Category Archives: CoCo

Tandy/Radio Shack TRS-80 Color Computer (CoCo)

Color BASIC challenge: ball bouncing WITHOUT any IFs?

Leave a reply

It seems like all I’m doing lately is regurgitating things that Robin of 8-Bit Show and Tell has already done.

So let’s do that again.

Don’t blame me. Blame YouTube!

YouTube did what YouTube does and it showed me another of Robin’s well-done videos. This one caught my attention because it dealt with the Commodore VIC-20 and its Super Expander cartridge.

The main thing that pulled me away from Commodore was seeing the TRS-80 Color Computer’s Extended Color BASIC. The CoCo had simple commands to play music and draw lines, boxes and circles. It also had this wondrous ELSE command I’d only heard rumors about.

On the VIC-20, it seemed you needed to use POKE and PEEK for just about anything graphics or sound related. Thus I gave up a computer with programmable characters and a hardware sound chip for a machine that had neither. On my new CoCo, at least I could draw a circle and play music without needing pages of DATA statements and cryptic PEEKS and POKEs.

Commodore was aware of this shortcoming, and they sold the Super Expander as a way to make up for it. Not only did it provide an extra 3K of memory (giving a whopping 6.5K for BASIC), it also added new commands to do “high resolution” graphics including drawing lines and circles, as well as ways to PRINT music using simple notation.

I used the Super Expander to do TV titles for fishing videos my father shot and edited. It was a thrill to see my VIC-20 graphics on TV screens at the Houston Boat Show.

But no one else could run my programs unless they had purchased the Super Expander as well.

But I digress.

(And besides, the Commodore 64 was $600 when it came out, and I was able to get a 64K CoCo 1 for $300 at the time.)

Don’t blame YouTube. Blame Twitter.

Robin’s video was making use of the Super Expander to let the VIC-20 solve a challenge initiated by Twitter user Dataram_57. On June 8th, 2019, they wrote:

This was, more or less, a classic bouncing ball program very much like the ones I have been writing about lately. But, all mine certainly made use of IF. Here’s how mine started:

0 REM bounce.bas
10 CLS:X=0:Y=0:XM=1:YM=1
20 PRINT@Y*32+X,"O";
30 X=X+XM:IF X<1 OR X>30 THEN XM=-XM
40 Y=Y+YM:IF Y<1 OR Y>13 THEN YM=-YM
50 GOTO 20

That’s not a very good example. It doesn’t erase itself, nor does it use the bottom line to avoid screen scrolling when the ball hits the bottom right position. It does show how I would use X and Y coordinates then an XM (X movement) and YM (Y movement) variable to increment or decrement them based on if they hit an edge.

The parameters of Dataram_57’s challenge were as follows:

  • Width: 90
  • Height: 80
  • Starting Position: 0,0
  • Time: ???

I wrote a quick graphical program do do this using my X/Y/XM/YM method:

0 REM dataram_57 twitter challenge
1 POKE 65495,0
10 W=89:H=79:X=0:Y=0:T=0:XM=1:YM=1
20 PMODE0,1:PCLS0:SCREEN1,1
30 LINE(0,0)-(89,79),PSET,BF
40 PSET(X,Y,0)
50 X=X+XM:IF X<0 THEN XM=1:GOTO 50 ELSE IF X>=W THEN XM=-1:GOTO 50
60 Y=Y+YM:IF Y<0 THEN YM=1:GOTO 60 ELSE IF Y>=H THEN YM=-1:GOTO 60
70 T=T+1:IF T=7031 THEN END
80 GOTO 40

The first thing to notice is that I draw a filled box from 0,0 to 89,79 and then set black pixels in it. This lets me visually verify my line is going all the way to the edge of the 90×80 target area. Also, I am using the CoCo 1/2 double speed poke since this is time consuming. If you do this on a CoCo 3, feel free to use POKE 65497,0 instead.

Twitter user Dataram_57’s challenge running on a CoCo.

Eventually the area should be entirely black when every dot has been erased.

How long has this been going on?

I did some tests and figured out that it takes 7032 iterations (0-7031) for the dot to cycle through the entire 90×80 area before it has erased all the other dots.

With that in mind, I propose we turn this into both a logic and optimization challenge. On the CoCo, let’s see if we can use the PMODE 0 screen (128×96 resolution with 1 color). We can put this in a modified version of benchmark framework for 7032 cycles and see how fast we can do it. (By modified, I am removing the outer “try this three times and average the results” loop.)

My example, using IFs, looks like this:

0 REM dataram_57 twitter challenge 2
1 POKE 65495,0
5 DIM TM,A
10 TIMER=0:TM=TIMER

15 W=89:H=79:X=0:Y=0:XM=1:YM=1
16 PMODE0,1:PCLS0:SCREEN1,1
17 LINE(0,0)-(89,79),PSET,BF

20 FORA=0TO7030

30 PSET(X,Y,0)
40 X=X+XM:IF X<0 THEN XM=1:GOTO 40 ELSE IF X>=W THEN XM=-1:GOTO 40
50 Y=Y+YM:IF Y<0 THEN YM=1:GOTO 50 ELSE IF Y>=H THEN YM=-1:GOTO 50

80 NEXT
90 PRINTTIMER:END

Mine, running in Xroar, displays 9808 at the end. And it’s not the correct way to meet the requirements of the challenge, so … the real versions may be faster, or slower.

Your challenge, should you decide to accept it…

Our challenge is to:

  1. Rewrite this to work WITHOUT using any “IFs”.
  2. Try to make it as fast as possible while keeping the benchmark code (lines 5, 10, 20, 80 and 90) intact. You can add variables to the DIM, but otherwise leave those lines alone.

