112 lines
5.5 KiB
Plaintext
112 lines
5.5 KiB
Plaintext
# CHAPTER 7 - CUSTOMIZING DOS
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Although DOS usually provides most of the functionality needed by the BASIC or
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assembly language programmer, at times a custom change is required. Making
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changes to your copy of DOS should only be undertaken when absolutely necessary,
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since new versions of DOS are released from time to time, and the job of moving
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several patches to a new version of DOS every few months can become a burden. In
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addition, wholesale modification of DOS without a clear understanding of the
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full implications of each change can result in an unreliable system.
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SLAVE VS MASTER PATCHING
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The usual procedure for making changes to DOS involves "patching" the object or
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machine language code in DOS. Once a desired change is identified, a few
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instructions are stored over other instructions within DOS to modify the
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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
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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 diskette
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while running the patched DOS. This procedure creates a slave diskette with a
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copy of OOS on tracks 0, 1, and 2 which contains the patch. Each time this
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newly created diskette is booted the patched version of DOS will be loaded.
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Also, any slave diskettes created by that diskette will also contain the patched
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version of DOS.
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3 - The patch is applied directly to a master diskette. This is somewhat more
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complicated. Either the patch may be made to the image of DOS on the first three
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tracks of a master diskette using a zap program, or MASTER CREATE may be used to
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write the changed copy of DOS to a new diskette. The following procedure may be
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followed to do this:
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BLOAD MASTER CREATE
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Get into the monitor (CALL -151)
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Store a $4C at location $80D (80D:4C)
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Execute MASTER CREATE (800G)
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When MASTER CREATE finishes loading the DOS image
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it will exit. You may use the monitor to make
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changes in the image. MASTER CREATE loads DOS
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into memory at $1200 such that Boot 2 (RWTS) is
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loaded first, followed by the main part of DOS
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starting at $1C00.
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When all patches have been made, reenter MASTER CREATE
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at location $82D (82DG).
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Complete the MASTER CREATE update normally. The
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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
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A rather annoying addition to DOS 3.3 was a patch to the Boot 2 code to store a
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binary zero in the first byte of the Language Card, forcing DOS to reload BASIC
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(either INTEGER or APPLESOFT) for every boot, whether or not the machine was
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just powered up. When the machine is first powered up this patch is not
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necessary, since the first byte of the Language Card does not appear to DOS to
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be either BASIC, and it will reload the card anyway. On subsequent reboots, more
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often than not, a good copy of BASIC already resides in the Language Card and
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this patch results in a LANGUAGE NOT AVAILABLE error message after booting a
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slave diskette. Presumably the patch was added for version 3.3 to allow for the
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eventual possibility that a language like PASCAL whose first byte of code just
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happens to match one of the BASICs would cause strange results in DOS. If the
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user always powers the machine off and on between using DOS and any other
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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.
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This instruction must be made into three no-operation instructions:
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BFD3:EA EA EA
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A slave diskette may then be INITed using this modified version of DOS and that
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diskette will have the patch in its DOS. The address of the store instruction
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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
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Once in a while it is useful to find a "safe" place to load a machine language
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program (a printer driver, perhaps) where BASIC and DOS can never walk over it,
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even if DOS is coldstarted. If the program is less than 200 bytes long, $300 is
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a good choice. For larger programs, it is usually better to "tuck" the program
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in between DOS and its buffers (assuming the program is relocatable and will run
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at that location). To do this, load the program into low RAM, copy it to high
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RAM right below $9D00 (for a 48K machine), over the top of DOS's buffers, change
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the first buffer address at $9D00 to point below your program, (remember to
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allow 38 extra bytes for the filename and link fields) and JMP to $3D3 (DOS
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COLDSTART). This will cause DOS to rebuild its buffers below your program and
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"forget" about the memory your program occupies until the next time DOS is
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booted. Of course, BASIC can not get at that memory either, since its HIMEM is
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below the DOS buffers.
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BRUN OR EXEC THE HELLO FILE
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Ordinarily, when DOS finishes booting into memory, it performs a RUN command on
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the HELLO file in its file name buffer (left there by the INIT command which
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wrote DOS to the diskette). To change the RUN command to a BRUN or an EXEC,
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apply the following patch to DOS (48K):
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9E42:34 (for BRUN)
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..or..
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9E42:14 (for EXEC)
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REMOVING THE PAUSE DURING A LONG CATALOG
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Normally, when a CATALOG command is done on a disk with many files, DOS will
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pause every time the screen fills with names to allow the user time to see them
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all. By pressing any key the CATALOG continues. If this pause is undesirable,
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apply the following patch to DOS (48K):
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AE34:60
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.nx ch8
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