From: Baltissen, GJPAA (Ruud) (ruud.baltissen_at_abp.nl)
Date: 2005-04-22 09:43:01
Hallo Spiro,
> Now, the format routine handles the same (writing a block to disk) a
> little bit differently:
>
> .8:fd1e 50 FE BVC $FD1E ; make sure the last byte is written
> .8:fd20 B8 CLV
> .8:fd21 50 FE BVC $FD21 ; write another (!) byte to disk
> .8:fd23 B8 CLV
> .8:fd24 20 00 FE JSR $FE00 ; set read mode and data
> port as input
The actual formating of the track starts at $FCAE with writing 10240 times
$55. This is more then enough to write the complete track. Then a loop
starts at $FCB1. This loop handles writing a sector including its header.
The loop itself goes as far as $FD1C. The last part of the loop is writing a
number of $55's to the disk. This number depends on the number of tracks and
is calculated at $FC17.
Only after ALL sectors have been written, your part shows up by writing two
extra $55's after the last $55's written by the loop. This bit of code isn't
used anywhere else, certainly it is not used for writing a block to disk as
you say.
The only reason these $55 bytes are there is to give the drive time to
switch from read- to write-mode (or vica versa) if needed, nothing else. So
why these extra two bytes are written, I don't know; I don't see any
advantage in doing so.
I hope this helped :)
--
___
/ __|__
/ / |_/ Groetjes, Ruud
\ \__|_\
\___| URL: Ruud.C64.org
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