Modify (hack) VIC-20 Cartridge "The Count" into "Sky Blazer" (2 x 8k images)

[LouisB]

I'd like to modify (hack) VIC-20 Cartridge "The Count" into a "Sky Blazer" cart by replacing the ROM with an EEPROM burned with these 8k images:

 

Sky Blazer (1983)(Broderbund)(NTSC)[6000][multipart].crt

Sky Blazer (1983)(Broderbund)(NTSC)[A000][multipart].crt

 

I've verified these images work in Vice emulation using "Add to generic cartridge..." 4/8/15KB image at $6000 and $A000

Cart w/ Original ROM

Other side of PCB

Understand that $A000 is the 8K cartridge space.  Is some memory mapping required to access the other 8k image in $6000, or can I just burn the two sequentially into a 16k eeprom?  Appreciate any VIC-20 guru comments!

Edited August 21, 2022 by LouisB

[carlsson]

You chose an unfortunate cartridge for your project. The adventure games contain 16K continuous ROM from $4000 to $7FFF, and none at $A000 which is one of the reasons there is a SYS address to start the game. The game you want to play instead is divided into two 8K blocks at $6000-$7FFF and $A000-$BFFF. Each pin on the cartridge connector has a different purpose, of which four of them are used to enable blocks 1 ($2000), 2 ($4000), 3 ($6000) and 5 ($A000). There are additional pins to enable RAM 3K expansion in the $0400 area, as well as pins for I/O blocks 2 and 3 at $9800 and $9C00 but that is beyond the question.

 

In theory it would be possible to modify the game you want to play into a continuous block of 16K but it would require you to enter a SYS address, probably something like SYS 16392 or similar. A lot of code would need to be disassembled and analyzed which calls and memory addresses in the $A000 block are used and need to be translated to $4000. Not impossible, but it depends how skilled you are and how strongly you want this to become reality.

 

The other way around would be to somehow make room for two different ROM chips or at least some additional circuitry that is able to split the upper addresses of a 16K EPROM into its own address pins. I think you'd rather etch a completely new circuit board than trying to modify this one.

 

If you had some other common 16K cartridge game that already was configured into two blocks at $6000 and $A000, I think your task would be much easier.

[Keatah]

Oh how cool. I had no idea SkyBlazer was on the VIC-20. I thought it was only on Apple II and Atari 400/800.

[LouisB]

8 hours ago, Keatah said:

Oh how cool. I had no idea SkyBlazer was on the VIC-20. I thought it was only on Apple II and Atari 400/800.

Yes!  Definitely one of my top 5 favorite games on Apple II back in the day.  I've been going back and playing it on all three platforms (on real hardware.)

 

Recently played it on an Atari 800XL for the first time, and unsurprisingly (due to obvious limitations with stock Apple II sound functionality) the sound effects on Atari are far superior!! 

[LouisB]

@carlsson TYSM for your expert reply, you've definitely saved me from spinning wheels with this cart.  About modifying the game, I don't have any experience decompiling or reverse engineering cartridge game code so that route is beyond my knowledge at this point.

 

So are you saying I should look for a cartridge that has two 8K ROMS in it?

 

I found this list of ROMS, anyone know what cart might fit the bill?

http://mhv.bplaced.net/cbmroms/cbmroms.php

 

Edited August 22, 2022 by LouisB

[Keatah]

9 minutes ago, LouisB said:

Yes!  Definitely one of my top 5 favorite games on Apple II back in the day.  I've been going back and playing it on all three platforms (on real hardware.)

 

9 minutes ago, LouisB said:

Recently played it on an Atari 800XL for the first time, and unsurprisingly (due to obvious limitations with stock Apple II sound functionality) the sound effects on Atari are far superior!! 

Won't argue there. I first played it on the Apple II right when it came out, so that's the version I will always prefer. The clicks and beeps bloops. Iconic in a grand way!

 

Same thing with Defender. I first played it on the Atari 400/800 and will always prefer that version over the arcades. Arcade Defender was simply too difficult to allow me any worthwhile game time. I always left the establishment with a sense of frustration. Too much to want to go back and keep trying. Space Invaders was similar in nature. I loved the 2600 version. Difficulty, colors, sounds, variety of options, yes. Space Invaders in VCS incarnation was superior all around. And I'm not the only one to say that.

 

Loved Gyruss on the C64, though of course, here, the arcade version trumps all. Gyruss was somewhat of a two-facer for me in that I liked both versions equally. The arcade with its multi-polyphonic sound. And the home C64 version which tried its best. Considering hardware limitations of home stuff, it was quite the thrill. Remains a fav today.

 

Pac-Man? Forget it. I suckedadickus at the Arcade. Various home versions were mixed. Just didn't take to the game. The clones (of Pac-Man) on Apple II were more exciting and varied. Microwave. Snack Attack. Others. Surprisingly I didn't have many (if any) knock-offs of it on the 400/800. The pirate scene for 8-bit was slim pickin's in my part of town. Nowhere near like Apple II which had a fire hose of games. And for better or worse I treated the 400/800 as a cartridge system.

