6502 Assembly programming for the Vic 20

The predecessor to the C64, the Vic-20 is an early 8 bit system, based around character graphics, it's far more limited than the C64, but still capable of some decent results, and is far better than the ZX-81 of the time.

Due to it's memory limitations, we won't be using the Vic-20 as much as some other systems, but there's still plenty of impressive stuff this early machine can do!

Cpu	1 mhz 6502
Ram	5k
Resolution	176184 max (2223 chars)
Sound	3 tone + 1 noise

Useful Documents

Programmers Reference Guide

Rom Disassembly

ChibiAkumas Tutorials

	Lesson P8 - Bitmap Functions on the VIC-20
	Lesson P16 - Joystick Reading on the VIC-20
	Vic 20 - ASM PSET and POINT for Pixel Plotting - 6502 Lesson Photon7
	Lesson S22 - Sprite Clipping on the VIC-20
	Lesson P29 - Sound on the on the VIC-20
	Lesson P50 - Sound on the VIC-20 (ChibiSound Pro)

Memory Map

Address	Purpose
$0000-$01FF	ZeroPage/Stack etc
$0200-$03FF	OS memory
$0400-$0FFF	3K RAM Upgrade
$1000-$1DFF	User Memory (screen memory with extra ram)
$1E00-$1FFF	Screen Memory
$2000-$7FFF	Rom area
$8000-$8FFF	Character Rom
$9000-$900F	VIC Registers
$9110-$911F	VIA 1 (Timer 1 causes NMI)
$9120-$912F	VIA 2 (Timer 1 causes IRQ)
$9400-$95FF	(Character colors with extra ram)
$9600-$97FF	Character Colors
$A000-$BFFF	Cartridge ROM
$C000-$DFFF	Basic ROM
$E000-$FFFF	Kernel ROM

Text Graphics
The characters shown onscreen are selected by the bytes in the memory range $1E00-$1FFF, the colors of the tiles are selected per 8x8 square, from the registers at $9600-$97FF

Character Map
The VIC does NOT use Ascii... as we don't have enough ram for a custom font, we're going to have to learn how to convert the charmap...
if we set the Character memory to $1C00 the Ascii set is offset by 128, and the inverted characters are lost... the result is the @ symbol is now character 128, and 0-127 are the custom characters

VIdeo Registers
The VIC20 has a range of video registers to set up it's screen

Address	Suggested Value	Meaning
$9000	$0C	horizontal centering
$9001	$26	vertical centering
$9002	$96 ($16)*	set # of columns, bit 7 is part of screen address
$9003	$AE	set # of rows
$9004	$7A	TV raster beam line
$9005	$FF ($CF)*	bits 0-3 start of character memory / bits 4-7 is rest of video address
$9006	$57	horizontal position of light pen
$9007	$EA	vertical position of light pen
$9008	$FF	Digitized value of paddle X
$9009	$FF	Digitized value of paddle Y
$900A	$00	Frequency for oscillator 1 (low)
$900B	$00	Frequency for oscillator 2 (medium)
$900C	$00	Frequency for oscillator 3 (high)
$900D	$00	Frequency of noise source
$900E	$00	bit 0-3 sets volume of all sound / bits 4-7 are auxiliary color information
$900F	$66+8	Screen and border color register

* Alternate settings to position screen base at $1000 and color base to $9400

Screen Colors
border and background color are set by memory address $900F... it's format is BBBISSS
Bits 0-2 (S) set the Screen background
Bit 3 sets Inverted or Normal mode - effectively this is usually on (so add 8 to all values)

Colors are defined as 0-7, as shown in the chart below... note, only color 0-7 can be used for background and border colors

In multicolor mode, the 'Auxilary' color is defined by the top 4 bits of $900E

Background colors
border and background color are set by memory address $900F... it's format is BBBISSS
Bits 0-2 (S) set the Screen background
Bit 3 sets Inverted or Normal mode - effectively this is usually on (so add 8 to all values)
Bits 4-7 set background color

The effective options are:

	Border
Background	Black	White	Red	Cyan	Purple	Green	Blue	Yellow
Black	08	09	0A	0B	0C	0D	0E	0F
White	18	19	1A	1B	1C	1D	1E	1F
Red	28	29	2A	2B	2C	2D	2E	2F
Cyan	38	39	3A	3B	3C	3D	3E	3F
Purple	48	49	4A	4B	4C	4D	4E	4F
Green	58	59	5A	5B	5C	5D	5E	5F
Blue	68	69	6A	6B	6C	6D	6E	6F
Yellow	78	79	7A	7B	7C	7D	7E	7F
Orange	88	89	8A	8B	8C	8D	8E	8F
Light Orange	98	99	9A	9B	9C	9D	9E	9F
Pink	A8	A9	AA	AB	AC	AD	AE	AF
Light Cyan	B8	B9	BA	BB	BC	BD	BE	BF
Light Purple	C8	C9	CA	CB	CC	CD	CE	CF
Light Green	D8	D9	DA	DB	DC	DD	DE	DF
Light Blue	E8	E9	EA	EB	EC	ED	EE	EF
Light Yellow	F8	F9	FA	FB	FC	FD	FE	FF

Bitmap Graphics

The Vic-20 has no bitmap graphics! however it can use a custom character set, so we can effectively fake 8x8 'tiles' using this option
the low nibble of $9005 defines the position of the characters, though some options are unusable, here are the options, and where in memory the characters will be read.
the high nibble is used to define the video address, so should not be altered... it's default value is $F , but you can read it in, and OR in your new character source

The best position for it is at $1C00... and example of how to do this is shown to the right.

