FileCore Disc Map Old (changes)

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Disc Map Old

Overview of Old Disc Map

A disc has a section of information defined as a Map. It controls the allocation of the disc to the files and directories.

Old Maps have the following format:

Notes

The 82 three byte entries in the FreeStart and FreeLen tables are in units of 256 bytes. The entries are sorted low addressed free areas first. Contiguous free areas will have been merged together.

The full disc name is the joining together of the bytes in OldName0 and OldName1. The name is interleaved, with OldName0 providing the first character, OldName1 the second, and so on.

OldId is the disc’s Id to identify when the disc has been modified.

If an old map does not end at a sector boundary, then it is padded with null bytes to the end of the sector. The sector immediately following the old map always holds the start of the root directory.

Calculating Check0 and Check1

These are checksums for the previous bytes in the map. They are calculated using repeated 8-bit ADC’s on bytes of the relevant map block, starting with a value of 0:

If R0 is the accumulated checksum, then it starts at 0, and each byte is added as follows:

ADC r0, r0, r1						;r1 is the byte picked up
MOVS r0, r0, LSL #24					;Shifts bit 8 into the carry bit
MOV r0, r0, LSR #24					;Not MOVS here to preserve the carry bit

Note that the check byte itself isn’t included in the checksum; its value equals the checksum of the previous bytes.