Commit 0dbb7645 authored by Ben Avison's avatar Ben Avison

Correction to documentation

parent 38a141ba
......@@ -83,11 +83,19 @@ available from FileCore_MiscOp 6) that if the disc has a sector size of
2K or more (1K or more if long filenames are not used), that the disc can
only be allocated 2 or more zones.
A slightly higher limit on sector size is imposed by the zone_spare field
in the disc record. This must be able to represent any number up to
(nearly) the number of bits in a map block, and is itself a 15-bit field.
There remains a sector size limit imposed by the FreeLink fragment in the
map block header. Because this is in the form of a fragment block (albeit
often shorter than the remaining fragment blocks in the map block), one of
the bits is used as a terminator. The remaining 15 bits must be able to
represent any number up to the number of allocation bits in a map block.
This limits map blocks, and therefore sectors, to 32768/8 = 4K.
A similar issue is raised by the zone_spare field in the disc record. This
is a 16-bit field, so to retain the ability to mark any amount of the map
block as spare would impose an 8K limit on sector sizes (however, it is
rare in practice for zone_spare to reserve much more than 32 non-allocation
bits between map blocks, so this is unlikely to be a problem in practice).
Re-entrancy
-----------
......@@ -461,7 +469,7 @@ By contrast, bits 0-3 of the results flags - "not changed", "maybe changed",
ADFS floppy drivers take different but equally valid approaches to the
relationship between these bits and the sequence number.
The 1770 driver, in "change doesn't work" mode, increments its internal copy
The 1772 driver, in "change doesn't work" mode, increments its internal copy
of the sequence number each time the motor is turned off. In "change works"
mode, it increments it whenever it polls the disc detect line, whether it is
diagnosed as drive empty or disc changed. Then in MiscOp 1, it constructs
......
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