ftp.elysium.pl/ftp.elysium.pl.tar/ftp.elysium.pl/other/vic20/cartgrab.txt

Text File | 2008-03-07 | 31KB | 555 lines

                        ======================
                       =                      =
                      =   Inside your Vic 20   =
                      =     By Ward Shrake     =
                       =                      =
                        ======================


This document is a collection of pinout diagrams and technical notes, meant
to assist anyone who wishes to experiment electronically with the "obsolete"
Commodore Vic20 computer. Specifically, this document addresses the cartridge
or Memory Expansion port and related items, such as ROM & EPROM memory chips.

This is a very preliminary version of a document which could be expanded, if
I decide to go even more ballistic, later on. All this text was written out in
one day, with no prior planning, so if its a little jumpy, that's why. Later
versions should be better. Right now, probably anything helps, so here it is.
(Additions, corrections or whatever can be sent via email to the author.)

Ward Shrake, July 1995
email: [e-mail]


----------------------------------------------------------------------------
  Pinout diagram #1: Memory Expansion port connector of the Vic20 computer
----------------------------------------------------------------------------

Below is the pinout diagram of the Vic20 cartridge port, or Memory Expansion
Connector. Please note that this is NOT the pinout for a cartridge that might
fit into any port, nor is it a pinout of the User Port, which is something
completely different. This diagram shows the cartridge port's pinout, as you
face the rear of the Vic 20 computer. (The port is on the Vic20's left side.)

Bottom side of expansion port.       Top side of expansion port.
(This half is on bottom.)            (This half is towards the keyboard.)

          (This edge of the connector faces left,
          where the power switch and the LED are.)

                        _________
                        |  ___  |
         GND         A  |  | |  |   1       GND
         CA0         B  |  | |  |   2       CD0
         CA1         C  |  | |  |   3       CD1
         CA2         D  |  | |  |   4       CD2
         CA3         E  |  | |  |   5       CD3
         CA4         F  |  | |  |   6       CD4
         CA5         H  |  | |  |   7       CD5
         CA6         J  |  | |  |   8       CD6
         CA7         K  |  | |  |   9       CD7       (Memory Location:)
         CA8         L  |  | |  |   10      BLK 1       ($2000 - $3fff)
         CA9         M  |  | |  |   11      BLK 2       ($4000 - $5fff)
         CA10        N  |  | |  |   12      BLK 3       ($6000 - $7fff)
         CA11        P  |  | |  |   13      BLK 5       ($a000 - $bfff)
         CA12        R  |  | |  |   14      RAM 1       ($0400 - $07ff)
         CA13        S  |  | |  |   15      RAM 2       ($0800 - $0bff)
         I/O 2       T  |  | |  |   16      RAM 3       ($0c00 - $0fff)
         I/O 3       U  |  | |  |   17      V R/W
         S02         V  |  | |  |   18      C R/W
         NMI         W  |  | |  |   19      IRQ
         RESET       X  |  | |  |   20      NC
         NC          Y  |  | |  |   21      +5 Volts
         GND         Z  |  | |  |   22      GND
                        |  ---  |
                        ---------

          (This edge of the connector is on the
          right, where all the other ports are.)


Pinout Notes:
-------------
BLKxx =  8K decoded RAM/ROM block xx, active low. See chart above for area.
CAxx  =  Address bus line xx
CDxx  =  Data bus line xx
C R/W =  Read/Write line from CPU. (Read = high, Write = low)
GND   =  System ground
I/O 2 =  Decoded I/O block 2, starting at $9130
I/O 3 =  Decoded I/O block 3, starting at $9140
IRQ   =  6502 Interrupt Request line (active low)
NC    =  No connection
NMI   =  6502 Non-Maskable Interrupt line (active low)
RAMxx =  1K decoded RAM blockxx, active low. See chart above for memory area.
RESET =  6502 reset line (active low)
S02   =  Phase 2 system clock
V R/W =  Read/Write line from Vic chip. (Read = high, Write = low)



-----------------------------------------------------------------------
  Pinout diagram #2: A standard Vic20 cartridge's card edge connector
-----------------------------------------------------------------------

Below is a pinout diagram of a standard Vic20 cartridge, seen facing its card
edge connector. Please note that this is NOT the pinout of the port it plugs
into, which is shown above. The two pinouts are exact opposites, because you
are facing the port head-on as is, but you have to flip a cartridge around
180 degrees to be able to look at it head-on, facing its card edge connector.


