See also: part 1, part 2, part 3, part 4 or part 5 (with more coming).

Hey hey! It’s another VIC-20 Tuesday!

I have completed my code walk-through of one of my earliest computer programs, a Donkey Kong-inspired VIC-20 game called Sky-Ape-Er. But, the version I presented was not the only version of the game I created. I have dozens of saved copies of this game in various stages of completion, but one in particular stood out. It used completely different graphics.

Sky-Ape-Er: The Prototype

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As a reminder, here is what the graphics were later changed to in the release version:

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The ape was completely different, and the platform graphics were meant to resemble the ones used in the arcade game Donkey Kong. But why were there pinwheels? I have so many questions for my junior high self.

When I first uncovered these tapes I was unaware of how many variations of my programs were on them. When I started this article on my Sky-Ape-Er game, I discovered that the earlier version with different graphics was also using some different code — most notably in how it read the keyboard input.

I thought it might be fun to look at the programming choices I originally made, and speculate on why I changed them.

Sky-Ape-Er: The Mystery

The first thing I want to discuss is a mystery I am currently trying to solve. My VIC-20 games that used custom character sets seem to come in three forms:

DATA STATEMENTS

The BASIC program reads the character set from DATA statements and POKEs it into memory. This is what the Sky-Ape-Er INSTRUCTIONS program does. I believe these character sets may have been designed by the Eight by Eight Create program I previously mentioned. If true, I don’t envy my junior high self having to manually copy down the numbers to paper and type them in to my own program later. From the documentation:

“Once you have created, designed and examined enough characters, you can copy their associated numbers on paper to be used in any programs you make.”

Eight by Eight Create instruction from January 1983 Creative Computing magazine (Volume 9, Number 1), page 270.

Yipes.

LOADABLE CHARACTER SET

A standalone binary “program” of the custom character set that can be loaded into memory, presumably at the address of where the character data goes. I have found several programs called things like CHARS and TNTCH that do not have any BASIC code in them. I suspected these were character set data (especially TNTCH which was on the tape after the main program TNT) but I had no idea how to use them. I was finally able to see what was inside by importing them into the CBM prg Studio‘s characters set editor. CBM prg Studio is a Windows integrated development environment (IDE) for making Commodore programs in BASIC or assembly. It has some great features and is worth checking out.

This let me see that these were indeed character sets, though my first attempt to import them had all the graphics off by a few lines. I needed to use an offset (bytes to skip in the file) of 2 for the characters to load properly. That told me that whatever type of file this was had some 2 byte header at the start (perhaps memory location where to load the data?).

ALL-IN-ONE PROGRAM WITH CHARACTER SET

And this is the mystery! My early prototype version of Sky-Ape-Er was just one program, and it loaded up with the custom character set. There was no font in DATA statements. There was no pre-loader that did it. It just loaded and “just worked.” I have no idea how I created this, nor do I know why, for the release version, I change it to use two programs and DATA statements.

But I have theories.

Dissecting the data

Thanks to suggestions from a VIC-20 group on Facebook and the Denial Commodore forum, I looked at the contents of the mystery SKY-APE-ER program file.

Using a free hex editor, I opened the file and looked for the end of the BASIC program. I could tell it ended around byte 2612 because the last line was a “SYS xxxxx” command that would reboot the VIC-20. The xxxxx numeric value was visible as plain text in the tokenized BASIC file, so it was easy to spot.

After this was a bunch more data. Somewhere in there must be the character set. But where? I decided to try opening the entire program file in the character set editor and using the 2612 offset where the BASIC program ended.

Doing this showed garbage between the BASIC program and character data, but scrolling down let me visibly see where the font data began.

I now knew that approximately 58 characters (each character is 8×8, so 8 bytes per) into the file was the start of the font data. A little math (which was hard) and some trial and error (which was easy) and I came up with 3073 as the offset to use from the stat of the .prg to where my custom characters were. I imported using that value and got this:

Tada!

If I knew what the font data was to begin wish (from DATA statements), I could have just scanned the HEX file looking for those values. But I didn’t, so I couldn’t.

Now I have a BASIC file for the game, as well as a character set file in the CBM prg Studio editor. But how did I combine them together in the first place?

Where does the data go from here?

The clue is in these POKEs found on the first line of the program:

POKE45,56:POKE46,26:POKE51,0:POKE52,28:POKE55,0:POKE56,28:CLR

They reminded me of similar POKEs in Color BASIC that track where the program starts in memory as well as where variables and strings go. I expected CBM BASIC would be similar, so I went searching for a VIC-20 memory map.

I found this one archived on Bo Zimmerman’s site. He’s the guy behind the incredible Zimodem firmware that lets you wire up a WiFi serial modem for under $10.

http://www.zimmers.net/cbmpics/cbm/vic/memorymap.txt

I want to do a deep dive into this later, but for now, here are what those POKEs are doing:

*002D-002E 45-46 Pointer: Start of Variables
*0033-0034 51-52 Pointer: String storage (moving down)
*0037-0038 55-56 Pointer: Limit of memory

The “*” notes “Useful memory locations” in the memory map. I agree. I seem to be changing where variables and strings start, as well as where the end of memory is on startup.