What says you?

Credit where credit is due…

And lastly, for those who want to cheat, here is the solution that Robin came up with using the VIC-20 Super Expander cartridge…

Is his the only way? The best way? The fastest way?

Let the regurgitated challenge begin!

Until next time…

Understanding and Using the CoCo 3 MMU

2 Replies

This document was originally written in 1987 and was available on my BBS. It was originally in some kind of text editor format that I do not remember, so I have reformatted it for WordPress and made it available here for archival purposes. If you want the original file, here it is:

mmu.txt.zipDownload

Understanding and Using the MMU

A “Hopefully Helpful” Text file by
Allen C. Huffman

The 6809 CPU can only access 64K at a time. In order for an 8-bit CPU to access memory beyond that range, some sort of Memory Management Unit must be used. The CoCo 3’s MMU does the job nicely by breaking all of the memory into 8K chunks. The CoCo 3 is set up to handle up to 512K of memory which is addressed from &H0000 to &H7FFFF (0 – 524287). The 6809 will recognize the last 64K in this map (&H70000 to &H7FFFF) as “normal” memory for it. (In Basic, when you do a POKE 1024,42 it is actually putting a 42 in physical memory &H70000 + 1024. (If you do an LPOKE 1024,42 you will actually be putting the 42 in physical memory location 1024.) This may seem strange at this time, but hopefully it will soon be clear.

In a 128K Color Computer, the RAM is located from &H60000 to &H7FFFF. (All memory from &H00000 to &H5FFFF is unusable unless you have 512K in which case your extra memory resides there.) The 128K is broken into two parts. One is the normal 64K workspace that the CoCo uses. The other 64K is used for Hi-Res graphics, screens, etc. (For more detail on where everything is, refer to your CoCo 3 owners manual Memory Map on Page 311.)

Meanwhile, back to the MMU, all of the CoCo’s memory is handled internally as 8K blocks. Thus, a 64K workspace is actually 8 MMU blocks. (8 x 8K is 64K, right?) The MMU can access any of these 8K blocks by using the corresponding block number. The blocks are numbered as follows:

##    Memory Range      ##   Memory Range      ##   Memory Range
00    00000 - 01FFF     10   20000 - 21FFF     20   40000 - 41FFF
01    02000 - 03FFF     11   22000 - 23FFF     21   42000 - 43FFF
02    04000 - 05FFF     12   24000 - 25FFF     22   44000 - 45FFF
03    06000 - 07FFF     13   26000 - 27FFF     23   46000 - 47FFF
04    08000 - 09FFF     14   28000 - 29FFF     24   48000 - 49FFF
05    0A000 - 0BFFF     15   2A000 - 2BFFF     25   4A000 - 4BFFF
06    0C000 - 0DFFF     16   2C000 - 2DFFF     26   4C000 - 4DFFF
07    0E000 - 0FFFF     17   2E000 - 2FFFF     27   4E000 - 4FFFF
08    10000 - 11FFF     18   30000 - 31FFF     28   50000 - 51FFF
09    12000 - 13FFF     19   32000 - 33FFF     29   52000 - 53FFF
0A    14000 - 15FFF     1A   34000 - 35FFF     2A   54000 - 55FFF
0B    16000 - 17FFF     1B   36000 - 37FFF     2B   56000 - 57FFF
0C    18000 - 19FFF     1C   38000 - 39FFF     2C   58000 - 59FFF
0D    1A000 - 1BFFF     1D   3A000 - 3BFFF     2D   5A000 - 5BFFF
0E    1C000 - 1DFFF     1E   3C000 - 3DFFF     2E   5C000 - 5DFFF
0F    1E000 - 1FFFF     1F   3E000 - 3FFFF     2F   5E000 - 5FFFF

That covers the 512K area. On a 128K CoCo, you can’t even use blocks 00 to 2F, so let’s look at what you can use:

30    60000 - 61FFF
31    62000 - 63FFF     The first 4 blocks (32K) is where Basic puts the
32    64000 - 65FFF     HSCREEN graphics.
33    66000 - 67FFF

34    68000 - 69FFF     This is where the HGET/HPUT buffer is. (8K)

35    6A000 - 6BFFF     This is the secondary stack area. (8K)

36    6C000 - 6DFFF     This is where the 40/80 column screen goes. (8K)

37    6E000 - 6FFFF     And this one is unused by Basic. (8K)

That takes care of the 64K you don’t have direct access to on startup. Here is the 64K you do have access to:

38    70000 - 71FFF     The first 32K is normal system RAM.  (System use,
39    72000 - 73FFF     32 column text screen. PMODE graphics, Basic program
3A    74000 - 75FFF     and variable storage)
3B    76000 - 77FFF

3C    78000 - 79FFF     The last 32K is where system ROM goes.   (Note that
3D    7A000 - 7BFFF     on the CoCo 3, all ROM is copied into RAM so these
3E    7C000 - 7DFFF     locations are indeed RAM.)
3F    7E000 - 7FFFF

(Notice that all you have to do to convert physical locations to normal 8 bit locations on the older CoCo’s is to remove the 7.)

