Rainbow Electronics AT91CAP9S250A - Manual (Page 62)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
12.5.2 Translation Look-aside Buffer (TLB)
The Translation Look-aside Buffer (TLB) caches translated entries and thus avoids going
through the translation process every time. When the TLB contains an entry for the MVA (Modi-
fied Virtual Address), the access control logic determines if the access is permitted and outputs
the appropriate physical address corresponding to the MVA. If access is not permitted, the MMU
signals the CPU core to abort.
If the TLB does not contain an entry for the MVA, the translation table walk hardware is invoked
to retrieve the translation information from the translation table in physical memory.
12.5.3 Translation Table Walk Hardware
The translation table walk hardware is a logic that traverses the translation tables located in
physical memory, gets the physical address and access permissions and updates the TLB.
The number of stages in the hardware table walking is one or two depending whether the
address is marked as a section-mapped access or a page-mapped access.
There are three sizes of page-mapped accesses and one size of section-mapped access. Page-
mapped accesses are for large pages, small pages and tiny pages. The translation process
always begins with a level one fetch. A section-mapped access requires only a level one fetch,
but a page-mapped access requires an additional level two fetch. For further details on the
MMU, please refer to chapter 3 in ARM926EJ-S Technical Reference Manual, ref. DDI0198B.
12.5.4 MMU Faults
The MMU generates an abort on the following types of faults:
• Alignment faults (for data accesses only)
• Translation faults
• Domain faults
• Permission faults
The access control mechanism of the MMU detects the conditions that produce these faults. If
the fault is a result of memory access, the MMU aborts the access and signals the fault to the
CPU core.The MMU retains status and address information about faults generated by the data
accesses in the data fault status register and fault address register. It also retains the status of
faults generated by instruction fetches in the instruction fault status register.
The fault status register (register 5 in CP15) indicates the cause of a data or prefetch abort, and
the domain number of the aborted access when it happens. The fault address register (register 6
in CP15) holds the MVA associated with the access that caused the Data Abort. For further
details on MMU faults, please refer to chapter 3 in ARM926EJ-S Technical Reference Manual,
ref. DDI0198B.
12.6 Caches and Write Buffer
The ARM926EJ-S contains a 16 KB Instruction Cache (ICache), a 16 KB Data Cache (DCache),
and a write buffer. Although the ICache and DCache share common features, each still has
some specific mechanisms.
The caches (ICache and DCache) are four-way set associative, addressed, indexed and tagged
using the Modified Virtual Address (MVA), with a cache line length of eight words with two dirty
bits for the DCache. The ICache and DCache provide mechanisms for cache lockdown, cache
pollution control, and line replacement.