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CHAPTER 7 - CUSTOMIZING DOS
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Although DOS usually provides most of
the functionality needed by the BASIC
or assembly language programmer, at
times a custom change is required.
Making changes to your copy of DOS
should only be undertaken when
absolutely necessary, since new
versions of DOS are released from
time to time, and the job of moving
several patches to a new version of
DOS every few months can become a
burden. In addition, wholesale
modification of DOS without a clear
understanding of the full
implications of each change can
result in an unreliable system.
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SLAVE VS MASTER PATCHING

The usual procedure for making
changes to DOS involves "patching"
the object or machine language code
in DOS. Once a desired change is
identified, a few instructions are
stored over other instructions within
DOS to modify the program. There are
three levels at which changes to DOS
may be applied.
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1 - A patch can be made to the DOS
in memory. If this is done, a later
reboot will cause the change to "fall
out" or be removed.
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2 - A patch of the first type can be
made permanent by initializing a\x1f?O7O?K'7s';;\x44\x4d\x08\x5a\x5d\x46\x46\x41\x46\x4f\x08\x5c\x40\x4d\x08\x58\x497#+''O\x87\x8f\x93oc\x97++3c77+'##37#g'7#'3#7g'g7+'3#K#/7/#33#g3/3/g'g'+7?g+#g\x83\x8b\x97g++g37''/7Kwkgwkg'+#gwg3//'/g'++3'/+7g3/#g7'3'h.
Each time this newly created diskette
is booted the patched version of DOS
will be loaded. Also, any slave
diskettes created by that diskette
will also contain the patched version
of DOS.
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3 - The patch is applied directly to
a master diskette. This is somewhat
more complicated. Either the patch
may be made to the image of DOS on
the first three tracks of a master
diskette using a zap program, or
MASTER CREATE may be used to write
the changed copy of DOS to a new
diskette. The following procedure may
be followed to do this:
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    BLOAD MASTER CREATE
    Get into the monitor (CALL -151)
    Store a $4C at location $80D (80D:4C)
    Execute MASTER CREATE (800G)
    When MASTER CREATE finishes loading the DOS image
       it will exit. You may use the monitor to make
       changes in the image. MASTER CREATE loads DOS
       into memory at $1200 such that Boot 2 (RWTS) is
       loaded first, followed by the main part of DOS
       starting at $1C00.
    When all patches have been made, reenter MASTER CREATE
       at location $82D (82DG).
    Complete the MASTER CREATE update normally. The
       resulting diskette will have the patches applied.
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This procedure will work for versions 3.2, 3.2.1, and 3.3 of
DOS.
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AVOIDING RELOAD OF LANGUAGE CARD

A rather annoying addition to DOS 3.3
was a patch to the Boot 2 code to
store a binary zero in the first byte
of the Language Card, forcing DOS to
reload BASIC (either INTEGER or
APPLESOFT) for every boot, whether or
not the machine was just powered up.
When the machine is first powered up
this patch is not necessary, since
the first byte of the Language Card
does not appear to DOS to be either
BASIC, and it will reload the card
anyway. On subsequent reboots, more
often than not, a good copy of BASIC
already resides in the Language Card
and this patch results in a LANGUAGE
NOT AVAILABLE error message after
booting a slave diskette. Presumably
the patch was added for version 3.3
to allow for the eventual possibility
that a language like PASCAL whose
first byte of code just happens to
match one of the BASICs would cause
strange results in DOS. If the user
always powers the machine off and on
between using DOS and any other
system, the patch may be removed as
follows.
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At $BFD3 (48K) is a STA instruction which
stores a zero on the Language Card.
This instruction must be made into
three no-operation instructions:
        BFD3:EA EA EA
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A slave diskette may then be INITed
using this modified version of DOS
and that diskette will have the
patch in its DOS. The address of the
store instruction for a 32K DOS is
7FD3 and for a 16K DOS is 3FD3.
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INSERTING A PROGRAM BETWEEN DOS AND ITS BUFFERS

Once in a while it is useful to find
a "safe" place to load a machine
language program (a printer driver,
perhaps) where BASIC and DOS
can never walk over it, even if DOS
is coldstarted. If the program is
less than 200 bytes long, $300 is a
good choice. For larger programs,
it is usually better to "tuck" the
program in between DOS and its
buffers (assuming the program is
relocatable and will run at that
location). To do this, load the
program into low RAM, copy it to high
RAM right below $9D00 (for a 48K
machine), over the top of DOS's
buffers, change the first buffer
address at $9D00 to point below your
program, (remember to allow 38 extra
bytes for the filename and link fields)
and JMP to $3D3 (DOS
COLDSTART). This will cause DOS to
rebuild its buffers below your
program and "forget" about the memory
your program occupies until the next
time DOS is booted. Of course, BASIC
can not get at that memory either,
since its HIMEM is below the DOS
buffers.
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BRUN OR EXEC THE HELLO FILE

Ordinarily, when DOS finishes booting
into memory, it performs a RUN
command on the HELLO file in its file
name buffer (left there by the INIT
command which wrote DOS to the
diskette).  To change the RUN command
to a BRUN or an EXEC, apply the
following patch to DOS (48K):
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     9E42:34         (for BRUN)
     ..or..
     9E42:14         (for EXEC)
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REMOVING THE PAUSE DURING A LONG CATALOG

Normally, when a CATALOG command is
done on a disk with many files, DOS
will pause every time the screen
fills with names to allow the user
time to see them all.
By pressing any key
the CATALOG continues.  If this pause
is undesirable, apply the following
patch to DOS (48K):
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     AE34:60
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