Hi again (we keep trying),
> > My idea: why not indicate a big sector number with the highest bit of
> > the drive number (passed with A register)?
(chop, chop...)
New calling convention:
--------------------------------
(drive numbers: for BIOS-routine 0144h: 0=A, 1=B, etc.,
for driver entry 4010h: drivenumber = 'local' drive)
For the programmer:
- 'old' disk-I/O: as normal
- 'Big disk-I/O':
LD A,drivenumber
SET 7,A
LD DE,pointer to 32-bit sectornumber
CALL DSKIO
(other parameters as usual)
The pointer simply points somewhere in memory.
Although diskROM is switched into 4000-7FFFh during the call, I think
this pointer can point anywhere. For the BIOS-routine (0144 or
FFA7h), DOS can copy the contents at the pointer address to an
internal save address (in page 3-workspace, for example).
For calls from disk-system to driver via 4010h, this pointer will
already point somewhere else.
If diskROM is directly called at 4010h by an old program, pointer
will be 16-bit sector-number, and things will work as usual.
If called for 'Big I/O' directly from a (new) non-standard-obeying
proggie, only problem will occur if pointer address is in 4000-7FFF
range, but is no problem either, because this would be -illegal-
(directly calling 4010h for 'Big I/O', that is)
For the DSKIO entry (4010h) in the driver ROM:
(check read or write)
Bit 7 of A register:
if 0, higher bits of sector number = 0, or other 'base'
lower bits are in DE register
if 1, memory address (DE) contains big sector number
(treat equally from here)
For those of you who worry about performance:
Suppose you need an extra of say, 20 Z80-instructions, with an
average time-consumption of say, 10 clockcycles, that would take
about 200/3.58e6 is about 0,06 milliseconds (@3.58 MHz).
(there go these 10 milliseconds access times...)
So don't complain about this, any single interslot-call consumes a
big multiple of that.
With an extremely fast driver (like one using a HDD with read-ahead
cache memory), this could even allow sector-reading from an
interrupt-routine (!!)
Probably not allowed, but this calling method method wouldn't stand
in the way.
So, this calling method is:
-100% Compatible: if you call an old driver with it, nothing will
happen, and an error gets returned. Any old calling methods have the
normal effect
-'Interslot-call proof'
-Fast
-Easy for programmers
I challenge you to come up with a better way.
Greetings,
Alwin Henseler ([EMAIL PROTECTED])
http://huizen.dds.nl/~alwinh/msx (MSX Tech Doc page)
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