This demo is fascinating to watch a C64 using their online software library in action:

C64 on Internet using the Comet64 Modem

I also purchased one of these 1541 Ultimate 2 drives, although I might not even see it this calendar year as these units are severely back-ordered:

The 1541 Ultimate 2 cartridge for the Commodore 64

Cool.

Despite having some fantastic clones of classic coin-operated games, the VIC 20 has a remarkably small list of straight arcade conversions to its name, so a version of something loud and shooty like Berzerk was almost guaranteed to be popular here at Retro Gamer.  And since the developer is Robert Hurst, the man behind the excellent unexpanded Pac-Man clone Quikman 2008, we were pretty eager to get our hands on Berzerk MMX.

The VIC graphics are, unsurprisingly, chunkier than the original version’s but serve their purpose well, in-game sound effects are simple but again appropriate, and the action rapidly becomes frantic as the poor player-controlled humanoid fights for his life.  And while Berzerk MMX runs on an 8K expanded VIC 20, for the users blessed with 16K expansion there’s a second version, which ships with most of the speech from the original in place and goes even further towards replicating the arcade experience.

87%

I could not be more pleased with that — cool!

Fear not, it’s Boray to the rescue!  He provides a nice link to a Windows utility, PRG Starter, that will deter those PRG loading blues — particularly for PET, VIC 20, and C64 BASIC programs — although there is quite a bit of logic for the more esoteric program loads, too, and for the other VICE emulated machines.

After permission to examine his source code, I decided I could use a simply elegant utility such as this for my Fedora desktop — after all, like Windows, GNOME offers the same customized launching feature via an associated filetype extension.  You can find the C source archive to the simple launcher dubbed run.

After associating filetypes, such as .prg, p00, .a0, .crt, .d64, .g64, and .rom, to launch a customized command, run, you simply double-click the Commodore 8-bit binary file and it will run the correct VICE emulator with any associated command-line options, such as memory expansion, attach as a cartridge, or boot from a floppy or tape.

You can even do this from the command-line.  Here are some examples (with the verbose option on):

$ run -v quikman2k8.prg
**** Commodore VIC 20 ML program ****
xvic -memory none “quikman2k8.prg”

$ run -v berzerk-mmx+.prg
**** Commodore VIC 20 ML program with 16kb memory expansion ****
xvic -memory all “berzerk-mmx+.prg”

$ run -v Super\ Zaxxon.crt
**** Commodore 64 game cartridge ****
x64 -cartcrt “Super Zaxxon.crt”

And of course my favorite:

$ run -v mega-cart.rom
**** Commodore VIC 20 with Mega-Cart ****
xvic -cartmega “mega-cart.rom”

And I got all of it to fit with only the minimal 8kb memory expansion required for those software sprites — with BASIC reporting only 1-byte free after LOAD.  Funny how that always makes me feel good.

Berzerk MMX on VIC= 20

I thought I was really clever at the time writing my random dungeon level generator — it was my first practical use outside of a classroom to implement a recursive function.  Because the dungeon ‘walls’ were probability driven, a function was needed to validate that every ’tile’ in the arbitrarily-sized dungeon floor was accessible by the hero.  Without this kind of validation, the hero would eventually find himself ‘trapped’ on a floor with no means to escape.

The recursive function was simply invoked as chkroom(0,0), whereas the 0,0 passed were parameters to the starting point in the dungeon map matrix.  Upon entering each tile, a flag is cleared for that element in the matrix.  Next, a condition is checked if the tile to its right is accessible.  If it is, call chkroom() with +1 added to X.  If not, do the same condition checks for down, left, and up directions calling chkroom() passing values Y+1, X-1, and Y-1 respectively.

And at the conclusion of all this recursion, do a loop through the dungeon matrix to check if ALL of the flags had been cleared.  If any persist, do the dungeon regeneration and this validation again until it has passed.  The higher the frequency of random walls generated increases the pattern complexity of the floor, which exponentially increases the probability that the floor will not pass with an ‘all clear’.

Today, I am working on a port of an old arcade favorite, Berzerk, for the Commodore VIC 20.  The playfield for the game is a fixed 3-row by 5-column matrix for each floor.  Given that the game needs to be written using 8-bit assembler, I quickly dismissed the idea that using my C algorithm to validate a floor could be feasible.  After all, VIC 20 is a relic machine running at 1mHz and limited memory of only a few thousand bytes available for the program, graphics, and sound all the while using a 256-byte system stack.  Ah, but the challenge by it all . . .