Why was I changing the start of variables, the end of string storage, and limiting the end of BASIC? I have a theory, which parallels something I’ve done on the CoCo.

In Color BASIC, we use the CLEAR command to allocate more string space (“CLEAR 500” for 500 bytes for strings). It looks like CBM BASIC doesn’t do that, and allows strings to use as much memory as is available (the memory between the end of the BASIC program + variable arrays, and the limit of memory).

CLEAR can also limit how much memory BASIC can use (“CLEAR 200,&H3F00”). That’s useful when you are wanting to use some of that memory for machine language and don’t want BASIC to overwrite it. I am betting POKE 51/52 is like CLEAR x,XXXX.

VIC-20 Memory Map

To better visualize this, let’s take a quick look at where the 5K of RAM in the VIC-20 is located.

   0 -> +------------------------------+
....    | 1K of System Memory          |
1024 -> +------------------------------+ <- 1023
        | 3K Expansion RAM (cartridge) |
4096 -> +------------------------------+ <- 4095
        | User BASIC Area (3583 bytes) |
7680 -> +------------------------------+ <- 7679
        | Screen Memory (512 bytes)    |
        +------------------------------+ <- 8191

Hey, look at that! The memory range used by BASIC (4096-7679) is the “3583 BYTES FREE” value shown on the startup screen:

Notice the 3K gap (1024-4095) which is where the 3K RAM expansion cartridge goes if you have one. I never did, though I did have the Super Expander cartridge which gave extra memory as well as enhanced graphics and sound commands.

Side Note: When memory expansion cartridges are plugged in, more memory becomes available and some things shift around. But for this discussion, we will talk only about the stock 5K VIC-20. It was only in recent years that I learned the VIC-20 was a 5K computer. I’d always thought it was 4K. That now makes the weird 3K memory expansion make more sense, since that would boost it to a nice even 8K. But I digress…

Now let’s zoom in on just the memory BASIC is using:

43/44 -> +---------------+
         | BASIC program |
45/46 -> +---------------+
         | Variables     |
47/48 -> +---------------+  
         | Arrays        |
         +---------------+ <- 49/50 End of Arrays
         |               |
         |               |
51/52 -> |---------------+  
         | Strings       |
         +---------------+ <- 55/56 Mem Limit (7679)

When a new numeric variable is added, it goes into the Variables section, which grows larger downward. When a new array is added, it goes into the Arrays area (and likely the entries there point to the Variable) and it grows larger downward. When a string is added, it gets an entry in the Variable section (“A$”) which has a pointer into the actual string content in the Strings section, which grows upwards.

So why was I changing the start of variables, the end of string storage, and limiting the end of BASIC? I believe I was making BASIC think the program was larger than it really was so it would SAVE out (and thus LOAD back later) the program PLUS some custom character data. When the program would run, it would need to reset the pointers to be at the actual end of the BASIC program, and limit memory so BASIC did not write over the character data.

In order to explain this, we need to look at how the VIC-20 custom characters worked.

How the VIC-20 custom characters worked

The VIC-20 character set was 4K of data stored in ROM starting at 0x8000:

8000-83FF 32768-33791 Upper case and graphics
8400-87FF 33792-33815 Reversed upper case and graphics
8800-8BFF 33816-35839 Upper and lower case
8C00-8FFF 35840-36863 Reversed upper and lower case

Each character was 8 pixels wide (one byte) by 8 pixels tall (8 bytes total). There is room for 512 characters in that 4K. Normal printable ASCII characters are 0-127, so it looks like the Commodore PETASCII was similar, with 128 special Commodore characters per bank (128 characters * 8 bytes per character = 1024 bytes).

On power up, the VIC’s video chip is programmed to use the first of those four 1K blocks of ROM for its character set. There is a register with four bits that can be changed to select which of those four ROM blocks it uses, or point it to four 1K RAM blocks in RAM. By loading a character set in to one of those RAM areas and setting the register, the VIC will now display the custom character set rather than the one built in to the ROM. Here are the important four bits:

9005 36869 bits 0-3 start of character memory (default = 0)
                     bits 4-7 is rest of video address (default= F)
                     BITS 3,2,1,0 CM starting address
                                  HEX   DEC
                     0000   ROM   8000  32768
                     0001         8400  33792
                     0010         8800  34816
                     0011         8C00  35840
                     1000   RAM   0000  0000
                     1100         1000  4096
                     1101         1400  5120
                     1110         1800  6144
                     1111         1C00  7168

Memory location 36869 can be one of these 8 values:

• 0xF0 / 240 / 11110000 – Use 8000-83FF (Upper case and graphics)
• 0xF1 / 241 / 11110001 – Use 8400-87FF (Reversed upper case and graphics)
• 0xF2 / 242 / 11110010 – Use 8800-8BFF (Upper and lower case)
• 0xF3 / 243 / 11110011 – Use 8C00-8FFF (Reversed upper and lower case)
• 0xfc / 252 / 11111100 – Use 1000-13FF RAM area #1
• 0xfd / 253 / 11111101 – Use 1400-17FF RAM area #2
• 0xfe / 254 / 11111110 – Use 1800-1BFF RAM area #3
• 0xff / 255 / 11111111 – Use 1C00-1FFFF RAM area #4

The four RAM locations all are within the 4K that is used by BASIC and screen memory:

• 0x1000 – 0x1dff – 3583 bytes used by BASIC programs.
• 0x1e00 – 0x1fff – 512 bytes used by screen memory.

