5
Instruction Cache (I-Cache)
The I-cache can contain high-use, multiple-code segments or entire programs, allowing the core
access to instructions at core frequencies. This prevents core stalls caused by multi-cycle accesses
to external memory.
The 32-Kbyte I-cache is 32-set/32-way associative, where each set contains 32 ways and each way
contains a tag address, a cache line of instructions (eight 32-bit words and one parity bit per word),
and a line-valid bit. For each of the 32 sets, 0 through 28 ways can be locked. Unlocked ways are
replaceable via a round-robin policy.
Datasheet
March 2005
Document Number: 252479, Revision: 005
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