This document describes the format of D format free space maps.

Sector 0:
Name            Bytes
FreeStart       82*3    Table of free space start sectors
Reserved        1       Reserved
OldName0        5       1st half of disc name
OldSize         3       Disc size in (256 byte) sectors
Check0          1       CheckSum on sector 0

Sector 1:
Name            Bytes
FreeLen         82j3    Table of free space lengths
OldName1        5       Second half of disc name
OldId           2       Disc id
OldBoot         1       boot option
FreeEnd         1       Pointer to end of free space list
Check1          1       Checksum on sector 1

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. OldName0 comes before OldName1 in the name.

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

Calculating Check0 and Check1:
These are checksums of the previous bytes in the map. They are calculated
using repeated 8-bit ADCSs on the bytes if the relevant map block, starting
with a value 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.