We can’t use RAM area #1 for characters because that is where our BASIC program is. If we kept our BASIC program and all its variables very small (1K, 0x1000-0x13FF), we could use area #2. But, it makes more sense to use area #4 and give as much memory as possible to BASIC.

In my programs I see POKE 36869,255 and POKE 36869,240. The first POKE makes the video chip start using characters in RAM starting at 0x1c00 (bit pattern 1111). This means a BASIC program and all its variables can’t be any larger than 3072 bytes (0x1000-0x1bff). The second poke switches the characters back to using the standard ROM location for uppercase and graphics characters (bit pattern 0000).

My Sky-Ape-Er INSTRUCTIONS program would READ character data and then POKE it into memory starting at 0x1c00 (7168). It then did POKE 36869,255 to start using them. To display a normal text screen, or at the end of the program, it would POKE 36869,240 to get back to the ROM character set. (This is the part I actually mostly remembered.)

I do want to point out that the character RAM area #4 overlaps with the screen memory:

• 0x1c00 – 0x1fff – Character RAM area #4.
• 0x1e00 – 0x1fff – 512 bytes used by screen memory.

This tells me that you really only have 0x1c00 to 0x1dff (7168-7679) for custom characters. That’s 512 bytes, and at 8 bytes per letter, there is only room for 64 custom characters. Assembly language programs that did not need BASIC could move things around and use one of the other blocks in its entirety, but this last block shares memory with the screen so not all of it can be used for character data.

Character sets

The PETSCII character set starts at zero with an “@” symbol, followed by the alphabet characters “A-Z” (1-26), then various punctuation and symbols (27-47), then numbers (48-57), then more punctuation and symbols (59-63). This covers the basic characters and uppercase alphabet just like standard ASCII does for characters 32-96. This means being limited to just 64 characters is not bad at all.

And, as I was working through this, I discovered why my Factory TNT game is not displaying the score correctly! I did not realize it also contained updated number characters (48-57) that I was not properly loading during my “restoration” of the game:

I’m so glad I am writing this article and figured that out. It was driving me mad!

But I digress…

Stay on target… Stay on target…

This means that you could have a BASIC program from 0x1000 to 0x1bff followed by custom character data from 0x1c00 to 0x1dff. If you had the characters in memory for your BASIC program to use, and you SAVEd your BASIC program, those custom characters would NOT be saved with it since BASIC doesn’t know anything about them.

But … you could lie to BASIC and tell it your BASIC program actually ENDS at 0x1dff, then when you SAVE it should write out the entire range of memory (0x1c00 to 0x1dff) thinking it’s just one large BASIC program…

Then, when you loaded it back it, BASIC would start loading it into memory at 0x1c00 and keep going until it got to the end of the program. You now have loaded memory that is part BASIC, and part character set!

But, if you tried to RUN it, you wouldn’t get very far because BASIC would think there is no memory left for variables. You would need to un-lie to BASIC and tell it where the program really ends, and do so without using varaibles.

That is what my six POKEs were apparently doing.

In part 5, we’ll see if this theory works.

Until next time…

See also: part 1, part 2, part 3, part 4 or part 5 (with more coming).

Its time for another VIC-20 Tuesday!

In this installment, I will walk through the code for my VIC-20 game Sky-Ape-Er. Thanks to a wonderful utility I mentioned in part one, I have this ASCII program listing. The utility replaces the special PETASCII control characters with {words} so they can be viewed on non-Commodore systems.

Sky-Ape-Er program listing

Initialization

1 POKE 808,100

• That poke would “disable the RUN/STOP key, the RESTORE key, and the LIST command” to keep someone from being able to save a copy of it.

5 REM  ************          *SKY-APE-ER*        *******BY*******
6 REM*ALLEN  HUFFMAN*      ****************{$cc}

• Comments, but with no dates. Thanks, me. One of my tapes had the game in various stages of completion, named “SKY-APE-ER 1”, “SKY-APE-ER 2” and “SKY-APE-ER 3”, but all my other tapes just called it SKY-APE-ER, and I’ve found at least two distinct versions of the game so far.
• I am not sure what the {$cc} is at the end. From looking at the c64list.exe command I used to make this listing, it seems to be an unknown token. This might have been something I placed in that line to prevent listing, even after a LOAD without the POKE being executed. But, you can LIST 10- and still see it ;-)

10 S=1:POKE36879,26:POKE36878,15:S1=36875:POKE775,200:POKE36869,255

• S is the screen (level) to display. Sky-Ape-Er contained three screens, and a “game won” screen.
• 36879 is the border color, with 26 being red.
• 36878 is something to do with the music chip.
• S1 is being set to the sound note location 36875.
• 775,200 disables LIST. Or maybe not. There was conflicting information in the magazine I looked that up in.
• 36869 enables the custom font characters.

