Module mmu 

Address translation: trit-sliced page tables.

Why these numbers

A page is 3^6 = 729 trytes, matching the emulator’s allocation granule. A page table entry is one word, three trytes, so a page holds 3^5 = 243 entries and an index into a table is five trits.

Five-trit indices are what make the layout work: a page table is exactly one page. Allocating a table is allocating a page, with nothing left over and nothing spanning a boundary. Seven-trit indices would give a three-level walk instead of four, but each table would then be nine pages.

Four levels of five trits is twenty trits of page number, which with the six-trit offset makes a 26-trit virtual address. That is one trit short of the architectural word, so the top trit of a virtual address must be zero while translation is on. This is the same bargain x86-64 strikes in requiring canonical addresses, and it costs a third of a space nothing is close to filling.

  trits  0..6    offset within the page
  trits  6..11   level 0 index
  trits 11..16   level 1 index
  trits 16..21   level 2 index
  trits 21..26   level 3 index
  trit  26       must be zero

Entry format

  trits  0..20   physical page number
  trit  20       valid
  trit  21       readable
  trit  22       writable
  trit  23       executable
  trit  24       user-accessible
  trit  25       leaf: maps a page rather than naming the next table
  trit  26       reserved, must be zero

A leaf above level 0 maps a superpage: the index trits not yet consumed join the offset, so a leaf at level 1 maps 3^11 trytes.

Modules

flag

Trit positions of the entry’s flags.

Structs

Entry

One page table entry.

Enums

Access

What an access is for. Each needs a different permission.

Constants

INDEX_TRITS

Trits of index per level.

LEVELS

How many levels the walk descends.

OFFSET_TRITS

Trits of offset within a page.

PAGE_TRYTES

Trytes per page: 3^6.

PPN_TRITS

Trits of physical page number in an entry.

Functions

index

The index this level takes from a virtual address.

is_canonical

True if the address fits the 26 trits translation can express.

page_span

The size in trytes of a page mapped by a leaf at level.

passthrough

The part of a virtual address a leaf at level passes through unchanged.