So now you should have some idea of how memory is looked at, but how do you actually use it? Eight memory locations control what physical block of memory goes where. Let’s have another chart…

FFA0    0000 - 1FFF      Remember those block numbers given earlier?
FFA1    2000 - 3FFF      On startup, FFA0 - FFA7 contain:
FFA2    4000 - 5FFF
FFA3    6000 - 7FFF      78, 79, 7A, 7B, 7C, 7D, 7E, & 7F.
FFA4    8000 - 9FFF
FFA5    A000 - BFFF      WHY?  Shouldn't they be 38, 39, 3A, etc, as the
FFA6    C000 - DFFF      block numbers would indicate?  Read on...
FFA7    E000 - FFFF

The MMU locations have bit 6 set high which adds &H20 (64) to their actual value. Therefore, to the MMU 38 is the same as 78, 3E is the same as 7E, etc. The CoCo 3 Service Manual indicates that programmers may use any blocks in the range of 00-3F so we will go by that. Please try not to let this confuse you! Just accept it.

In order to use the MMU from Basic, you simply use the POKE command to tell the MMU what “block” goes where. For instance, location &HFFA2 contains 7A on startup. Anytime you peek memory between 4000-5FFF it is actually PEEKing from block 3A which is physical locations 74000-75FFF. You can change this by POKEing to it. If you POKE &HFFA2,&H30 then anytime you PEEK or use memory in the range of 4000-5FFF it will actually be coming from physical memory 60000-61FFF. Whatever was in block 3A is still there, but when you try to use that memory, the MMU points the CPU to somewhere else. The CPU never knows the difference! To get it back as before, POKE &HFFA2,&H3A.

You may now have a slight understanding of how those Basic HSCREEN save/load routines work. Take a look:

10 INPUT "Save picture as";N$
20 FOR A = &H30 TO &H33
30 POKE &HFFA2,A
40 SAVEM N$+"/HR"+CHR$(A-&H2F),&H4000,&H5FFF,0
50 NEXT A
60 POKE &HFFA2,&H3A

Line 10 simply asks for what to save the picture as. Line 20 does a loop. If you notice 30-33 is the physical location of the hi-res HSCREENs. The poke in line 30 switches the physical location pointed to by “A” into CPU location 4000-5FFF. (The first time through, block 30 is there, then 31, then 32, etc.) After this, line 30 does a SAVEM of whatever is now at that location. (It tags the extension of HR at the end followed by a number 1-4). Line 50 continues the loop and line 60 restores the memory like it was before this routine.

Programmers out there might think this is strange seeing a program save the same contents of memory (4000-5FFF) four times, but that is where the magic of the MMU comes in. Thanks to the POKE, each time the program gets to that line the MMU points to a different area. To load a file back in, a similar routine must be used that flips the MMU to that area then, after loading, flips it back where it goes. It might look something like this:

10 INPUT "Load picture";N$
20 INPUT "HSCREEN #";H
30 HSCREEN H
40 FOR A = &H30 TO &H33
50 POKE &HFFA2,A
60 LOADM N$+"/HR"+CHR$(A-&H2F)
70 NEXT A
80 POKE &HFFA2,&H3A

If you can understand why this program works, you are in good shape. By theory, someone could save the entire memory of the Color Computer to disk in 8K chunks using the above method. (Why? I don’t know, but they could…)

Now if you still don’t see how this works, try this experiment. Enter the following line which will fill up physical block &H37 (6E000-6FFFF) with the value of 42.

FOR A=&H6E000 TO &H6FFFF:LPOKE A,42:NEXT A

Okay. So all the memory from 6E000 to 6FFFF should have 42s in it, right? Right. (To verify, you could do FOR A=&H6E000 TO &H6FFFF:PRINT LPEEK(A):NEXT A and you would get 42s back.)

Somewhere in memory we have a block of 42s. We can make that block appear to be somewhere else. Try a POKE &HFF2A,&H30. You have just set it so whenever the CPU looks at locations 4000-5FFF it will really be looking at 60000-61FFF. Confirm this and try:

FOR A=&H4000 TO &H5FFF:PRINT PEEK(A);:NEXT A

You will see a bunch of 42s. Magic, right? No, just amazing. Still not convinced? Type POKE&HFF2A,&H3A and try that previous line again. What happened to the 42s? They are still where they were originally but, the CPU is being directed back to where it was supposed to be.

This is just the start of what the MMU can do. There is another bank of MMU registers from FFA8 to FFAF. These are being used by the CoCo itself while in basic, but any machine language programmer could define each set of MMU registers and then by toggling a single bit (bit 0 of FF91) he could select between which MMU map would be used.

If you still haven’t got it, leave a message for me and I will do my best to answer questions or to create another file that further explains the MMU.

CoCoFest Chronicles book by Allen Huffman

Leave a reply

In honor of the 40th anniversary of the Radio Shack Color Computer, I am making my book, CoCoFest Chronicles, available as a free download. This book was first published in 1998 and has been out-of-print for many years.

The book contains updated versions of my CoCoFest reports covering events from 1990 to 1997. It also contains behind-the-scenes tidbits that explain various inside jokes and things that went on that were not included in the original reports I uploaded to online services back then.

Download it for free here:

CoCoFestChronicles_AllenHuffmanDownload

Happy 40th birthday, CoCo!

CoCoWiFi in the UK

Leave a reply

Thanks to Rob Inman for sharing this link over on Discord. This was supposed to have been posted in July 2019, but I just found it in my drafts folder. I think I was going to write an article about it, but forgot.

Someone in the UK is selling an all-in-one RS-232 to WiFi adapter. They use Bo Zimmerman’s excellent Zimodem firmware, though the version they use is based on my fork of the project with the defaults set to standard RS-232 rather than Commodore’s inverted RS-232.

https://www.simulant.uk/shop/retro-vintage-computer-wifi-modem-rs232-serial-hayes-compatible

What ELSE can you do when you don’t have ELSE?