Set up game screen

15 PRINT"{clear}":L=8118:B=7772:M=15

• L is the starting memory location for the player.
• B is the starting memory location of the chimp (B for barrel, I suppose, as this was inspired by Donkey Kong).
• M is the character to POKE to the screen for the player (man). 15 is the letter”O”.

20 FORA=38400TO38905:POKEA,0:NEXTA

• This clears the color for each character on the screen. The characters for the screen are stored in one block of memory, and the color of each character is stored in a different block of memory.

Display ape

25 PRINT"{home}{down}{right}{black}ABC{down}{left:3}DEF{down}{left:3}GHI{down}{left:3}JKL"

• This clears the screen, then draws the letters that make up the ape character. The VIC-20 has no PRINT@ or LOCATE function, but you could embed cursor movement commands in the PRINT statements. Thus, it would “home” to the top fo the screen, move the cursor {down} one, then {right} one, set the color to {black}, print ABC, then go {down} one and {left} three times (cursor is now back under the A) and print DEF, and repeat for the other lines. The ape was 3×4 characters in size!

Display game screen

30 ONSGOTO35,45,55,500

• This goes to the appropriate routine to display the current screen (level) based on the value of S. There were three screens. If S is 4, it goes to a “win” screen.

Screen 1

35 PRINT"{red}@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@{down}{left}";
40 PRINT"@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@{up}{left}@{up}{left}@{up}{left}@":GOTO70

That would draw this:

Screen 2

45 PRINT"{blue}@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}";
50 PRINT"@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@@{up}{left}@{up}{left}@{up}{left}@":GOTO70

That would draw this:

Screen 3

55 PRINT"{purple}@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{up:3}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}";
60 PRINT"@@@{up:4}{left}@@@{down}{left}@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@{up}{left}@{up}{left}@{up}{left}@{up}{left}@{up}{left}@"

That would draw this:

Begin game

70 T=0

• T is used for time. It starts at zero and increments, so the goal is to get to the top as fast as possible.

Main game loop

Display player and check to see if we made it to the top, or were hit by a chimp.

80 POKEL,M:POKEB,32:POKES1,0
83 IFL=7772ORL=7749THEN400
84 IFL=BTHEN250

• Line 80 pokes the Man on the screen at its Location. The Barrel is poked to a space, so erased. The S1 sound is stopped. Nothing would be playing the first time this code is ran.
• If the Location is 7772 or 7749, we go to 400.

Move the chimp

85 B=B+23:IFPEEK(B)=0THENB=B-22:POKEB,16

• This controls the movement of the enemy (chimp). B is the location. The VIC-20 screen is 22 characters per line, so incrementing B by 23 would move the position down and to the right.
• The new location of B is checked to see if it is 0 (the brick). If it is, it moves the location back 22 (up one). Thus, the chimp always tries to go down and to the right, and if it can’t it just goes right.
• The POKE is what puts the chimp character on the screen at that position.
• From looking at this, it seems the chimp is not displayed if it goes down and to the right successfully. I wonder why? Bug? It would get displayed the next time. Or inteitonal? I’ll look into this.

Handle player falling

87 IFPEEK(L+22)=32THENPOKEL,32:L=L+22:POKEL,M

• This looks at the block under the player. If it is 32 (empty), it erases the player and moves it down, then displays it at the new location.
• This code is what makes the player “fall” when it jumps or, on screen 3, when it falls off the edge. Cool.

Make “tick” sound

90 POKES1,240:POKES1,0

• S1 is the address of one of the sound channels (the VIC-20 has three musical channels, plus noise). This turns one on and off, making the “tick” sound.
• I did not even consider being able to do background music for a game back then. I bet I could today, memory permitting, even in BASIC.

Collision detection (player and chimp)

95 IFB=LTHEN250

• This is the collision detection. If the chimps location (B) is the same as the players Location, we go to 250 (death).

Reset chimp back to top

100 IFB=8118THENPOKEB,32:B=7772:POKEB,12

• This checks to see if the chimp is at the bottom right of the screen.
• If it is, it gets erased, and the position is reset back to the top left starting position and it is displayed there.

Update and check timer

105 PRINT"{home}{blue}{rvrs on}{right}TIME:";T:T=T+1
110 IFT>200THEN300

• This homes the cursor, changes the color to blue, turns on reverse video, then displays “TIME:” and the current time value. The reason it goes reverse is because we are using a special character set that replaces the lower alphabet characters. In this mode, there is some memory thing that happens that wraps around the inverse video characters to the standard characters. When using special characters, you have access to the basic letters by using reverse. Apparently.

Keyboard input

115 K=PEEK(197):IFK=64THEN80

• Memory location 197 is the current key being pressed. If no key is pressed, 64 is returned and the program goes back to the top and continues. (Move the chimp, update the time, etc.)

120 POKEL,32:POKES1,230

• If there was input, then this line will erase the player, and turn on the sound chip to start making a tone.