 Bottom side of cartridge      Top (label or component) side of cartridge

                         _-_
         GND         Z   | |   22    GND
         NC          Y   | |   21    +5 Volts
         RESET       X   | |   20    NC
         NMI         W   | |   19    IRQ
         S02         V   | |   18    C R/W
         I/O 3       U   | |   17    V R/W     (Memory location:)
         I/O 2       T   | |   16    RAM 3       ($0c00 - $0fff)
         CA13        S   | |   15    RAM 2       ($0800 - $0bff)
         CA12        R   | |   14    RAM 1       ($0400 - $07ff)
         CA11        P   | |   13    BLK 5       ($a000 - $bfff)
         CA10        N   | |   12    BLK 3       ($6000 - $7fff)
         CA9         M   | |   11    BLK 2       ($4000 - $5fff)
         CA8         L   | |   10    BLK 1       ($2000 - $3fff)
         CA7         K   | |   9     CD7
         CA6         J   | |   8     CD6
         CA5         H   | |   7     CD5
         CA4         F   | |   6     CD4
         CA3         E   | |   5     CD3
         CA2         D   | |   4     CD2
         CA1         C   | |   3     CD1
         CA0         B   | |   2     CD0
         GND         A   | |   1     GND
                         -_-


--------------------------------------------------
   Pinout diagram #3: EPROM # 2764A
   (This is a standard, 8K x 8 bit memory chip)
--------------------------------------------------

            ____    ____
            |   !__!   |
        Vpp | 1     28 | Vcc  (+5 Volts)
        A12 | 2     27 | PGM  (Active low)
        A7  | 3     26 | N.C. (No connection)
        A6  | 4     25 | A8
        A5  | 5     24 | A9
        A4  | 6     23 | A11
        A3  | 7     22 | OE  (Output Enable; Active low)
        A2  | 8     21 | A10
        A1  | 9     20 | CE  (Chip Enable; Active low)
        A0  | 10    19 | D7
        D0  | 11    18 | D6
        D1  | 12    17 | D5
        D2  | 13    16 | D4
        GND | 14    15 | D3
            |__________|

 See Note 6 below, for some tips on using this as a replacement memory
 chip, on a modified cartridge body, for experimentation purposes. Note
 that there are differences between it and the standard Vic20 chip below,
 that will have to be accounted for before it can be wired to the boards.


--------------------------------------------------------
    Pinout diagram #4: "MPS 2364" ROM chip
    (Commodore-standard, 24-pin ROM chip, 8K x 8 bit.)
--------------------------------------------------------

            ____    ____
            |   !__!   |
       CA7  | 1     24 | +5 Volts
       CA6  | 2     23 | CA8
       CA5  | 3     22 | CA9
       CA4  | 4     21 | CA12
       CA3  | 5     20 | CS (Chip select, active low)
       CA2  | 6     19 | CA10
       CA1  | 7     18 | CA11
       CA0  | 8     17 | CD7
       CD0  | 9     16 | CD6
       CD1  | 10    15 | CD5
       CD2  | 11    14 | CD4
       GND  | 12    13 | CD3
            |__________|


This pinout was derived from a Vic20 schematic, found in the book the
"Vic20 Programmer's Reference Guide". (Great book!) Please note that while
this is an 8k-by-8-bit Commodore memory chip (its actually the Kernal chip,
located at $E000-$FFFF), and that while it could normally be assumed safely
that a company would standardize and use the same chips in their cartridges
that they used in their computer's motherboards, this is Commodore we're
talking about. A bit of paranoia might be in order. Having said that, I'll
note that as of this writing, the author has not compared this diagram and
an actual memory chip from a Commodore-produced Vic20 cartridge. 'Nuff said!


---------------------------------------------------------------------------
  Assorted notes on the diagrams above, and related subjects. (No order.)
---------------------------------------------------------------------------


Note 1: Information for the diagrams above was taken from the Vic20 reference
        book "The Vic Revealed" by Nick Hampshire, 1982, Hayden Book Co, Inc.
        That info was verified by checking it against information found in
        the "Vic20 Programmer's Reference Guide," 1982, by Commodore Business
        Machines, Inc. and Howard W. Sams & Company, Inc.

Note 2: You are looking at the cart pinout as if you were holding an unopened
        cartridge, label-side up (or right, in these diagrams) with the gold
        fingers pointing at you. If you are looking at a bare circuit board
        once it has been taken out of its outer plastic case, the chip is on
        top (or right, as shown here) again with the gold fingers facing you.
        (The diagrams were drawn sideways, as the ASCII drawings are somewhat
        clearer this way, believe it or not. There is less confusion this way
        over which pin is which. Horizontally, each name takes up more space.)