17 Replies

NOTE: I have a follow-up to this ELSE thing already written which “changes everything” for me. There’s stuff about ELSE I never knew, which makes me want to never use it. More on that in the next installment. And now back to your regularly scheduled blog post…

First, a quick BASIC memory saving tidbit.

Shaun Bebbington left a comment about my VIC-20 dissection article asking if I knew you could omit zeros from DATA statements like this:

100 DATA 128,0,0,50,239,0,123,42,0,4

If you leave them out (comma comma), READ will still get a zero! It saves a byte each time you do that, which could be important on a machine with 3584 bytes free on startup like my VIC-20.

100 DATA 128,,,50,239,,123,42,,4

Good tip, Shaun! Thanks!

Is there anything ELSE I can help you with?

And while I had his attention, I asked him a few Commodore BASIC questions. I was wondering how you did IF/THEN/ELSE without ELSE! One of the many things I liked about the BASIC in the CoCo was it had ELSE and commands like PLAY, DRAW, CIRICLE, etc. My VIC-20 had no ELSE. In my Sky-Ape-Er game, I’d see my young self doing things like this:

125 IF K=17 THEN L=L-1:M=14
130 IF K=41 THEN L=L+1:M=13
135 IF K=39 THEN 180

In that code snippet, K represented the key value that was pressed. If it was 17 (left), I’d move the player left one and change its graphic character. If it was 41 (right), I’d move the player right one and change its graphics character. If it was 39 (F1, jump), I’d go to a routine that would handle the jump.

Without ELSE, if K was 17, it would do that, then check K two more times, even though it didn’t need to. Maybe I did not “see” it back then, but If I had the bytes to spare, I probably should have done something like this:

125 IF K=17 THEN L=L-1:M=14:GOTO 140
130 IF K=41 THEN L=L+1:M=13:GOTO 140
135 IF K=39 THEN 180

That way, if K=17 was true, it would do it’s thing, then GOTO would skip over the next two lines. This could be a huge time saver if there were a bunch of conditions (up, down, left, right, diagonals, jump, fire, punch, windshield wipers, etc.)

Someone has already suggested that I may have done it my original way to get consistent timing in the game loop. Somehow I doubt my junior high self was that clever. But I digress…

With ELSE, it could have been written like this:

125 IF K=17 THEN L=L-1:M=14 ELSE IF K=41 THEN L=L+1:M=13 ELSE IF K=39 THEN 180

Less code (the ELSE token takes up less memory than an addition line number and/or the extra GOTO to skip lines), and faster execution, maybe.

Side Note: Maybe? There is still the issue of, when completing a successful IF, BASIC’s parser still having to run through the rest of the line characters to find the end and the next line. Adding a “GOTO” wouldn’t help, either, since it would have to parse that and then STILL have to scan to the end of the line. (At least on Color BASIC, which does not remember line pointers once it is parsing a line.) It may actually be faster to break up a long set of IF/ELSE into small lines with a GOTO.

But Shuan mentioned a way of using ON to simulate an ELSE. A quick search led me to a forum post discussing this very thing.

It supposedly works like this… since a compare (K=42, A$=”RED”, G>100) will return either -1 (false) or 0 (true), you can do an ON GOTO based on the result of that compare. Since it’s a -1 or 0, you can just make the result negative (thus, -1 becomes 1, and 0 becomes -0 which is still 0):

10 ON -(A=42) GOTO 20:PRINT "NOT 42":END
20 PRINT "42!"

Er… what? I thought BASIC did not execute anything after a GOTO. It doesn’t, does it?

Nothing on a line after GOTO will be executed. WIll it?

But… but… How can ON/GOTO with something after it work, then? It turns out, ON/GOTO is special since the conditions of the “ON” may not be met, and thus the GOTO part may not get executed and BASIC will continue.

ON A GOTO 100,200,300:PRINT "A WAS NOT 1, 2 or 3"

Looking at it like that makes perfect sense to me. If A is not 1, 2 or 3, it won’t GOTO anywhere and the line continues to be processed.

Thus, this odd code serves as a simple “ELSE” when you don’t have ELSE:

10 INPUT "VALUE";A
20 ON -(A=42) GOTO 30:GOTO 10
30 PRINT "YOU KNOW THE ANSWER!"

…would be like…

10 INPUT "VALUE";A
20 IF A=42 THEN 30 ELSE 10
30 PRINT "YOU KNOW THE ANSWER!"
If no ELSE, use ON GOTO?

Interesting! I have not benchmarked this to see if it’s faster than using GOTO to skip lines, but it might be smaller due to not needing another few bytes for each line number.

Would this have helped my “left, right, jump” code? Maybe not. You can string these together like this:

124 ON -(K=17) GOTO 125:ON -(K=42) GOTO 130:ON -(K=39) GOTO 180
125 L=L-1:M=14:GOTO 140
130 L=L+1:M=13:GOTO 140

Since I was doing code in response to the IF, the ON/GOTO approach would just let me GOTO a line that does that code, which then still needs a GOTO to skip the lines after it that shouldn’t be executed. Not great for that use.

But, if I were dispatching to routines based on direction (like I do with the “jump” check to line 180), it would have worked just fine. Instead of this:

125 IF K=17 THEN 200
130 IF K=41 THEN 300
135 IF K=39 THEN 400
...
200 REM Handle LEFT
...
300 REM Handle RIGHT
...
400 REM Handle JUMP
...

Those three lines could be combined into one like this:

124 ON -(K=17) GOTO 200:ON -(K=42) GOTO 300:ON -(K=39) GOTO 400

But, doing a quick test typing in JUST those lines on a VIC-20 emulator, I got 3530 bytes free after each approach. No penalty, but no savings, and all the parsing with parens and the negatives is probably slower.