Move player or jump

125 IFK=17THENL=L-1:M=14
130 IFK=41THENL=L+1:M=13
135 IFK=39THEN180
140 IFPEEK(L)=0THEN150
145 GOTO80

• K is the value of the key being pressed.
• 17 is “A”, so if that is pressed, we move the player’s location to the left and change the player’s character to the “facing left” letter.
• 41 is “S”, so if that is pressed, we move the player’s location to the right and change the player’s character to the “facing right” letter.
• 39 is the F1 function key. If that is seen, we go to the jump routine at line 180.
• If the player’s position (which may or may not have been updated) is 0, it is in a wall, so go to line 150 to fix that location.
• If still here, we go back to line 80 and continue with the main loop.

Handle running into wall collision

150 IFK=17THENL=L+1
155 IFK=41THENL=L-1
160 M=15:GOTO80

• This is kind of silly, but if we are here, we hit a wall, so we look at what the last key was pressed and reverse the direction that it did.
• We set the player’s character to the “facing forward” character.

Jump routine

180 POKES1,235:IFM=14THENL=L-23
183 IFM=15THENL=L-22
185 IFM=13THENL=L-21
190 IFPEEK(L)=0THEN200
195 M=15:GOTO80

• First we start to make a tone.
• Then we check to see if the player’s character is 14 (facing left). If it is, we move the player’s position up and to the left (22 characters per row, so -23).
• If the player’s character is 15 (facing forward), we move the location up,
• If the player’s character is 13 (facing right), we move the location up and to the right.
• If the new location is in a wall, we go to 200 to fix that.
• We set the player’s character to 15 (facing forward) so the jump will land facing that way.
• We go back to 80 to continue the main game loop.

Handle jumping into wall collision

200 IFM=13THENL=L+21
210 M=15:GOTO80

• This code takes care of jumping into walls.
• If the character is facing right, then we move the position down and to the left back where it came from.
• Else, we know the only other place it can hit a wall is to the right (wow, I was clever here!) so we just make the character face forward and go back to the main loop.

Death screen

250 FORA=1TO11:READN,L:POKES1,N:FORP=1TOL:NEXTP:POKES1,0:NEXTA
255 PRINT"{clear}{black}{rvrs on}{down} GUESS WHAT! YOU HAVE {down}JUST BEEN STRUCK DOWN {down}  BY A LUCKY CHIMP!"
260 PRINT"{rvrs on}{down}({blue}THAT MEANS {red}GAME OVER{black})"
265 GOTO450

• The first line reads musical note and duration values from DATA statements, then makes each note sound for that amount of time.
• The screen is then cleared, and a game over message is displays.
• It then goes to 450 to see if the player wants to play again.

270 DATA202,250,202,250,202,150,202,350,210,250,208,150,208,200,202,200,202,250,200,200
275 DATA 202,400

• Those DATA statements contain the note value and length of the tune that plays when you die (funeral march). I did not know anything about music at the time, so I figured it out by ear. The timing is awful.

Time exceeded

285 PRINT"{down:2}{rvrs on}PRESS ANY KEY TO START"
300 PRINT"{clear}{black}{rvrs on}{down:3}WELL WHAT DO YOU KNOW?"
305 PRINT"{rvrs on}{down:2}WHILE YOU WERE DOWN ONTHE GROUND JUMPING A- ROUND,YOUR TIME CLOCK"
310 PRINT"{rvrs on}    JUST RAN OUT!"
315 PRINT"{rvrs on}{down:2}  NEXT TIME {red}WATCH{black} THE CLOCK!"
320 GOTO450

• This message is displayed when time runs out. It then goes to the “play again?” screen.

Screen completed

400 S=S+1:GOTO15

• This increments the screen, and then goes back to display it.

Play again prompt

450 PRINT"{black}{rvrs on}{down:2}{right}WILL YOU TRY AGAIN?"
455 GETA$:IFA$=""THEN455
460 IFA$="N"THEN550
465 IFA$="Y"THEN480
470 GOTO455

• This prompts the user to ask if they want to play again. It waits for a key press, then if it is “Y”es, it goes to 480 (start a new game). If it is “N”o, it goes to 550 (end the game).

Press any key to play again prompt

480 PRINT"{clear}{black}{rvrs on}{down:2}AT LAST! SOMEONE BRAVE ENOUGH TO {red}TRY{black} TO STOP  KING KING'S COUSIN!"
490 GETA$:IFA$=""THEN490
495 RUN

If the player chooses “Y”es to play again, this messages is displayed. It waits for any key to be pressed, then re-runs the program.

Game won – Falling ape animation

500 PRINT"{clear}{black}"
505 FORZ=0TO16:PRINT"{home}":FORR=0TOZ:PRINT:NEXTR
510 PRINT"   {down}{left:3}ABC{down}{left:3}DEF{down}{left:3}GHI{down}{left:3}JKL"
515 NEXTZ

• This cleans the screen, draws the ape, and then makes it “fall” to the bottom of the screen. Remember how Donkey Kong would fall at the end of the rivets level? Kinda like that. Just not as good. I had forgotten about this!

525 POKE36874,128
530 FORX=15TO0STEP-.1:POKE36878,X:NEXTX:POKE36874,0

• That plays a sound effect when the ape hits the bottom of the screen.

