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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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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.
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.
2 - A patch of the first type can be made permanent by initializing a diskette
while running the patched DOS. This procedure creates a slave diskette with a
copy of OOS on tracks 0, 1, and 2 which contains the patch. 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:
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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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.
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:
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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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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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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):
9E42:34 (for BRUN)
9E42:14 (for EXEC)
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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):
.nx ch8