Interesting, for sure. Useful? I guess I’ll find out if I get around to updating my VIC-20 code.

Bonus offer

I also saw this example in the Commodore forum:

10 rem if then else demo
20 input a
30 if a = 1 then goto 60
40 print "this is the else part"
50 goto 70
60 print "this is the if (true) part"
70 end

This is how I’d write that with ELSE:

20 INPUT A
30 IF A=1 THEN PRINT "THIS IS THE IF (TRUE) PART" ELSE PRINT "THIS IS THE ELSE PART"

So we could change the Commodore example to match this a bit closer:

20 input a
30 if a = 1 then print "this is the if (true) part":goto 70
40 print "this is the else part"
70 end

…which leads us back to just adding a GOTO at the end of each separate IF:

10 GET A$:IF A$="" THEN 10
20 IF K$="U" THEN Y=Y-1:GOTO 70
30 IF K$="D" THEN Y=Y+1:GOTO 70
40 IF K$="L" THEN X=X-1:GOTO 70
50 IF K$="R" THEN X=X+1:GOTO 70
60 GOTO 10
70 REM DRAW X,Y...
80 GOTO 10

…which on the CoCo’s Extended Color BASIC might look like:

10 GET A$:IF A$="" THEN 10
20 IF K$="U" THEN Y=Y-1 ELSE IF K$="D" THEN Y=Y+1 ELSE IF K$="L" THEN X=X-1 ELSE IF K$="R" THEN X=X+1 ELSE 10
70 REM DRAW X,Y...
80 GOTO 10

Ultimately, we are just trading “ELSE” with “GOTO xx” and a new line, with ELSE being smaller due it it just being a token, verses the overhead of a GOTO token and the line number characters after it, AND a new line for each additional “else” condition.

Until next time, ELSE maybe sooner…

Virtual CoCoFest this Saturday, April 18, 2020.

Leave a reply

News release:

CoCoTALK! is hosting a live, virtual CoCoFEST!

April 18th, 2020 would have been the 29th annual “last” Chicago CoCoFEST! celebrating the 40th anniversary of the Tandy RadioShack TRS-80 Color Computer. While the event itself has been postponed, the live retro podcast CoCoTALK! is hosting a live, virtual CoCoFEST! Saturday April 18th @ 2:00 PM EDT.You can view and chat live on YouTube, the friendly link is http://live.cocotalk.live which points to  https://www.youtube.com/c/Ogsteviestrow/live

CoCoFEST! is an annual event hosted by the Glenside Color Computer Club and has been running since the last Rainbow Fest ended.  This event is the K-FEST/VCF event of the Color Computer world. The virtual event is not an official Glenside Color Computer event, but is  being held, with their approval.  The goal is to get as many of the attendees, exhibitors, vendors, speakers, and presenters to join the live call present virtually, and turn the event into one that can be experience globally.

CoCoTALK! is a weekly live and interactive video talk show which multicast to YouTube and Facebook live, so there will be plenty of ways to join the celebration with us.  To learn more about CoCoTALK! go to http://cocotalk.live To learn more about the Glenside Color Computer club, visit http:///glensideccc.com

VIC-20: Sky-Ape-Er code dissection – part 2

Leave a reply

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

Welcome to my second VIC-20 Tuesday!

Previously, I began to dissect my old VIC-20 game Sky-Ape-Er. It was made up of two BASIC programs:

  1. INSTRUCTIONS – Display instructions, set up custom character set, then load and run the main game.
  2. SKY-APE-ER – The main game.

VIC-20 custom fonts

The VIC-20 did not have a true graphics screen. Instead, it used font characters that could be dynamically changed. Each character on the 22×20 screen was 8×8. You could create custom 8×8 characters to represent anything you wanted.

I believe I used a program called Eight by Eight Create by Robert Spahitz from the January 1983 issue of Creative Computing (Vol 9, Number 1). See page 270 (or 272 of this PDF scan):

https://archive.org/details/creativecomputing-1983-01/page/n271/mode/2up

Or this text version of just the article.

This also helps me confirm that I got my VIC-20 in 1982 since I initially did not have a Commodore Datasette tape deck for it and had no way to save programs until later. By the time this issue came out, I had the tape deck.

But I digress…

VIC-20 custom fonts on the CoCo

I have screen shots of what the custom characters looked like in the VIC-20 game, but thought it would be fun to take those DATA statements and display them on my CoCo. Using lines 100 to 125 from the INSTRUCTIONS program, I created this Color BASIC program:

0 REM skychars.bas
5 POKE 65495,0
10 CLS:XO=4:YO=2
15 DIM BM(7):FOR A=0 TO 7:BM(7-A)=(2^A):NEXT
20 FOR R=0 TO 7
25   READ V:IF V=-1 THEN 99
30   FOR C=0 TO 7
35     IF V AND BM(C) THEN SET(XO+C,YO+R,8) ELSE RESET(XO+C,YO+R)
40   NEXT
45 NEXT
50 XO=XO+10:IF XO>60 THEN XO=4:YO=YO+10
55 GOTO 20
99 GOTO 99
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
130 DATA -1

It now displays what those font characters look like:

VIC-20 Sky-Ape-Er font display on a CoCo.

These custom characters would replace “@ABCDEFGHIJKLMNOP”. The “@” symbol was the brick used to make the game playfield.

Letters A-L were used to make the ape, like this:

ABC
DEF
GHJ
JLK

Letters M-O were the player’s character, facing right, left and forward.

Letter P were the small apes that were running towards the player. They never turned back, so no additional characters we needed for them.

