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Commit a3051e86 authored by Moritz Schneider's avatar Moritz Schneider Committed by Florian Zaruba
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Add PMP documentation

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......@@ -285,6 +285,33 @@ The page table walker gives precedence to DTLB misses. The page table
walking process is described in more detail in the RISC-V Privileged
Architecture.
#### PMA/PMP Checks
The core supports PMA and PMP checks in physical mode as well as with
virtual memory enabled. PMA checks are performed only on the final
access to the (translated) physical address. However, PMPs must be
checked during the page table walk as well. During a page walk, all
memory access must pass the PMP rules.
The amount of entries is parametrizable under the
`ArianeCfg.NrPMPEntries` parameter. However, the core only supports
granularity 8 (G=8). This simplifies the implementation since we do
not have to worry about any unaligned accesses. There are a total of
three distinct PMP units in the design. They verify instruction
accesses, data loads and stores, and the page table walk respectively.
#### MMU Implementation Details
The MMU prioritizes instruction address translations to data address
translations. The behavior of the MMU is described in the following:
1. As soon as a request from the instruction fetch stage arrives, the
ITLB checked for a cached entry (combinatorial path). Upon a cache miss,
the PTW is invoked.
2. The PTW will perform the page table walk in multiple cycles. During
this walk, the PTW will update the content of the ITLB. The MMU checks
every cycle if a cache hit in the ITLB exists, and therefore, the page
table walk has concluded.
#### Multiplier {#ssub:multiplier}
The multiplier contains a division and multiplication unit. Multiplication
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