Note 3: Radio Shack currently (as of July 1995) carries a part that works as
        a Memory Expansion Port connector for the Vic20. However, they are
        discontinuing it, and are selling off all remaining stock. Their part
        number is 276-1551 for their "44-position card-edge connector." Once
        these are gone, you'll have to find another source for them. ($1.27)
        This part is useful for experimenting with this port on the Vic20, as
        it is a perfect fit, as is, to accept standard plug-in Vic cartridges.

Note 4: A perhaps useful experimenter's tip: If you plan to take the circuit
        board out of the cartridge case, then remove the ROM chip(s) from the
        circuit board, mark the two sides of the board first, or you may no
        longer be able to tell which is the top, and which is the bottom!
        This is especially true if one is removing chips to install sockets.
        (Any magic marker will do; just write "Bottom" on the "green stuff".
        Do not write on top of any exposed metal surfaces or the gold pins!)

Note 5: For experimenting purposes, it might be handy to find a cartridge you
        no longer care about, desolder the existing ROM chip from the circuit
        board inside the cartridge and install an IC socket to make it easier
        to change chips, later. However, note two things about this. One, the
        socket makes the whole thing taller, and it may no longer fit into a
        standard case. You can either (carefully!) use the new circuit board
        without using its outer case, at least for in-house testing, or you
        may be able to cut a "window" in the top of the case to clear it all.
        Second, some cartridges don't use standard IC ROM's, so try another
        cartridge, until you find one that does use a standard DIP package.

Note 6: On all the Vic20 carts the author has opened and seen so far, the ROM
        chip inside has 24-pins. Standard replacement EPROM's have 28-pins.
        This creates problems, but not insurmountable ones. It is a bit of a
        nuisance, but the dedicated experimenter can modify a 24-pin circuit
        board, to accept any 28-pin standard EPROM, such as the 2764 package.
        If one did not want the hassle of doing this, one other method exists
        to be able to put your own 8K eprom chips inside; however, it is not
        cheap. Motorola makes an 8K EPROM that is completely compatible with
        Commodore's "standard" 24-pin ROM chips. It even works as a direct
        pin-for-pin replacement for the 8K Kernal and Basic ROM's inside your
        Vic20 or your C64; the author has replaced both types in the past.
        The part number for this EPROM is MCM 68764. It is about $18.00 each.
        (Yes, it is programmable via the C64's "Promenade" eprom burner too!)
        I think I got my 68764 years ago, from Jameco Electronics; try them.
        (A 2764 adapter circuit board can be made to adapt the differences in
        24-to-28 pin sockets, but it will likely be too tall to fit inside.)

Note 7: A reset button can be wired into the Vic20 fairly simply. This might
        be a good idea, if one plans to experiment much. The Vic20, or any
        computer, might lock up when you've told it to do something odd while
        experimenting. Simply use a momentary-contact, normally-open switch,
        wired between the RESET line and any port GND line. When you press
        the new button for a second and let go, the computer will reset. This
        is because the RESET line is active low, and your switch grounds that
        line. If you decide to do this *inside* your Vic20, remember you are
        now looking at the *back* of the port, NOT the front. Therefore, use
        the pinout diagram for a cartridge, not the port, to find the lines.
        Radio Shack switch part number 275-1547 or equivalent will work fine.

Note 8: The memory area located at $A000-BFFF ("Block 5") is normally
        considered to be a ROM-only block of memory. RAM can be mapped into
        that space if you modify the cartridge's internal wires or jumpers,
        to redirect a standard RAM cartridge to load higher up in memory.
        However, BASIC cannot normally access that area, as it is meant to
        be for ROM's only. This is important, if one wishes to archive an
        8K autostart ROM cartridge (normally located in block 5), and run the
        resulting ROM image from within RAM. It can be done, but you have to
        modify a standard 8K RAM cartridge to do it. (Ground BLK 5, and cut
        any trace or jumper that tells the cart to load in a different area.)

        Note also that some carts may be copy-protected, and will not run in
        RAM without modification or "training," to alter the copy protection.

        It may be possible to get around this copy-protection (but the author
        hasn't tried it yet) by modifying the read/write lines, so that after
        loading the program's ROM image into RAM memory, you flip a switch
        first, then type in the SYS command to start the program up. If this
        idea works, it would emulate ROM by not being able to be written to,
        when the copy protection tries to determine if the copy is in RAM or
        ROM. Of course, when reprogrammed into an EPROM, it will act as if it
        were ROM anyway, so this may no longer matter in that case.