My program was written to just put those characters on the screen, which would look like this:

My VIC-20 Sky-Ape-Er game without the custom font.

But with the custom font in use, it looks like this:

In the next installment, I’ll walk through the actual SKY-APE-ER game code and see how it works.

Until next time…

IF AND/OR THEN versus IF THEN IF – part 1

1 Reply

See also: part 1 and part 2.

Or, “I’ll NEVER compare multiple values the same way again”

Here is a quick Optimizing Color BASIC quickie.

Tonight, I saw this YouTube video from Robin at 8-Bit Show and Tell. He was responding to someone who noticed a bit of code in an old Commodore magazine that did:

IF Z>192 THEN IF Z<219 THEN Z=Z-128

…instead of the more common…

IF Z>192 AND Z<219 THEN Z=Z-128

He performed a benchmark very much like the ones I have been doing on the CoCo (I’m glad I’m not the only one doing this). I decided to try it on the CoCo and see what happened here.

I first tried the normal “IF AND THEN” way:

0 REM IFAND.BAS
5 DIM TE,TM,B,A,TT
10 FORA=0TO3:TIMER=0:TM=TIMER
20 FORB=0TO1000
30 IF Z>192 AND Z<219 THEN Z=Z-128
70 NEXT
80 TE=TIMER-TM:PRINT,TE
90 TT=TT+TE:NEXT:PRINTTT/A:END

This produced 875 on my Xroar emulator.

Then I tried the “odd” IF AND THEN way…

0 REM THENIF.BAS
5 DIM TE,TM,B,A,TT
10 FORA=0TO3:TIMER=0:TM=TIMER
20 FORB=0TO1000
30 IF Z>192 THEN IF Z<219 THEN Z=Z-128
70 NEXT
80 TE=TIMER-TM:PRINT,TE
90 TT=TT+TE:NEXT:PRINTTT/A:END

This produced 472! A significant time savings just by changing the way you compare multiple things.

Thinking this through, I have some theories, based on some very inefficient C code I found in a work project once.

See A C tale of two ampersands for that story…

Could BASIC be doing the same thing?

IF THEN BASIC

“IF this AND that THEN” surely isn’t doing a logical AND for values, is it? It might be, since you can print a comparison like this:

PRINTing the result of a comparison in Color BASIC.

In Color BASIC, true is -1 and false is 0. So perhaps doing this:

IF A=1 AND B=2 THEN PRINT "BOTH ARE TRUE"

…is actually turning into…

IF -1 AND 0 THEN PRINT "BOTH ARE TRUE"

…with -1 or 0 being used based on the conditions. Let’s try:

Color BASIC must not be doing bitwise math for compares.

Oh my. Maybe it IS doing this, thus requiring the values to be saved and compared, making it slower just like C generating more code to do the same thing.

I know I could just look at the disassembly in Color BASIC Unravelled and know exactly what is going on, but it’s more fun to experiment. How can we test this?

Well, things that return true/false return -1 or 0. You can AND two numbers together and get mathematical results. Even a compare like:

IF A$="ALLEN" THEN

…will return a -1 if it is true, or a 0 if it is false:

Even string compares turn into -1 or 0.

It sure seems likely that BASIC is just taking the result of a compare (true or false, -1 or 0) and AND-ing or OR-ing those values. Thus, my theory is this:

Multiple AND/OR comparisons turn into multiple results of -1 or 0 (true or false) which are then mathematically processed:

IF A=1 AND B=2 AND C=3 THEN ...

…so all of it has to be processed. But, using this IF THEN IF approach:

IF A=1 THEN IF B=2 THEN IF C=3 THEN ...

…will be much faster since it does a compare, then if good, it does another compare, then if good, it does the final compare… rather than having to do three compares then do math on the three results of that compare then check that final result to know what to do.

What says you?

I suppose it gets much slower the more conditions you want to compare:

5 DIM TE,TM,B,A,TT
10 FORA=0TO3:TIMER=0:TM=TIMER
20 FORB=0TO1000
30 IF X=1 AND Y=1 AND Z=1 THEN Z=42
70 NEXT
80 TE=TIMER-TM:PRINT,TE
90 TT=TT+TE:NEXT:PRINTTT/A:END

763 for that one. Versus:

5 DIM TE,TM,B,A,TT
10 FORA=0TO3:TIMER=0:TM=TIMER
20 FORB=0TO1000
30 IF X=1 THEN IF Y=1 THEN IF Z=1 THEN Z=42
70 NEXT
80 TE=TIMER-TM:PRINT,TE
90 TT=TT+TE:NEXT:PRINTTT/A:END

335!

So obviously I have to do this:

5 DIM TE,TM,B,A,TT
10 FORA=0TO3:TIMER=0:TM=TIMER
20 FORB=0TO1000
30 IF G=1 AND H=1 AND I=1 AND J=1 AND K=1 AND L=1 AND M=1 AND N=1 AND O=1 AND P=1 AND Q=1 AND R=1 AND S=1 AND T=1 AND U=1 AND V=1 AND W=1 AND X=1 AND Y=1 AND Z=1 THEN Z=42
70 NEXT
80 TE=TIMER-TM:PRINT,TE
90 TT=TT+TE:NEXT:PRINTTT/A:END

If those 10 conditions are true, do something… Testing this produces: 4782

And doing it the weird way:

5 DIM TE,TM,B,A,TT
10 FORA=0TO3:TIMER=0:TM=TIMER
20 FORB=0TO1000
30 IF G=1 THEN IF H=1 THEN IF I=1 THEN IF J=1 THEN IF K=1 THEN IF L=1 THEN IF M=1 THEN IF N=1 THEN IF O=1 THEN IF P=1 THEN IF Q=1 THEN IF R=1 THEN IF S=1 THEN IF T=1 THEN IF U=1 THEN IF V=1 THEN IF W=1 THEN IF X=1 THEN IF Y=1 THEN IF Z=1 THEN Z=42
70 NEXT
80 TE=TIMER-TM:PRINT,TE
90 TT=TT+TE:NEXT:PRINTTT/A:END

…speeds up to 732!