Game won message

535 PRINT"{clear}{black}{rvrs on}{down:3}CONGRATULATIONS!!!!!!!{down} YOU NOW KNOW WHAT IT  FEELS LIKE TO BE ON"
540 PRINT"{rvrs on}TOP!{$a0}YOU PLAYED WELL!"
545 PRINT"{rvrs on}{down:2}{blue}BET YOU DON'T WIN NEXT       TIME!"
548 PRINT:GOTO450

• This displays the “game won” message, then goes to 450 to ask the player if they wish to play again.

Quit game message

550 PRINT"{clear}{black}{rvrs on}I DIDN'T THINK YOU HAD ENOUGH COURAGE TO TRY   TO STOP A MAD APE!"
560 PRINT"{down}{rvrs on}{red}I HOPE YOU ENJOYED NOT SAVING THE BUILDING!"
565 FORG=1TO10000:NEXTG
570 SYS64802

• This would display a message, wait, and then reboot the VIC-20.

Programmer’s thoughts

I actually expected this code to be far worse than it actually is. If I were writing it today, I see a number of things I could do to make it more efficient. I also have many ideas of ways I could use the VIC-20 features to make the game fancier (animated characters, better music and sounds).

But, overall, I am fairly pleased with how my junior high self programmed this. I was completely self-taught from reading the VIC-20 manual and articles in magazines.

But since I know I could do it better today, I think I might just give that a shot.

Here is the full listing of this version of the program. If you want to run it, I would do two things:

• Delete line 1 entirely.
• In line 10, remove the POKE772,200.
• And I’d get rid of the same POKEs in the INSTRUCTIONS loader.

And if you don’t want to type it in, here is a virtual tape for the VICE Commodore emulator. It has the above changes made, but otherwise is exactly as I last touched it in 1983. Enjoy!

Until next time…

Full SKY-APE-ER listing:

1 POKE 808,100
5 REM  ************          *SKY-APE-ER*        *******BY*******
6 REM*ALLEN  HUFFMAN*      ****************L
10 S=1:POKE36879,26:POKE36878,15:S1=36875:POKE775,200:POKE36869,255
15 PRINT"{clear}":L=8118:B=7772:M=15
20 FORA=38400TO38905:POKEA,0:NEXTA
25 PRINT"{home}{down}{right}{black}ABC{down}{left:3}DEF{down}{left:3}GHI{down}{left:3}JKL"
30 ONSGOTO35,45,55,500
35 PRINT"{red}@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@{down}{left}";
40 PRINT"@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@{up}{left}@{up}{left}@{up}{left}@":GOTO70
45 PRINT"{blue}@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}";
50 PRINT"@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@@@@@@@@@@{up}{left}@{up}{left}@{up}{left}@":GOTO70
55 PRINT"{purple}@@@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{up:3}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}";
60 PRINT"@@@{up:4}{left}@@@{down}{left}@@@@@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@{down}{left}@@@@@@@@@{up}{left}@{up}{left}@{up}{left}@{up}{left}@{up}{left}@"
70 T=0
80 POKEL,M:POKEB,32:POKES1,0
83 IFL=7772ORL=7749THEN400
84 IFL=BTHEN250
85 B=B+23:IFPEEK(B)=0THENB=B-22:POKEB,16
87 IFPEEK(L+22)=32THENPOKEL,32:L=L+22:POKEL,M
90 POKES1,240:POKES1,0
95 IFB=LTHEN250
100 IFB=8118THENPOKEB,32:B=7772:POKEB,12
105 PRINT"{home}{blue}{rvrs on}{right}TIME:";T:T=T+1
110 IFT>200THEN300
115 K=PEEK(197):IFK=64THEN80
120 POKEL,32:POKES1,230
125 IFK=17THENL=L-1:M=14
130 IFK=41THENL=L+1:M=13
135 IFK=39THEN180
140 IFPEEK(L)=0THEN150
145 GOTO80
150 IFK=17THENL=L+1
155 IFK=41THENL=L-1
160 M=15:GOTO80
180 POKES1,235:IFM=14THENL=L-23
183 IFM=15THENL=L-22
185 IFM=13THENL=L-21
190 IFPEEK(L)=0THEN200
195 M=15:GOTO80
200 IFM=13THENL=L+21
210 M=15:GOTO80
250 FORA=1TO11:READN,L:POKES1,N:FORP=1TOL:NEXTP:POKES1,0:NEXTA
255 PRINT"{clear}{black}{rvrs on}{down} GUESS WHAT! YOU HAVE {down}JUST BEEN STRUCK DOWN {down}  BY A LUCKY CHIMP!"
260 PRINT"{rvrs on}{down}({blue}THAT MEANS {red}GAME OVER{black})"
265 GOTO450
270 DATA202,250,202,250,202,150,202,350,210,250,208,150,208,200,202,200,202,250,200,200
275 DATA202,400
285 PRINT"{down:2}{rvrs on}PRESS ANY KEY TO START"
300 PRINT"{clear}{black}{rvrs on}{down:3}WELL WHAT DO YOU KNOW?"
305 PRINT"{rvrs on}{down:2}WHILE YOU WERE DOWN ONTHE GROUND JUMPING A- ROUND,YOUR TIME CLOCK"
310 PRINT"{rvrs on}    JUST RAN OUT!"
315 PRINT"{rvrs on}{down:2}  NEXT TIME {red}WATCH{black} THE CLOCK!"
320 GOTO450
400 S=S+1:GOTO15
450 PRINT"{black}{rvrs on}{down:2}{right}WILL YOU TRY AGAIN?"
455 GETA$:IFA$=""THEN455
460 IFA$="N"THEN550
465 IFA$="Y"THEN480
470 GOTO455
480 PRINT"{clear}{black}{rvrs on}{down:2}AT LAST! SOMEONE BRAVE ENOUGH TO {red}TRY{black} TO STOP  KING KING'S COUSIN!"
490 GETA$:IFA$=""THEN490
495 RUN
500 PRINT"{clear}{black}"
505 FORZ=0TO16:PRINT"{home}":FORR=0TOZ:PRINT:NEXTR
510 PRINT"   {down}{left:3}ABC{down}{left:3}DEF{down}{left:3}GHI{down}{left:3}JKL"
515 NEXTZ
525 POKE36874,128
530 FORX=15TO0STEP-.1:POKE36878,X:NEXTX:POKE36874,0
535 PRINT"{clear}{black}{rvrs on}{down:3}CONGRATULATIONS!!!!!!!{down} YOU NOW KNOW WHAT IT  FEELS LIKE TO BE ON"
540 PRINT"{rvrs on}TOP!{shft space}YOU PLAYED WELL!"
545 PRINT"{rvrs on}{down:2}{blue}BET YOU DON'T WIN NEXT       TIME!"
548 PRINT:GOTO450
550 PRINT"{clear}{black}{rvrs on}I DIDN'T THINK YOU HAD ENOUGH COURAGE TO TRY   TO STOP A MAD APE!"
560 PRINT"{down}{rvrs on}{red}I HOPE YOU ENJOYED NOT SAVING THE BUILDING!"
565 FORG=1TO10000:NEXTG
570 SYS64802