Note 9: Trying to relocate a machine language program from the memory area
        where it was written in and intended to be, and to have it actually
        work afterwards, is not advised. It won't work, without doing so
        many modifications to the machine language object code that you may
        as well just start over and reprogram it all. Don't bother trying.
        It seems the idea is to get the real, actual code as written, anyway.

        Similar problems confront the user who wishes to archive a tape or a
        diskette-based original. Trying to make a cartridge out of it, isn't
        worth the huge headaches. You are better off, in this case, trying to
        archive tape-based programs to floppy disk (which will still require
        a little modification, but not as much), and floppies to floppies.

Note 10: Some FTP archives exist on the Internet for the Vic20 enthusiast.
        ("ftp.funet.fi" in Finland, and "ftp.hrz.uni-kassel.de" in Denmark.)

        When downloading archived ROM images from these places, one should
        be aware that they are missing two bytes that the Commodore Vic20 and
        the Commodore 64 computers consider absolutely critical to load and
        use files with. These are the first two bytes of any Commodore file,
        which tells the computer a program's "Starting address," or where to
        load it into memory. If its not in the right place, it will not work.
        Period. Even one byte off, and it will not work. These raw image
        files include only the ROM's internal codes, and no starting address.

        The FTP site in Denmark has some instructions on how to add these two
        bytes to a file, using a Commodore Vic20 compatible disk drive. The
        author made an executable program that does this on the IBM, but it
        isn't very user-friendly at this point. It does work, however, and as
        soon as it will pass muster publicly, I plan to upload it to both FTP
        sites. This program, when available to the public, is meant to be used
        after a ROM image file has been downloaded onto an IBM computer, but
        before the user transfers it to a Commodore-compatible diskette, using
        whatever process the user likes most. (The "X-1541" user-built cable
        which connects the IBM printer port to the CBM serial port, is the
        author's favorite way of transferring files from IBM to CBM and back.)

Note 11: On all the NTSC Vic20 motherboards I looked at (4 or 5), the BASIC
        memory chip is labeled "UE11", while the Kernal chip is labeled as
        "UE12". My PAL schematic lists these two as UD5 and UD6, respectively.
        The Basic memory map area is at $C000-DFFF. Kernal is at $E000-FFFF.
        The only other 24-pin chip is the Character Generator; its 4K, not 8K.

Ward Shrake
email: [e-mail]
July 1995


-------------------------------------------------------------------------
    Short technical tutorial on Vic20 ROM cartridges, including notes
    on cartridge archiving methods, and the Vic autostart process.
-------------------------------------------------------------------------

A Vic20 cartridge is approximately 5.5 inches wide, 3 and 3/8ths inches deep,
and 5/8ths inches tall, when viewed as if ready to be installed. Standard
configurations are 8K and 16K of 8-bit ROM memory, using one or two chips of
ROM memory onboard. If other standard cartridge memory configurations exist,
the author is currently unaware of them. (But would like to hear of any, if
they do/did exist, and especially if they were once made commercially.)

Inside a typical Vic20 cartridge is one double-sided, etched circuit board,
with some form of memory chip installed. This may be a "standard" IC chip as
we are used to seeing (24 or 28 pin ROM, in a DIP package), or it could be
a blank circuit board, with a tiny blob of black epoxy material on it, under
which are presumably the internal components of a typical, normal ROM chip.

The standard circuit board size is approximately 3 and 9/16ths inches wide,
and 1 and 3/4 inches deep. A cart fits into a 44-position, double-sided card
edge connector, which is a permanent part of the Vic20 itself, in the back.

On the cart circuit board itself may (or may not) be several wire traces or
jumpers, which if there, are meant to configure the cartridge to a certain
memory arrangement. These jumpers are meant to connect the BLK or RAM lines
to the system ground. These lines, when connected, tell the Vic20 where to
place the cartridge within the Vic20's internal memory map scheme.

While all "normal," autostart game cartridge are located in one fixed area
of memory, this is not true with all cartridges, game or not. Some of the
earliest cartridges did not use the autostart procedure for whatever reason,
(presumably because they did not know it existed, at that early stage?) and
those are located in "non-standard" memory locations. The Vic20 was built
and designed when memory was expensive, and computers were designed to be
bare-bones memory-wise, but with the capability to expand on later. This may
take some getting used to at first, but understanding it gets easier in time.

