Rockford Magazine/Rockford_Magazine_033_19xx_-_de_Disk_3_of_5_Side_B.d64/15.1

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        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