 

I don't believe I spent more than 3-5 dollars on Pac-Man. Lesser on Defender. And, if memory serves, exactly fiddysents on Space Invaders. However it would take me spending hundreds or possibly thousands more on other games before I gave up the arcades for good and forever. Sometime in the mid-1990's give or take.

 

Retro can be a funny nonsensical thing at times. Logic would dictate that we gravitate toward the technically superior version of a game. But time and time again nostalgia and first experiences prove otherwise.

[carlsson]

6 hours ago, LouisB said:

I found this list of ROMS, anyone know what cart might fit the bill?

http://mhv.bplaced.net/cbmroms/cbmroms.php

Those are only the first party releases by Commodore themselves. It appears they didn't release a single 16K cartridge that autostarts.

 

I know that this cartlist is more than 20 years old and outdated, but it is a good start:

http://www.zimmers.net/anonftp/pub/cbm/vic20/Cartlist-2.10.txt

 

Most games listed as 12K or 16K should fit the bill, but then it depends what they look like on the inside, if they have two distinct ROMs (or ideally EPROMs as those tend to have slightly different pinout) or a single blob on the PCB. Of course you don't want to hack up any expensive game, in that case you might as well go for a multicart. For a possibly more up to date rarity guide, check Digital Press.

 

Another way is to get a switchable RAM expansion that allows you to map 16K RAM into those two blocks $6000 and $A000 and then load the files from e.g. disk or memory card device, just like you can load them in the emulator. Some ROMs have copy protection in terms of they try to overwrite themselves, but far from all have that property.

[LouisB]

16 hours ago, carlsson said:

Another way is to get a switchable RAM expansion that allows you to map 16K RAM into those two blocks $6000 and $A000 and then load the files from e.g. disk or memory card device, just like you can load them in the emulator. Some ROMs have copy protection in terms of they try to overwrite themselves, but far from all have that property.

@carlsson thanks for that, when you say "get a switchable RAM expansion..." is there a PCB or schematic publicly available that shows the memory mapping configuration and switches to enable eproms at the various memory access locations?  Sorry, technically I should research this more before asking and/or study the VIC20 Schematics to derive it myself but If it's been done already, well a schematic is worth a thousand thoughts

 

EDIT: Ok a quick google revealed this: https://github.com/dikdom/VIC-20-RAM-ROM-Expansion.  And appreciate any comments about other user experiences / preferences with VIC-20 RAM Expansion or Mapping Techniques

Edited August 23, 2022 by LouisB

[LouisB]

23 hours ago, Keatah said:

 

Won't argue there. I first played it on the Apple II right when it came out, so that's the version I will always prefer. The clicks and beeps bloops. Iconic in a grand way!

 

Same thing with Defender. I first played it on the Atari 400/800 and will always prefer that version over the arcades. Arcade Defender was simply too difficult to allow me any worthwhile game time. I always left the establishment with a sense of frustration. Too much to want to go back and keep trying. Space Invaders was similar in nature. I loved the 2600 version. Difficulty, colors, sounds, variety of options, yes. Space Invaders in VCS incarnation was superior all around. And I'm not the only one to say that.

 

Loved Gyruss on the C64, though of course, here, the arcade version trumps all. Gyruss was somewhat of a two-facer for me in that I liked both versions equally. The arcade with its multi-polyphonic sound. And the home C64 version which tried its best. Considering hardware limitations of home stuff, it was quite the thrill. Remains a fav today.

 

Pac-Man? Forget it. I suckedadickus at the Arcade. Various home versions were mixed. Just didn't take to the game. The clones (of Pac-Man) on Apple II were more exciting and varied. Microwave. Snack Attack. Others. Surprisingly I didn't have many (if any) knock-offs of it on the 400/800. The pirate scene for 8-bit was slim pickin's in my part of town. Nowhere near like Apple II which had a fire hose of games. And for better or worse I treated the 400/800 as a cartridge system.

 

I don't believe I spent more than 3-5 dollars on Pac-Man. Lesser on Defender. And, if memory serves, exactly fiddysents on Space Invaders. However it would take me spending hundreds or possibly thousands more on other games before I gave up the arcades for good and forever. Sometime in the mid-1990's give or take.

 

Retro can be a funny nonsensical thing at times. Logic would dictate that we gravitate toward the technically superior version of a game. But time and time again nostalgia and first experiences prove otherwise.

@Keatah I feel our gaming tastes are very similar because many of your comments resonate with me.  Especially those about Defender and Pac-Man.  Really glad you mentioned Snack Attack!  Definitely a pac-man type game, but faster and felt more alive.  The sort of game you can "get in the zone on" and start feeling it.  IIRC the sound effects were really good for an Apple II game.