16K cartridges, for example, generally have one 8K ROM block located from
$A000-$BFFF, and another 8K ROM block located at $6000-7FFF. If a cartridge
was factory-made not to autostart; in other words, if it needs you to type in
a "SYS" number to start it, its memory could be located in any block.

The "normal" spot for an 8K cartridge to be located in memory is in "Block 5"
(which is located at $A000 to $BFFF). Again, this is not the only possible
spot in memory for a cartridge to be located, but it is the only spot where
the Vic20 will look for a cartridge that automatically starts on power-up.

This is because (at power up) the Vic20 looks for a precise code in a precise
spot, to see if it should autostart a cartridge or not. If the Vic20 finds
this *exact* five-byte code, *exactly* where it is supposed to be located in
memory, the Vic20 turns control over to the cartridge. If not, it gives over
control to the user, via the normal, power-up Basic READY prompt screen.


        This 8K autostart sequence code is shown below:

        Address______Hex value_____Decimal value____ASCII values
        $A004        $41           65               Capitol "A"
        $A005        $30           48               Digit "zero"
        $A006        $C3           195              Reverse "C" character
        $A007        $C2           194              Reverse "B" character
        $A008        $CD           205              Reverse "M" character

If the computer finds this five-byte sequence exactly as shown, it
turns control over to the machine language program in the cartridge.
To do so, it needs to know where the program begins. There are four
bytes which determine this, as shown below: (Also within the cart.)

        Address______What this byte of information contains__________
        $A000        Low-byte of a 16-bit "hard start" address (to begin at)
        $A001        High-byte of the 16-bit "hard start" address

        $A002        Low-byte of 16-bit "warm start" address (restore key)
        $A003        High-byte of 16-bit "warm start" address

This is all very important when trying to archive an image of the information
contained on a ROM cartridge. Altering either of the two required parts of
this sequence, causes the cartridge to not autostart, but to be mapped into
memory properly, anyway. Assuming a certain level of technical expertise, a
user can do one of two (or more?) things to achieve this.

One is to move the cartridge's starting location temporarily, so that the
code is present, but not in the right place. This can be done by altering the
physical traces on each cartridge's circuit board. This is undesirable,
except perhaps for self-training purposes, as every single cartridge would
have to be individually modified, archived, then reassembled for normal use.
Permanently modifying the Vic20's internal wiring is also possible, but
hardly desirable, as permanent damage to the Vic20 might also result.

The best way I could recommend for this would be to have a qualified tech
build or wire a cartridge-port pass-through device, where nearly all the
traces re-connected where they are supposed to, except for a critical few.
Using a 44-position card edge connector, some wire and a junk cartridge (or
etched circuit board like a cartridge's pins), this could be done. Most wires
would start at the card edge, where you'd plug the device into the Vic20, and
pass-through harmlessly to the card edge connector, where carts would plug in.

This device reroutes the "BLK 5" trace, preferably by use a "normal/modified"
switch, so that even though the cartridge intends to load at $A000, it loads
to another 8K block of memory instead. Whichever one you wire it to; perhaps
Block 1 or 3. Make the switch(es) act as a simple pass-through, normally, but
disconnect the "Block 5 to ground line" while at the same time connecting the
"Block X line to ground", to remap where the cartridge will temporarily go.

This device would ideally be plug-in-able and removable, like a cartridge, so
as to require no modifications to either the Vic20 or cartridges. A reset
button could also be installed in the device, as well, while you're at it. If
you use an old cartridge as your plug-in on the Vic20 end, cut all traces that
still go to the existing chip(s). Solder wire ends onto the portions that exit
the Vic20, and route them to your external plug-in connector box. If you use
ribbon cable as your wiring, you can probably even sneak the wire out the back
of the existing cartridge's case, as there is a wide slot in most Vic cases!

This method will allow you to temporarily re-map where the cartridge image
will appear in memory, to defeat the autostart feature. Once this is defeated
the technically-inclined user who is familiar enough with Commodore computers
can copy the information to tape or diskette for storage. This takes some
knowledge and expertise, but can be learned. Some help is offered; see below.

The user should understand that if this method (memory relocation) is used to
defeat the autostart feature, and to thereby attempt to archive a cart's ROM,
the work is not over. Once archived to disk, the hard part is done, true, but
when you saved it to diskette, it was saved with a now-incorrect starting
address. Before the image can be used further, this has to be corrected.

