wAx the VIC-20

By Michael Doornbos

April 19, 2024 - 4 minutes read - 696 words

Wax

Over the years, I’ve looked for an Assembler for the VIC-20. Sure, there are great cross-assemblers like Kick Assembler, but I wanted something that would run on the VIC-20 itself.

Is the VIC-20 a good machine on which to write assembly? Maybe not ON the machine. The 22-column screen is a bit limiting, but after spending so much time in Turbo Macro Pro on the C64, I wanted to see what I could do on the VIC-20 natively.

I came across a cool assembler by Jason Justian a few years ago called wAx.

Original Wax

This is a previous version of the cart. The current version has some improvements; we’ll be using the current version, called wAX2.

wAx is a set of machine language monitor tools for the Commodore VIC-20. It works as a BASIC extension, allowing assembly, disassembly, and a comprehensive set of debugging and memory management features, all integrated with the BASIC environment.

Is it weird to mix assembly and BASIC? I don’t think so. I think it’s cool. I can hear some of you screaming about it though.

10PRINT

Let’s do my favorite thing, 10 PRINT to get a feel for wAx.

10 S=6400
20 .A 'S' @L JSR $E09B
30 .A * LDA #205
40 .A * ADC #0
50 .A * JSR $FFD2
60 .A * BMI @L 
1000 .D 'S' * 
1010 SYS 6400

S=6400 sets a variable we’ll use as the start of our assembly code.

.A 'S' tells wAx to assemble the following code at the address stored in S.

@L is a label we can jump to. Labels are prefixed with @ in wAx and seem to be limited to a single character.

Calling JSR $E09B is the same as RND(0), but we’re not actually using the value. We’re just calling it to randomize the carry flag. It creates “enough” randomness for our purposes.

Fun to try: If you were to CLC before the ADC, you’d get the same number every time and it would just print 205 which is / in PETSCII.

Fibonacci in WAX

Let’s try something slightly more complex. How about a Fibonacci sequence?

5  REM FIBONACCI 10 WITH WAX2 ASSEMBLER
10 S=6400:M=6500:REM S=START, M=STORE
15 D=4096:C=37888: REM SCREEN ON EXPANDED VIC
17 I=11
20 .A 'S' LDA #0
30 .A  *  STA $FC
40 .A  *  LDA #1
50 .A  *  STA $FB
60 .A  *  LDX #0
65 .A  *  SED 
70 .A  *  @L LDA $FB
90 .A  *  STA 'M',X
100 .A  * STA $FD 
110 .A  * ADC $FC
120 .A  * STA $FB 
130 .A  * LDA $FD
140 .A  * STA $FC
150 .A  * INX
160 .A  * CPX #'I'
170 .A  * BNE @L
200 .A  * LDY #$00
210 .A  * @Q
220 .A  * LDA 'M',Y
230 .A  * JSR @P
240 .A  * LDA #32
250 .A  * JSR $FFD2
260 .A  * INY
270 .A  * CPY #'I'
280 .A  * BNE @Q
290 .A  * RTS
300 REM HEX PRINT
305 .A  * @P
310 .A  * PHA
320 .A  * AND #$F0
330 .A  * LSR A
340 .A  * LSR A
350 .A  * LSR A
360 .A  * LSR A
370 .A  * JSR @C
380 .A  * JSR $FFD2
390 .A  * PLA
400 .A  * AND #$0F
410 .A  * JSR @C
420 .A  * JSR $FFD2
430 .A  * RTS
500 .A  * @C
510 .A  * SED
520 .A  * CLC
530 .A  * ADC #$90
540 .A  * ADC #$40
550 .A  * CLD
560 .A  * RTS
1000 .D 'S' * 
1020 PRINT CHR$(147)
1030 SYS 6400
1035 PRINT:PRINT:PRINT
1040 .M 1964 196E

Here, we store the Fibonacci sequence in memory starting at M and print it to the screen in hex.

Starting in line 300, we have a subroutine to print the hex value of the accumulator so we can see how to use labels and subroutines in wAx.

Extra Credit

The documentation is excellent. To get a quick overview of the commands, type.? in wAx.

Can you modify the Fibonacci program to generate the first 20 Fibonacci numbers?

Have Fun!