Stolen Data 8/Stolen Data 8.adf/TEXT/mfmarticle/mfmarticle

Text File | 1989-08-16 | 6KB | 206 lines

@^
@t4@f01@cTRACK LOADERS


@t3@f00Typed by OMICRON of Dual Crew

@t5@f00O.K,  I  am  writing  this article and
supplying   the   source  for  my  own
personal custom track loader for three
reasons.   Firstly,  because there are
so  many  coders who don't really know
how   tracks,   disks  and  MFM  work,
because there is no such documentation
in  any  of  the  technical  reference
manuals,  which is really a shame, and
secondly, because most people who code
trackloaders  who  use  the  Operating
System  functions, such as OPENDEVICE,
and  READ  etc..., which is not really
cool  because  reasons  which  I  will
explain later, and thirdly, people who
can  program  the disk hardware, don't
@^

@t5

really  code  it  the way it should be
done, this is why some loaders may not
work  on  Amigas  with  delicate  disk
drives,  and  drives  which are not as
fast  as  others, anyway all this will
be explained.
@t3
SO WHAT ABOUT THE TRACKDISK !

@t5
The  trackdisk  is  the  part  of  the
Kickstart  that  controls all the disk
hardware  from an almost user friendly
point of view.  You can do things like
reading  tracks,  check for protection
tabs,  etc.   And  you  can do alot of
advanced  things  like read individual
disk  blocks, and write to disks.  The
@#
@^
@t5disadvantages are tenfold, firstly, as
you   may   know,  to  use  the  Amiga
Operating  system,  the system must be
in  a fully working state, which means
that  memory is going to be gobbled up
lots and you cannot really rely on the
state  of  your INTerupts+DMA and your
Status  Register.   Everything on your
disk  must really use the SAFE way, eg
using   special   EXEC  functions  for
creating  interupt  vectors, disabling
interupts, memory management, and disk
calls.   If  you start writing spurios
data into the INTERUPT AUTOVECTORS, or
change  your SR, or start fidling with
copper  lists  and  DMA,  you may find
that  your greatest demo does not work
on as many Amigas as you would like it
to  !  (LSD "DESPAIR" is an example of
a  badly  coded  loader, coz the music
cuts  when some new tracks are loaded,
@^
@t5most  likely  because this uses the OS
in conjuction with INTERUPTS not setup
in  a  friendly  manner).  Another big
disadvantage   is   the  trackdisk  is
terribly ssllooww and prone to errors.
The  way  the  Operating  System deals
with  disks  is totaly lame the way it
is  supposed  to  (eg.  not reading on
SYNC  marks and the timing of the disk
step).  I don't know whether the error
proneness  is  a bug in the OS or just
my  machine,  coz sometimes the tracks
loaded    with   the   trackdisk   are
sometimes mashed, so don't quote me on
that !!

@t3HARDWARE LOADER
@t6---------------
@t5
OK  you've  coded  a  hardware loader,
what are the advantages ??.  There are
@#
@^
@t5quite alot !!  Firstly, you have total
control  over  all  the memory and the
system,  coz a real trackloader should
not  use  the  OS, so there is no need
for  the  OS  to  be  resident, so you
don't  have  to  worry about the Amiga
Hardware    conflicting    with    the
operating  system.   Secondly, you can
customize your drive speeds (you could
have  a menu where the user can select
a  fast/slow  trackload).  and you can
customize your own disk format for say
copy  protection  on  your latest cool
game  to beat all the crackers.  There
are  lots  of other advantages such as
turning  the  trackloader  into an MFM
file  loader,  which would work on the
same  basis  as the OS deals with Data
files,  you could even code the loader
to  load  in executable files, and you
could  write  your  own LIBRARY to use
@^
@t5all   these   functions  within  other
programs.   You  may  even  change the
loader  into  a  GCR  trackloader, and
have  2X  as much data on your disks !
THE SKYS THE LIMIT !!

@t3THE SOURCE
@t6----------
@t5
On  this  magdisk is my own customized
CYLINDER   loader  (a  CYLINDER  is  2
tracks).   I  coded this loader, coz I
used  C-MON  to write the executeables
to  my  disk, and C-MON only writes in
CYLINDERS.   But this should be no big
deal  to  change  (I  have not done it
personaly,   coz   I   haven't   coded
anything  that really realies on cramp
data).   I  will give ya a some of the
advantages  of  using  this  loader in
prods.
@#
@^
@t5
@t61.@t5 The loader source is very small
   (around 540 bytes), and is written
   to fit on a bootblock.  You can
   easily fit a small boot intro with
   it on the  BOOT as a BOOTBLOCK(S)
   is just 1K, or you could write in
   some boot text,to waste the 0's.

@t62.@t5 Uses real CIA timing for a 3
   millisecond wait for diskstep
   (no DBRA's !).

@t63.@t5 Header and Checksum verify (almost
   guaranteed to load the specified
   track  without any errors !

@t64.@t5 Uses SYNC read + reasonable decoding
   method.

@t65.@t5 Code is position independant.
@^
@t5
@t66.@t5 Does not assume anything on boot,
   eg state of the SR, DMACON, INTENA
   or even the ADKCON register !

@t67.@t5 Uses no part of the OS at all !

@t68.@t5 Tries to load the track in 10
   times, if failed flashing lights
   occur and the computer hangs !

I  am  not  saying that this loader is
the  coolest  in the world, but it has
taken me around 5 months on/off to get
it  to  this stage, so I sure hope you
use it and improve on it.  It may also
not  be without bugs, but I am sure if
you  are  gonna  use a trackloader for
anything,  you  must be cool enough to
fix    these    bugs.     Heres   some
improvements that I thought of making.
@#
@k370,003,03
@^
@t5
@t61.@t5 Blitter decoding
@t62.@t5 4 Drive Capability
@t63.@t5 Data Encrypting
@t64.@t5 Decode one track while reading
   another
@t65.@t5 Do something constructive during
   STEP waits
@t66.@t5 Find some clever way of using
   SYNC + DISKBLK Interupts
@t67.@t5 THE LIST GOES ON !! LOOK AT THE
   SPACE ACE LOADER !!!

To  know  more about Amiga Diskdrives,
read   the  Abacus  Book  "AMIGA  DISK
DRIVES  INSIDE AND OUT", which is lame
in  parts, but should help you alot to
know how it all really works !!.

Anyways I got to finish  this article
LOVE ON YA !! @t2 Dual Crew 92
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