Caple C871i Manual de usuario descargar pdf (Pagina 147)

100

101

Apple II Computer Info

It's a bit more complicated than that.

There are, in effect, four buffers. They get filled something like this:

1st byte --> 6 bits to byte 1 of buffer 1, 2 bits to byte 1 of buffer 4

2nd byte --> 6 bits to byte 1 of buffer 2, 2 bits to byte 1 of buffer 4

3rd byte --> 6 bits to byte 1 of buffer 3, 2 bits to byte 1 of buffer 4

4th byte --> 6 bits to byte 2 of buffer 1, 2 bits to byte 2 of buffer 4

5th byte --> 6 bits to byte 2 of buffer 2, 2 bits to byte 2 of buffer 4

and so forth.

All the while a three-byte checksum is being computed, in a very

hard-to-understand way. Watch out! The checksum computation modifies the

data, just like it does on the 5.25-inch drive.

Once the buffers are filled, they're written:

1st byte of buffer 1

1st byte of buffer 2

1st byte of buffer 3

1st byte of buffer 4

2nd byte of buffer 1

2nd byte of buffer 2

etc.

So the byte containing the extra two bits from three other bytes is written

immediately following those three bytes. The three-byte checksum expands

to four bytes in exactly the same fashion.

The data field looks like this:

$D5, $AA, $AD, track, 699 data bytes, 4 checksum bytes, $DE, $AA

The "track" number is identical to the "track" number from the address

field.

Remember that the original data is 524 bytes, not 512.

>- is the book "Beneath Apple ProDOS" still available? :-))))

I think Byteworks still sells it. But it was written before the 3.5-inch

drive was available for the Apple II, so its contents are only relevent in

a very general sort of way.

- Neil Parker

P.S. I recommend having a look at the 3.5-inch drive I/O routines in the

IIGS ROM. You can find them by looking in the SetHook/GetHook table, which

is at $E10F6F in ROM 1 (note that there are two identical JMP instructions

for each vector). It's best to do this without booting GS/OS, because

GS/OS patches the vectors. See the file on undocumented Smartport info on

my home page, http://cie-2.uoregon.edu/~nparker.

P.P.S. Are you aware that something keeps adding control-P's to the subject

lines of your postings? I've seen it in at least three unrelated posts from

you.

Neil Parker | No cute quote, no cute ASCII art, no cute

Apple II Computer Technical Information : Apple II Family Hardware Info

ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 147 of 572

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Caple C871i Manual de usuario Pagina 147

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Caple C871i Manual de usuario Pagina 147

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