VIC-20 joystick - which is the real way?

[allenhuffman]

I recently obtained a copy of my first-ever published program (in a newsletter, heh) and found it odd I chose to use "Up" and "Down" on the joystick to move left and right. I did some research and discovered the routine I was using was incorrect, and never read the byte that contained the "right" direction. I must not have been able to get it to work, and just switched to using up and down, since those read for me.

I tried to figure out what magazine I got my joystick code from, and the only hit in Archive.org was some Toronto Pet (The TORPET) newsletter. It was so early in the VIC-20 era that they had an article mention that "15,000" had been sold so far Wild. Here is the VIC-20 column I found (page 31, or 46 in the PDF):

https://archive.org/details/TORPET_The_ ... 0++4+%2B32

And their code:

Code: Select all

9000 POKE 37154,127
9010 JO= (NOT((PEEK(37152)AND128) /8+( PEEK(37151)AND60)/4)) +32
9020 POKE 37154,255
9030 RETURN

This was identical to the code I had found, except they used 37152 and 37151 and my program had a typo where it was using 37152 twice.

BUT, as I searched to figure out the problem, I found an Atari Magazine reference to reading these two values:

Code: Select all

10 PRINT PEEK(37137), PEEK(37152) : GOTO 10

Those two match what I'd expect from the memory map. I see that 37137 is "Port A output register" and 37152 is "Port B output register".

37151 is listed as "Port A (Sense cassette switch)" and I don't see anything about the joystick there.

BUT ... the code works when using these two locations, OR the 37151/37152 locations. My test:

Code: Select all

10 rem fixed torpet code
20 gosub 9000
30 if jo and 1 then print "up ";
40 if jo and 2 then print "down ";
50 if jo and 4 then print "left ";
60 if jo and 8 then print "fire ";
70 if jo and 16 then print "right";
80 if jo<>0 then print
90 goto 20
9000 poke 37154,127
9010 rem jo=(not((peek(37152)and128)/8+(peek(37137)and60)/4))+32
9015 jo=(not((peek(37152)and128)/8+(peek(37151)and60)/4))+32
9020 poke 37154,255
9030 return

I have found examples showing the use of both sets of locations. The wiki here shows it that way as well:

http://sleepingelephant.com/denial/wiki ... p/Joystick

Code: Select all

1 S=PEEK(37151):POKE37154,127:T=PEEK(37152):POKE37154,255
2 IF (128ANDT)=0 THEN PRINT "RIGHT"
3 IF (16ANDS)=0 THEN PRINT "LEFT"
4 IF (8ANDS)=0 THEN PRINT "DOWN"
5 IF (4ANDS)=0 THEN PRINT "UP"
6 IF (32ANDS)=0 THEN PRINT "FIRE"
7 GOTO 1

I did not know much about VIC-20 hardware during the time I had one, but on my Radio Shack Color Computer I am aware that there were certain I/O registers that were ghosted, and would appear in multiple places. Is something like this happening here?

After finding so many places saying "use A and B" and other places saying "use A and C" and they both work, I'm a bit confused

Thanks, much.

[Soloman]

I saw a ping-pong-game with paddles. That is a step further in coding.

[Mike]

I did not know much about VIC-20 hardware during the time I had one, but on my Radio Shack Color Computer I am aware that there were certain I/O registers that were ghosted, [...]

... or mirrored ...

[...] and would appear in multiple places. Is something like this happening here?

Not here, in a certain sense. The addresses in question belong to the same 16 byte register block of VIA #1, $9110..$911F.

37137 ($9111) and 37151 ($911F) both contain the ORA, i.e. the port A data register, with the same meaning of the port bits. Their difference is that $9111 controls the data handshake, whereas $911F has no effect on the data handshake. For more details, please refer to one of the original data sheets.

I prefer the use of 37151 in a joystick routine simply because it is in closer proximity to the other two necessary registers in VIA #2, 37152 (ORB, $9120, contains the joystick right direction) and 37154 (DDRB, $9122, bit 7 - value 128 - temporarily set to input).

[allenhuffman]

I saw a ping-pong-game with paddles. That is a step further in coding.

I remember using my Uncle’s Atari VCS (as it was known back then) joystick in my VIC, but I never owned a joystick for it. I do not recall ever playing any paddle games, or even realizing the VIC-20 could support the Atari paddles. I believe they were rotary encoders—was that even possible to use from BASIC on the VIC?

[allenhuffman]

I did not know much about VIC-20 hardware during the time I had one, but on my Radio Shack Color Computer I am aware that there were certain I/O registers that were ghosted, [...]

... or mirrored ...

[...] and would appear in multiple places. Is something like this happening here?

Not here, in a certain sense. The addresses in question belong to the same 16 byte register block of VIA #1, $9110..$911F.

37137 ($9111) and 37151 ($911F) both contain the ORA, i.e. the port A data register, with the same meaning of the port bits. Their difference is that $9111 controls the data handshake, whereas $911F has no effect on the data handshake. For more details, please refer to one of the original data sheets.

I prefer the use of 37151 in a joystick routine simply because it is in closer proximity to the other two necessary registers in VIA #2, 37152 (ORB, $9120, contains the joystick right direction) and 37154 (DDRB, $9122, bit 7 - value 128 - temporarily set to input).

I love how terms are sort of localized such as “ghosted” versus “mirrored”. When I had my VIC, I remember a kid in Houston showing me his VIC-20 “monitor” cartridge. On the CoCo, it was an “assembler” cartridge. I wish I had learned assembly/machine language earlier. I don’t think I touched it until 1984-85 on my CoCo’s 6809.

For the data sheet, which part is this going to? I’ll go find it.

And if I understand: Both officially and correctly have that I/O bit, but one is a R/W and the other is just a R?

[Merytsetesh]

I remember using my Uncle’s Atari VCS (as it was known back then) joystick in my VIC, but I never owned a joystick for it. I do not recall ever playing any paddle games, or even realizing the VIC-20 could support the Atari paddles. I believe they were rotary encoders—was that even possible to use from BASIC on the VIC?

Yes. $9008 (36872) gives paddle X and $9009 gives Y. Denial page all about paddles.