Module image 

Building a TFS image on the host.

The guest driver reads and writes an image; this builds one. A filesystem needs to exist before a driver can be tested against it, and formatting a disk from inside a machine that cannot yet read a disk is a bootstrap problem better solved on the host.

Why fields are stored in base 243

A stored word is three trytes, and every tryte crossing to the image must be a byte: 0 to 255. That leaves a choice of radix for spreading a value across the three.

The obvious choice is 256, which is what a binary filesystem would use. It is a poor one here. Reassembling a field would mean computing t0 + 256*t1 + 65536*t2, and 256 is not a power of three, so each of those is a genuine multiply on every metadata access.

Base 243 is used instead. It is 3^5, so it is a power of three, and it is below 256, so every digit is still a valid byte. Reassembling a field becomes t0 + (t1 << 5) + (t2 << 10) in trit shifts, which the machine does with shl in one instruction each.

The cost is range: a stored word reaches 243^3 - 1 rather than 256^3 - 1, a loss of about 15 per cent. The fields this format stores are block numbers, inode numbers and file sizes, all far below either limit.

Structs

Builder

Builds a TFS image.

Enums

BuildError

Something the builder could not do.

Constants

MAX_STORED

Largest value a stored word can hold.

RADIX

How a value is split across the trytes of a word.

RADIX_TRITS

Trits per stored digit: RADIX is 3 to this power.