VIC-20

The Commodore VIC-20 (1980) was Commodore's first million-seller and the little sibling of the C64. It uses the same MOS 6502 CPU, runs the same Commodore BASIC V2, and uses a similar Kernal — but the hardware is quite different, and RAM is very tight without expansion.

Don't underestimate it. The VIC-20 has a passionate community, and its constraints make programming it a genuine puzzle worth solving.

Languages in the IDE

Language	Use for
Commodore BASIC V2	Quick experiments, small programs
CC65 C	Structured programs with cc65's VIC-20 target

Hardware overview

CPU — MOS 6502A at 1.108 MHz

Essentially the same 6502 as the C64, running slightly faster. The same assembly instructions work on both — but the hardware around it is completely different.

VIC chip — Video

The MOS 6560/6561 (the "VIC" chip the machine is named after) is simpler than the C64's VIC-II:

Text mode: 22 columns × 23 rows of characters (not 40×25)
Graphics mode: accessible via the VIC chip's registers
Colour: 8 colours + 8 bright variants for foreground; 8 for background
No hardware sprites — sprites must be software-drawn
Sound: 3 square wave channels + 1 noise channel (built into the VIC chip)

Key VIC registers at $9000–$900F:

Address	Register
$9000	Interlace, screen origin X
$9001	Screen origin Y
$9002	Video columns (bits 0–6), screen memory bit 8 (bit 7)
$9003	Video rows (bits 1–6), double height (bit 0)
$9005	Screen and charset memory address
$900E	Auxiliary colour and sound volume
$900F	Screen colour and border colour

RAM — the tight part

Unexpanded VIC-20: only 5 KB of RAM total, of which BASIC gets about 3.5 KB after system overhead.

That sounds brutal, but people have written impressive games in 3.5 KB. With memory expansion:

Expansion	Free BASIC RAM
None	~3.5 KB
3K expansion	~6.5 KB
8K expansion	~11.5 KB
16K+ expansion	~27+ KB

The IDE emulates the unexpanded machine by default. Check your preset's settings if you need expansion RAM.

Memory map (unexpanded)

Range	Contents
$0000–$00FF	Zero page
$0100–$01FF	Stack
$0200–$03FF	System workspace
$0400–$07FF	Unmapped (colour RAM mirrors here)
$1000–$1DFF	User RAM (main BASIC area, ~3.5 KB)
$1E00–$1FFF	Screen character RAM (22×23)
$8000–$8FFF	Character ROM
$9000–$93FF	VIC chip registers
$9400–$97FF	Colour RAM
$A000–$BFFF	BASIC ROM
$C000–$FFFF	Kernal ROM

Notice that the user RAM starts at $1000 — not $0800 like the C64. And the screen is at $1E00, not $0400.

Screen differences from C64

Feature	C64	VIC-20
Character columns	40	22
Character rows	25	23
Screen RAM	$0400	$1E00
Colour RAM	$D800	$9400
Border/bg colour	$D020/$D021	$900F (combined byte)

The border and background colour share one register on the VIC-20:

POKE 36879, 27   : REM BORDER=WHITE (high nibble), BG=BLUE (low nibble)
                 : REM High nibble 0-7 + bright: 8*bright + colour

The combined byte format: bits 7–4 = border colour, bits 3–0 = background colour.

Writing to the screen

With only 22 columns, the calculation changes:

RO% = 5 : CO% = 3
POKE 7680 + RO%*22 + CO%, 65    : REM 7680 = screen RAM base ($1E00)
POKE 38400 + RO%*22 + CO%, 7    : REM 38400 = colour RAM base ($9600)

Wait — the colour RAM base address ($9600 = 38400) is different from most references. The VIC-20's colour RAM is at $9400, but the accessible colour RAM for the default screen position starts at $9600. Always verify with your specific memory configuration.

Sound

The VIC chip includes a simple sound section: 3 square-wave channels and 1 noise channel. Volume is controlled by the low nibble of $900E.

10 POKE 36878, 15   : REM MAX VOLUME ($900E bits 0-3)
20 POKE 36876, 200  : REM VOICE 3 FREQUENCY (high freq channel)
30 FOR I=1 TO 500 : NEXT
40 POKE 36876, 0    : REM VOICE 3 OFF

The VIC chip's sound is much simpler than the C64's SID — but it has its own charm.

Programming tips for the VIC-20

Save every byte — with 3.5 KB, every REM statement costs memory. Delete them from finished programs (not while developing, though!).

Use short variable names — in Commodore BASIC V2, only the first two characters of a variable name matter anyway. S% not SCORE%.

Use integer variables everywhere — % variables use 2 bytes; float variables use 7 bytes. On a 3.5 KB machine, the difference matters.

Watch the column count — 22 columns means printing calculations are different. TAB(23) wraps to the next line.

Expansion RAM opens the machine up — if you can emulate or use an 8K expansion cart, the VIC-20 becomes far more capable.