Well I’ll be. It looks like, even though its longer code, it’s much faster. I never knew that. Or if I did, I forgot it long ago.

Thanks, Robin, for posting that video a few years ago. Very cool! Here’s his original video:

Until next time…

Extended Color BASIC plays non-existent notes.

2 Replies

Just for fun…

When I was working on my SirSound project, I had to create my own implementation of the Extended Color BASIC “PLAY” command. I did this by going through the Extended BASIC Unravelled book and looking at the 6809 assembly for the command.

I made a workalike function in C for the Arduino:

https://github.com/allenhuffman/PlayParser

…and while doing so, discovered an interesting thing about the PLAY parser and how it handled the notes.

The PLAY command operates by reading a command followed by an option modifying. For instance, “V10” is the command “V” for volume, and a modifier “10” for volume level 10. “T8” is command “T” for tempo and “8” for the speed.

When it comes to playing musical notes, PLAY understands the standard scale notes of C, D, E, F, G, A and B. When it parses a command letter, it then looks to see if a modifier is after it. The modifiers include “#” and “+” for sharp, and “-” for flat.

This allows you to play the full 12 note scale:

Using sharps: C C# D D# E F F# G G# A A# B (or C C+ D D+ E F F+ G G+ A A+ B)

Using flats: C D- D E- E F G- G A- A B- B

These notes correspond to the notes found on a piano keyboard:

Piano keyboard, one octave.

I don’t understand music theory, but I know enough to say the white keys are the natural notes (CDEFGAB) and the black keys are a half step in between and are either sharps of the note before it (C# D# F# G# A#) or flats of the notes after it (Db Eb Gb Ab Bb). There is no E# or Fb or B# or Cb on a keyboard.

But, the BASIC PLAY parser does not add any code to prevent you from playing them anyway :-)

  • PLAY “E# F” (E sharp and F natural) will play the same note.
  • PLAY “E F-” (E natural and F flat) will play the same note.
  • But you can’t PLAY “C-” (C flat) or “B#” (B sharp) because it knows to reject those modifiers from the notes at each end of the scale.
CoCo can play notes that are not on a piano keyboard :-)

To me, this seems like a bug, but I implemented in my PLAY PARSER code anyway, just in case anyone used it. But since I lack any music theory background, I am probably wrong, as I have been told there is a reason for this.

For for someone like me with limited music background, it seemed like an odd thing to stumble upon in the code. I even documented in my SirSound documentation:

NOTE
—-
N (optional) followed by a letter from “A” to “G” or a number from 1 to 12.
When using letters, they can be optionally followed by “#” or “+” to make it
as sharp, or “-” to make it flat. When using numbers, they must be separated
by a “;” (semicolon).

C C# D D# E F F# G G# A A# B (sharps)
1 2 3 4 5 6 7 8 9 10 11 12
C D- D E- E F G- G A- A B- B (flats)

Due to how the original PLAY command was coded by Microsoft, it also allows
sharps and flats that would normally not be allowed. For instance, E# is the
same as F, and F- is the same a E. Since notes are numbered 1-12, the code
did not allow C- or B#. This quirk is replicated in this implementation.

SirSound documentation.

Ah, the things that amuse those of us without the brainpower to understand them.

Until next time…

Porting 10 PRINT RACER by 8-Bit Show And Tell to the CoCo

10 Replies

YouTube decided to show me a video by 8-Bit Show And Tell. It is a BASIC driving game called “10 PRINT RACER” for the Commodore PET. For those that don’t know, “10 PRINT” is a one line Commodore BASIC program that generates a maze by randomly printing slash and backslash PETASCII characters.

10 PRINT CHR$(205.5+RND(1)); : GOTO 10

There is even a book about it (which you can also download free). See the official site:

https://10print.org

I actually ported this to the CoCo years ago:

10 PRINT CHR$(47+(RND(2)-1)*45); : GOTO 10

…but using the ASCII “/” and “\” characters just doesn’t have the same effect:

CoCo version of the famous Commodore “10 PRINT” program.

But I digress…

10 PRINT RACER

The game, 10 PRINT RACER, was based on this maze program. You drive a car through a passage in the middle of a random maze. The video demonstrates the game and then walks through the code, line by line.

I was impressed with his awareness of speeding up BASIC, such as using a FOR/NEXT loop with STEP 0 to create an infinite loop that is faster than using a GOTO. He also removes unnecessary spaces and combines lines together for faster execution. Heck, he even knows about using a period as a faster way to make zero!

My PET experience

I’ve actually written programs for the PET. My high school had some, and I wrote a banner printing program for my typing teacher. She’d already written PRINT statements for each large letter based on needlepoint patterns, but there wasn’t enough memory in the PET to load them at the same time.

I solved this problem by having a main program input the message to print, then POKEing it to screen memory along with a “which character are we printing” counter. If the character to print was not in the current program, it would load and run the one that contained it. Those programs would then PEEK that screen memory to see what character it was supposed to print next.