This can be changed, fairly easily, by someone who knows how, but it requires
knowledge of using a software tool called a "disk sector editor" to change
the now-incorrect high-byte of the relocated starting address, to the correct
unrelocated high byte: where it would have been, before you moved it. To do
so, you find the first disk sector of the ROM image file, and change the
fourth byte in (not byte $04!) to be $A0, resave the sector, and you're done.
After this modification, your ROM image files will now load where they should,
(into $A000 RAM, if you have it), and are ready for further processing.

Even the common C64 "Fastload" utility cart has a very servicable sector
editor in it, so it isn't as bad as it may have sounded. Still, you need to
know how to use it, and what to change, so it is best left to a professional.

To actually save the block of memory to tape/disk, you either use the memory
save feature of a machine language monitor program (not built in) to save the
8K block of ROM memory temporarily, or you can change certain POKE's in the
Vic20's memory, to tell it your cartridge is located in the Basic program
memory area, so that the normal SAVE command works. The beauty of the latter
is that no additional programs are needed to make this work; just type in four
POKE commands, then tell it to SAVE as it would normally save a Basic program.

For those of you already familiar enough with Commodore's style and memory
arrangement schemes to make sense of this, bytes 43 and 44 (decimal, not hex)
are the pointers to the Start of Basic memory area, and bytes 45 and 46 are
the pointers to the Start of Variables, or in other words, the End of Basic.
Follow this chart, to save a block of memory from these areas via pokes:


   Block #___Hex Address____Poke 43,x___Poke 44, x___Poke 45, x__Poke 46, x
     1       $2000-3fff      x = 0       x = 32       x = 255     x = 63
     2       $4000-5fff      x = 0       x = 64       x = 255     x = 95
     3       $6000-7fff      x = 0       x = 96       x = 255     x = 127
     5       $a000-bfff      x = 0       x = 160      x = 255     x = 191


The above is only one possible way to archive an 8K block of Vic20 memory.
However, a second method exists, requiring some Vic20 modifications. A truly
hard-core Vic user could alter the copy of the startup code that is stored in
the Kernal ROM chip, so that no normally-correct autostart sequence code ever
matches it as long as your new Kernal is in place.

Remember, there must always be *two* copies of the start-up code; one in the
cartridge itself and one stored in the computer for comparison. If either of
these two seperate codes are altered, even slightly, it will not autostart!

The cartridge gets mapped into memory at its correct location, but the Basic
"READY" screen comes up instead, giving the user full control. Once you've
achieved this, you simply do four pokes (to change the start and end of BASIC
memory to match where the cart code is), or save the info with an ML monitor.

With some work and an "MCM 68764" 8K EPROM, this can be done. It is not just
a vague theory; the author has done it. It works, as does a similar technique
for doing the same general thing with the Commodore 64 computer. (With the
appropriate changes, of course, to reflect the C64's different architecture
and memory map; the C64 codes are "CBM80" instead of "A0CBM", and the Kernal
expects that code to be in a cartridge at locations $8004-$8008 instead of
$A004-$A008. Starting address is in the same exact format as the Vic's.)

This a much more involved modification, and not for the squeemish or
those who only own one Vic20 (or C64) without any spares to fall back on!

To do it, you need a heck of a lot of computer knowledge specific to the
computer you're working on, an eprom burner, an eprom eraser, and patience.
You have to search in the Kernal's memory image, for the start-up code, and
modify those bytes to be something different than they are, while leaving all
the other bytes just as they are. This alone takes some serious skills, no?
You also need skills and tools in desoldering and removing IC chips, as well
as soldering in IC sockets, and not destroying anything in the process.

I won't go into this much, because I don't imagine many people will bother to
actually attempt this method, since the other has some serious pro's to it,
and since this should be about enough info to go on; if you can't figure out
how to do this modification with this info, you probably should not try it,
until you've progressed enough at a later time. Or, have someone else do it
for you, with these instructions, if you really want it done badly enough.

This way may appeal to some people, but I recommend the other for most folks.
The overly-technical types might be tempted to go ballistic however, and more
power to you! A thought, in parting; one could put 2 copies of the Kernal ROM
chip, one stock and one modified, into one 16K eprom, with an external switch
to flip between the two versions. (Or just go lower tech, and just manually
swap out Kernal chips as needed; most Vic20's seem to have sockets anyway.)

Whatever you do, I hope you've enjoyed this article! Later, all!

Ward Shrake
[e-mail]
July 1995