See also: part 1, part 2, part 3, part 4 or part 5 (with more coming).

NOTE: This article was originally started on February 29, 2016, shortly after I discovered my cigar box of VIC-20 cassette. When I say “recently,” that now means “four years ago…”

---

Welcome to VIC-20 Tuesday!

I have slowly been importing my old VIC-20 programs in to the VICE emulator. This has been a tricky process. The tapes are over 30 years old and I no longer posses any real hardware to read them on. (I let my dad take my VIC-20 in exchange for getting me a Radio Shack TRS-80 Color Computer.) I played them on a higher end late 1990s Radio Shack dual cassette deck, and record them at 96khz stereo in Audacity on my Mac. The original tapes were mono, but some are stronger in one channel so I have been preserving both tracks as stereo audio files. I then use wav-prg to convert them to image files I can load in the VICE Commodore emulator.

My Sky-Ape-Er game has caught my attention. On the tape was a program called INSRTUCTIONS (with that typo I apparently never noticed) that will display instructions, wait for a keypress, then load the game as a second program called SKY-APE-ER. There were actually three copies of the main program, and at least two were different versions (oh, if only I had added version numbers to the file name). I also found several other copies of the main game program on other tapes in various forms of completion. I am hoping that this one is the “final” complete version.

The loading process looks like this:

…then you would type “RUN” (assuming you didn’t use the shortcut “RUN” key, which would do the “LOAD” and “RUN” automatically):

Then you press any key…

…and the program will have you “PLEASE WAIT” for a moment, then will erase itself with “NEW” and the “LOAD” the actual game.

I scanned the code to see how it did this, but I saw nothing obvious. No “NEW” or “LOAD” commands or anything. How did it work? Thanks to the Internet, I believe I figured it out.

First, let’s look at the source code of the program:

• Lines 5-6 – REMarks (comments).
• Line 10 – POKE stores a value at a memory location. But what is 36879, 808 and 775? A quick search lead me to some answers. 36879 ($900f) is “Screen color / Reverse mode / Border color”. In the VIC-20 manual, Appendix E shows that 26 would be red border, and white screen. Another search led me to a Compute magazine article showing that  808 ($328) is used to disable STOP, RESTORE and LIST (to prevent the user from BREAKing out of the program). 775  ($307) showed up in a correction article from the same magazine, stating that it is what disabled LIST. (Not sure about 808, then. Maybe there were two POKEs required and the first article had a typo or left the second one out by mistake.)
• Line 15-25 – The Commodores allowed embedded special control characters, and the inverted heart is clear screen. The others are likely color codes and cursor movements, to position the text where I wanted it to be.

• Line 25-50 – More things that print the text on the screen.
• Line 55 – Gets a keypress then, if the keypress is nothing (“”), go back and get another keypress. This causes the program to spin in a loop until a key is pressed on the keyboard.

