Tandy 1000 Programmer's Reference Manual page 183

Chapter 5 I Device Drivers
A pointer to a BPB (BIOS Parameter Block) pointer array is also
returned. There is one table for each unit defined. These blocks
will be used to build an internal DOS data structure for each of
the units. The pointer passed to the DOS from the driver points
to a n array ofnword pointers to BPBs, where n is the number of
units defined. In this way, if all units are the same, all of the
pointers can point to the same BPB, in order to save space.
Note that this array must be protected (below the free pointer
set by the return), since an internal DOS structure will be built
starting at the byte pointed to by the free pointer. The sector
size defined must be less than or equal to the maximum sector
size defined at default BIOS INIT time. If it is not, the install
will fail.
The last thing that INIT of a block device must pass back is the
media descriptor byte. This byte means nothing to MS-DOS, but
is passed to devices so that they know what parameters MS-DOS
is currently using
for
a particular drive unit.
Block devices may take several approaches; they may be dumb or
smart. A dumb device defines a unit (and therefore an internal
DOS structure) for each possible media drive combination. For
example, unit 0
=
drive 0 single side, unit 1
=
drive 0 double
side. For this approach, media descriptor bytes mean nothing. A
smart device allows multiple media per unit. In this case, the
BPB table returned at INIT must define space large enough to
accommodate the largest possible media supported. Smart driv-
ers will use the media descriptor byte to pass information about
what media is currently in a unit.
Function Call Parameters
All strategy routines are called with ES:BX pointing to the Re-
quest Header. The interrupt routines get the pointers to the Re-
quest Header from the queue in which they are stored by the
strategy routines. The command code in the Request Header
tells the driver which function to perform.
All DWORD pointers are stored offset first, then segment.
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