 

Agree about the scene, in my neck of the woods there was also a plethora of Apple II software titles going around.

 

Nostalgia is a powerful thing (probably not for everyone) but there's something very comforting in reviving and re-living those old brain paths still laid down like wax in our grey matter.

[carlsson]

If you would happen to come across a 16K VIC-1111 expansion, it already has pads on the PCB to install switches or hardwire which blocks it should map. Default is blocks 1+2 but it supports blocks 3 and 5 too. Or buy a ready-made modern expansion, they're quite affordable unless you see added value in DIY.

[LouisB]

37 minutes ago, carlsson said:

If you would happen to come across a 16K VIC-1111 expansion, it already has pads on the PCB to install switches or hardwire which blocks it should map. Default is blocks 1+2 but it supports blocks 3 and 5 too. Or buy a ready-made modern expansion, they're quite affordable unless you see added value in DIY.

Thanks, I'll keep an eye out.  I did some searching but I don't see any ready-made boards in stock.  Would need shipping to US.

Edited August 23, 2022 by LouisB

[carlsson]

There used to be several inexpensive ones on the market just some 2-3 years ago, but I agree that at the moment it looks kind of dry/expensive.

 

This one has 35K RAM and apparently an EPROM socket to have up to four 8K ROMs available. It doesn't help you with Sky Blazer since it is 16K, but the memory expansion would let you soft load the ROM files.

 

This one is cheaper and looks more DIY but I don't know if it has all the advertised functionality. There is even an entire eBay store based on it.

Edited August 23, 2022 by carlsson

[oracle_jedi]

 

A simpler solution might be to just buy some ready made 8K/16K PCBs from "thefuturewas8bit".

 

https://www.thefuturewas8bit.com/vic-8k-16k-pcb.html

 

These boards will fit inside the standard Commodore case without modification, and allow standard 27cxx EPROMS to be used.  The jumpers allow the first and second 8K blocks to be positioned within the VICs memory map.

 

As I recall, some Commodore cartridge PCBs requires special 2364 ROM chips and could not support a "standard" 27cXX chip.  I don't know if this was some early anti-piracy thing or if Jack just had a pile of weird chips on the shelf he needed a use for.

 

If you use one of the later VIC cartridges with the smooth faceplate, you can also experiment with making your own labels for a really professional looking finish.

 

 

[Keatah]

Man I think I got a liking to those big VIC-20 carts. Substantial. Not unlike wood-paneled station wagons of the 70's.

[LouisB]

1 hour ago, oracle_jedi said:

 

A simpler solution might be to just buy some ready made 8K/16K PCBs from "thefuturewas8bit".

 

https://www.thefuturewas8bit.com/vic-8k-16k-pcb.html

@oracle_jedi That board is perfect!  But the shipping costs 3x what the PCB does   I'm going to spend some more time researching "open source" boards that might fit the bill because I can have 5 of those made & shipped (without components of course) by JLCPCB for less than $6 USD

 

p.s. thanks for the tip on making custom labels, indeed very professional looking!

Edited August 23, 2022 by LouisB

[oracle_jedi]

You could always make your own:

 

http://sleepingelephant.com/~sleeping/ipw-web/bulletin/bb/viewtopic.php?f=11&t=8724

 

 

[LouisB]

7 minutes ago, oracle_jedi said:

You could always make your own:

http://sleepingelephant.com/~sleeping/ipw-web/bulletin/bb/viewtopic.php?f=11&t=8724

Agree, thank you for the link!!

[carlsson]

2 hours ago, oracle_jedi said:

As I recall, some Commodore cartridge PCBs requires special 2364 ROM chips and could not support a "standard" 27cXX chip.

Um.. aren't pretty much all 8K ROM chips of the 2364 variety, which has fewer pins and a different pinout than the 2764 EPROM chips? I wouldn't call it an anti-piracy measure, rather that is how mask programmed ROM chips were laid out. The same can be said about the 4k ROM chips 2332, which are different than the 2732. In that case however TI made a custom 2532 EPROM that is at least read compatible with the 2332 ROM. There are such read compatible 8K EPROMs too, but those are rather uncommon to find and to locate a programmer that takes it.

 

Of course many cartridge manufacturers making smaller batches, used EPROM instead of getting mask programmed ROMs so in that case an existing cartridge would work. I wasn't aware of TFW8B's empty boards but it sounds like a good deal. I wonder if @brain has something similar in the Go4 Retro store?

[LouisB]

13 hours ago, oracle_jedi said:

You could always make your own:

http://sleepingelephant.com/~sleeping/ipw-web/bulletin/bb/viewtopic.php?f=11&t=8724

p.s. I imported the panelized version (Vic16K-G_P.brd) of the VIC-20 16K ROM Cartridge into EasyEDA and ordered a batch of 5 PCB's to play with

 

Thanks All for helping a VIC-20 Newb (me) with this!!