• Line 60 – Please wait … for what? It looks like what happens next may take a few seconds.
• Line 65 – This is clearing out memory locations 7424 to 7432. But why? All I can tell so far is this is some memory location in RAM with other places saying it’s in the User BASIC area. A helpful forum post gave me a hint. This is actually clearing out the 8 bytes that will make up the “space” character so once I customize the character set with game graphics, that character will still be a space. More on this next…
• Line 70 – This walks through memory locations 7168 to 7303 and POKEs them with the values read from DATA statements below. This is storing values in that range of memory. But what is 7168? According to this page, that is the start of “half RAM, half ROM of upper case normal characters since the 14-bit address space of the VIC wraps around”. I believe this is the font/character set, and the DATA statements are the custom graphics for the Sky-Ape-Er program (such as the bricks for the levels, the pieces that make up the ape, the chimps, and the player).
• Line 80 – This is the magic! According to this page, memory location 198 is the number of characters in the keyboard buffer. The VIC-20 has a keyboard buffer from 631-640 (9 bytes?). I am stuffing the buffer with 78 (“N”), 69 (“E”), 87 (“W”), 13 (“ENTER”) and then 131 (the “RUN” key). Thus, it’s like I typed “NEW” and pressed ENTER (clearing out the BASIC memory), followed by hitting the “RUN” key, which does a “LOAD/RUN” sequence, loading the next file from tape and running it. Huzzah! The autostart with done by stuffing the keyboard buffer as if the user typed commands. This tells me just putting in “LOAD:RUN” in a line of BASIC wouldn’t have worked (probably because it would overwrite the program, and you could never get to the RUN part). Cool

• Line 110-END – The rest of the code are the DATA statements that make up the custom character set. I will have to dissect them so I can see what the bitmaps look like.

I have no recollection of how this works, so I assume these POKE values were things I found in the Commodore VIC-20 manual or read in magazines such as Compute’s Gazette. There was very little information around back then. In fact, when I got this computer, I knew of only one store in all of Houston that sold software for it (other than Commodore cartridges at some places that sold the computer). I had my grandmother drive me across town to go there once, and that is the store where I bought the Krazy Kong game that inspired me to write Sky-Ape-Er.

There was a Commodore Houston’s User’s Group (CHUG) that I attended a meeting of, but that was the only time I ever met other VIC-20 users (other than going to meet the publishers of the FOX-20 cassette magazine at their house). It was a whole different world back then, and it’s amazing that I can go from from “what does this do?” to writing this article in just a few web searches.

Up next: A look at the actual Sky-Ape-Er game itself, including how it uses this custom character set for game graphics.

Converted Source Code

Here is the INSTRUCTIONS program converted from PETASCII to ASCII. I found a utility that does this thanks to this blog post. The utility is found here:

https://www.commodoreserver.com/Downloads.asp

It does a nice job of translating the PETASCII characters to {words} so I can better understand what was supposed to be there.

5 REM  ************          *SKY-APE-ER*        *******BY*******
6 REM*ALLEN  HUFFMAN*      ****************{$cc}
10 POKE36879,26:POKE808,100:POKE775,200
15 PRINT"{clear}{black}{right:2}-=<<SKY-{red}APE{black}-ER>>=-{right:5}BY:{blue}ALLEN HUFFMAN"
20 PRINT"{purple}{$a4:22}{$a5}    INSTRUCTIONS    {$a7}{$a3:22}"
25 PRINT"{black}{down}CLIMB THE BUILDINGS TO CAPTURE THE MAD APE!"
30 PRINT" JUMP OVER CHIMPS AND UP STEPS TO GET TO THETOP. YOU MUST FACE THE";
35 PRINT"  DIRECTION TO JUMP."
40 PRINT"{down}{blue}      CONTROLS :{down}"
45 PRINT"{red}<LEFT>='A'{right}<RIGHT>='S'{right:4}<JUMP>='F1 KEY'"
50 PRINT"{down}{black}{rvrs on}    PRESS ANY KEY.    "
55 GETA$:IFA$=""THEN55
60 PRINT"{clear}{right:4}PLEASE WAIT..."
65 FORA=7424TO7432:POKEA,0:NEXTA
70 FORA=7168TO7303:READB:POKEA,B:NEXTA
80 POKE198,6:POKE631,78:POKE632,69:POKE633,87:POKE634,13:POKE635,131
100 DATA223,223,223,0,253,253,253,0,0,0,0,0,1,3,3,7,0,60,126,219,129,165,165,129
105 DATA0,0,0,0,128,192,192,224,31,63,127,255,252,254,127,63,0,126,126,0,255,0,231,231
110 DATA248,252,254,255,63,127,254,252,15,7,8,30,31,31,31,31,231,219,60,255,126,126
115 DATA189,195,240,224,16,120,248,248,248,248,31,31,31,15,15,63,127,0,231,129,0,0,129
120 DATA129,129,0,248,248,248,240,240,252,254,0,12,12,24,47,8,15,82,36,48,48,24,244,16
125 DATA240,74,36,28,8,28,42,8,20,20,54,12,76,40,47,40,159,82,36

I normally just use the WordPress “preformatted text” box, but it did not like this listing. Hopefully this shows up.

Until next time…