A klunky solution, but it worked. (And in case you wondered why I used screen memory… I did not have a PET memory map, so I didn’t know where I could store the message data. I knew I could find screen memory by printing something on the screen and then PEEKing through memory until I found it. Once I located where the screen was stored, I used that for temporary memory. Funny enough, I did the same thing years later for my *ALLRAM* BBS when I needed a safe spot to temporarily load and execute PCLEAR 0 code.)

But I digress… Again.

From Commodore PET to Radio Shack CoCo

Inspired by the video, I decided to port the game over to Color BASIC. This involved a few things:

  1. Screen Size: The PET screen is 40×25, so I had to scale everything down to fit on the CoCo’s 32×16 screen.
  2. PETASCII: The game uses two special characters which are not part of the standard ASCII that the CoCo has. I substituted with a forward slash (“/”) and a back slash (“\”). It doesn’t look as maze-like as the PET version, but it is the same general idea. There is also a PETASCII character that you print to clear the screen. I replaced that with CLS.
  3. Input: Commodore uses GET instead of INKEY$. I changed “GET A$” to “A$=INKEY$”.
  4. Memory Map: The game uses POKE and PEEK to put the racer on the screen, and check to see if it hit something. I had to change the PET memory location to be 1024, the start of the CoCo screen. I was surprised to hear him say the Commodore 64’s screen also starts at 1024.
  5. Spaces: Commodore BASIC has the SPC() keyword which prints that many spaces. Color BASIC has TAB() but it moves to columns, so it couldn’t be used. I created an S$ of 32 spaces using STRING$() and then used MID$() to get just the number of them I needed. This is likely much slower than if I had SPC().
  6. Controls: The PET has a numeric keyboard, so the PET version used 4 for left, and 6 for right. The CoCo has arrow keys, so I changed it to use the left and right arrow keys. It now looks for CHR$(8) for left and CHR$(9) for right. Parsing CHR$ is slower than the original which looked for “4” and “6”.
  7. Random Differences: I thought I was going to have to change the random numbers. The Commodore version of RND would return a value from 0 to 1. You would then multiply the result by a number (X) which gave you a number from 0 to X. On the CoCo, doing RND(X) would return 1 to X. But, I realized RND(0) on the CoCo would do the same thing, so I ended up not changing it.
  8. Key Repeat: The Commodore has key repeat and a small typeahead buffer. At the end of the PET code there was a POKE to clear out that buffer so it wouldn’t instantly restart the game if key presses were still in the buffer. I removed that POKE since the CoCo has no such buffer.

Here is what I came up with:

CoCo port of 8-Bit Show And Tell’s “10 PRINT RACER” BASIC game.

And here is the code:

0 ' 10 PRINT RACER
1 ' BY WWW.8BITSHOWANDTELL.COM
2 '
3 ' PORTED FROM PET TO COCO
4 ' BY SUBETHASOFTWARE.COM
5 R$="":CLS:PRINT"INIT:";:FORX=1TO75:M$=CHR$(47+45*(RND(2)-1)):R$=R$+M$:PRINTM$;:NEXT
6 S$=STRING$(32," ")
10 CLS:C=16:R=10:W=12:D=0:S=1024
20 L=0:FORZ=0TO1STEP0:X=RND(.)*10
30 IFX<4THENR=R-1:IFR<1THENR=1
40 IFX>5THENR=R+1:IFR+W>29THENR=29-W
50 RN=RND(.)*28+1:PRINTMID$(R$,RN,R);MID$(S$,1,W);MID$(R$,RN,31-R-W)
60 D=D+1:L=L+1:IFL>49THENL=0:W=W-1:IFW<3THENW=3
70 IFD<16THENNEXT
75 A$=INKEY$:IFA$=CHR$(8)THENC=C-1
80 IFA$=CHR$(9)THENC=C+1
90 P=PEEK(S+C):IFP<>96THEN200
100 POKES+C,106:NEXT
200 PRINTTAB(13)"CRASH!":IFD>H THENH=D
205 PRINTTAB(6)"SCORE:"D"  HIGH:"H
210 FORX=1TO2000:NEXT:A$=INKEY$
220 A$=INKEY$:IFA$=""THEN220
230 GOTO10

I tried to keep the code as close to the original as I could, so there are some more optimizations that we could do on the CoCo. For instance:

  • Instead of using “RND(0)*10” for a value of 0-9, it might be faster to do RND(10) and adjust the IF/THEN to look for 1-10 instead of 0-9. This would save the overhead of doing the multiplication.
  • If Extended BASIC can be used, then most constants can be changed to HEX values and they will be slightly faster. (Thought after seeing some code from Jim Gerrie recently, it may still be faster to parse some single digits as decimals over their HEX version. More on this in a future article.)
  • Line 30 and 40 could be combined using ELSE, saving time each time the value is less than 5. Currently, if X is 1, it processes line 40 and then goes to line 50 which checks X again. Using ELSE would at least omit that step in that condition.
  • The screen position variable (S) gets the car position (C) added to it each time it is POKEd or PEEKed. To save that math, the position of the car could be initialized as S+C and the two “+C”s in the code could be removed.
  • There is a special IF in line 70 that is used to initially draw the screen before letting the car drive on it. Once the screen is drawn, it still checks this IF every time through. Time could be saved by drawing the initial screen outside of the main loop and then not needing to check that again.

What else do you see that might help speed things up? Please leave your comments, or take this code and see what you can do with it.

PET emulator online

If you never got the pleasure of using one of these early Commodore Business Machines, I found a PET emulator that runs from a web browser:

https://www.masswerk.at/pet/

You can use that to type in the original Commodore “10 PRINT” program and see it run it all its glory:

Commodore PET running the 10 PRINT program.

Until next time…