Generating the random walls for each floor is a simple matter, but validating that all its ‘rooms’ are accessible was turning out to be very problematic.  Every time I thought I could solve this puzzle using bit operators (OR, AND, XOR) to assure that its rooms and corridors could not trap the hero, I would only later stumble upon an instance where even my most elaborate of schemes failed.  This was getting a bit frustrating, pun intended.

So I wandered back to my dungeon floor validation and decided it was so simple of an algorithm that it may be possible to translate it to assembler and still keep it within reason of the machine’s constraints.  After some writing and whittling, the resulting code turned out perfect!  It is called by simply zero-ing the starting cell (R0) and invoking CRAWL.  Here is its listing:

CRAWL:
  LDX R0
  LDA MAZE,X
  CMP #$80
  BCC @fini        ; been here?
  AND #$7F         ; clear accessible bit
  STA MAZE,X

@right:
  LSR
  BCS @down        ; right wall?
  PHA              ;++ push walls
  LDA R0
  PHA              ;++ push cell
  INC R0
  JSR CRAWL        ; explore right cell
  PLA              ;-- pop cell
  STA R0
  PLA              ;-- pop walls

@down:
  LSR
  BCS @left        ; bottom wall?
  PHA              ;++ push walls
  LDA R0
  PHA              ;++ push cell
  CLC
  ADC #5
  STA R0
  JSR CRAWL        ; explore cell below
  PLA              ;-- pop cell
  STA R0
  PLA              ;-- pop walls

@left:
  LSR
  BCS @up
  PHA              ;++ push walls
  LDA R0
  PHA              ;++ push cell
  DEC R0
  JSR CRAWL        ; explore left cell
  PLA              ;-- pop cell
  STA R0
  PLA              ;-- pop walls

@up:
  LSR
  BCS @fini        ; top wall?
  PHA              ;++ push walls
  LDA R0
  PHA              ;++ push cell
  SEC
  SBC #5
  STA R0
  JSR CRAWL        ; explore cell above
  PLA              ;-- pop cell
  STA R0
  PLA              ;-- pop walls
@fini:
  RTS

This old dog is always happy to re-implement an old trick!

OpenCBM provides an API library and some useful command-line tools.  The following is an example of how to format and copy Commodore files to the floppy drive:

$ sudo modprobe cbm

$ dmesg | tail
cbm_init: using passive (XM1541) cable (auto), irq 7
cbm: resetting devices
cbm: waiting for free bus...

$ cbmctrl detect
 8: 1540 or 1541 

$ cbmctrl command 8 "N0:VIC20 3-JAN-2010,10"
$ cbmctrl status 8
00, ok,00,00
$ cbmctrl dir 8
0 ."vic20 3-jan-2010" 10 2a
664 blocks free.
00, ok,00,00

$ cbmwrite 8 quikman+8k.prg
[Info] writing quikman+8k.prg -> QUIKMAN+8K.PRG,P,W
[Info] identified a 1540 or 1541 drive
............-
[Info] 00, OK,00,00
$ cbmwrite 8 omega-fury.prg 
[Info] writing omega-fury.prg -> OMEGA-FURY.PRG,P,W
[Info] identified a 1540 or 1541 drive
....................\
[Info] 00, OK,00,00

$ cbmctrl dir 8
0 ."vic20 3-jan-2010" i1 2a
47   "quikman+8k.prg"   prg
76   "omega-fury.prg"   prg
541 blocks free.
00, ok,00,00

Of course, I can now hook-up this drive to a real VIC 20 computer and play what’s on the diskette(s).  But for giggles, I can even use the Commodore 8-bit emulator, VICE, to test drive my floppies, for example:

$ xvic -memory 8k -device8 2 +truedrive \
-keybuf 'LOAD "QUIKMAN+8K.PRG",8\0D'
...
sucessfully loaded libopencbm.so
/dev/cbm opened.
...

VICE 2.2 on a 24" HP display

Nice!

Nevertheless, I am perfectly fine with a backup dump of my Mega-Cart, to be played using the new release of VICE 2.2 which supports the 2mb image, 8kb NvRAM, and all the appropriate bank-switching.  Cool!!

VICE Mega-Cart

xvic -cartmega mega-cart.rom -mcnvramwriteback -mcnvramfile mega-cart.nv