This will give you 128 definable characters between 0-127... the normal A-Z char map will start from character 128

    lda $9005
    and #$F0
    ora #$0F
    sta $9005    ;Set character Ram Location ----LLLL

$9005 Low Nibble	value in bits	Type	Hex addr	Dec Addr
0	0000	ROM	$8000	32768
1	0001	ROM	$8400	33792
2	0010	ROM	$8800	34816
3	0011	ROM	$8C00	35840
8	1000	RAM	$0000	0
9	1001	N/A	xxxx	xxxx
A	1010	N/A	xxxx	xxxx
B	1011	N/A	xxxx	xxxx
C	1100	RAM	$1000	4096
D	1101	RAM	$1400	5120
E	1110	RAM	$1800	6144
F	1111	RAM	$1C00	7168

Color Map
The color map contains one byte per character, and defines the foreground color... the character colors is defined from $9600 onwards

Multicolor mode
Multicolor mode can be enabled for a screen character by adding 8 to its color number in the memory area $9600+
in multicolor mode, horizontal resolution is halved, and the color of the tile will be defined depending on it's bitpair...

The source of the color depends on the bit combination - 3 of the colors are screen-wide... only one is 'per tile'

Bits	Source	Address
00	Screen Color	$900F CCCC----
01	Border Color	$900F ----CCCC
10	Character color	$9600 - $97FF
11	AuxColor	$900F CCCC----

Interrupt Vectors

The standard 6502 interrupt vectors from $FFFA+ are ROM, however these jump to vectors in low memory addresses. IRQ and BRK interrupts push A,X and Y onto the stack in that order.

Timer 1 on VIA 1 causes an NMI interrupt
Timer 1 on VIA 2 causes an IRQ interrupt

Note: It seems there is no VBLANK interrupt on the VIC20!

From	To	Function	Registers Pushed
$0314	$0315	INT	A X Y
$0316	$0317	BRK	A X Y
$0318	$0319	NMI

Sound

There are 3 sound channels for the 3 different frequencies, one for random noise, and a volume setting...
As well as volume The top 4 bits of the $900E also handles color

Address	Meaning	Bits	Details
$900A	Frequency for oscillator 1 (Bass)	OFFFFFFF	O=On F=Frequency
$900B	Frequency for oscillator 2 (medium)	OFFFFFFF	O=On F=Frequency
$900C	Frequency for oscillator 3 (high freq)	OFFFFFFF	O=On F=Frequency
$900D	Frequency of noise source	OFFFFFFF	O=On F=Frequency
$900E	Volume of all sound / Auxiliary color information	CCCCVVVV	V=Volume C=Aux color

File Header

If we're creating a PRG file which we're going to run on the VIC, we need a 'basic header', this is effetively a 'launcher' for the program... our code immediately follows.	* = $1001 ; BASIC program to boot the machine language code db $0b, $10, $0a, $00, $9e, $34, $31, $30, $39, $00, $00, $00
If we want to create a ROM cartridge, we can do this with a simple header... however there is a problem... the screen will not be set up, so we'll get a black screen until we set up the screen properly.	* = $A000 dw ProgramStart dw ProgramStart db $41,$30,$C3,$C2,$CD ;ROM Header ProgramStart:

VIAs

Address	Meaning	Bits	Details
$9110	VIA1: Port B output register
$9111	VIA1: Port A output register
$9112	VIA1: Data direction register B	DDDDDDDD	Direction 0=read 1=write
$9113	VIA1: Data direction register A	DDDDDDDD	Direction 0=read 1=write
$9114	VIA1: Timer 1 low byte	LLLLLLLL	Set Timer (60 hz) (Timer1=NMI)
$9115	VIA1: Timer 1 high byte & counter	HHHHHHHH	Set Timer (60 hz)
$9116	VIA1: Timer 1 low byte	LLLLLLLL
$9117	VIA1: Timer 1 high byte	HHHHHHHH
$9118	VIA1: Timer 2 low byte	LLLLLLLL
$9119	VIA1: Timer 2 high byte	HHHHHHHH
$911A	VIA1: Shift register	SSSSSSSS
$911B	VIA1: Auxiliary control register	1O2SSSPP	1=T1 Control PB7 Out / O=One shot free run / 2=T2 control PB6 sense / S=Shift register control / P=PB PA Latch control
$911C	VIA1: Peripheral control register	BBBbAAAa	B=CB2 (Pup) control in/out / b=CB1 in casette #2 / A= CA2 graphics lowecase in out / a=CA1 in polarity .... 12=Graphic
$911D	VIA1: Interrupt flag register	ITtBBSAa	I=IRQ status / T=T1 interrupt / t=T2 interrupt / B=CB1 Casette / S=SR Interrupt / A=CA1 Interrupt / a=CA2 Interrupt ... Write 255 to clear
$911E	VIA1: Interrupt enable register	ETtBbSAa	E=Enable or disable following ints / T=T1 interrupt / t=T2 interrupt / B=CB1 interrupt / b=CB2 Interrupt / S=SR interrupt / A=CA1 Interrupt / a=CA2 interrupt
$911F	VIA1: Port A	R-------	Joystick Right
$9120	VIA2: Port B output register	--FLDU--	Joystick Fire,Up, Down, Left
$9121	VIA2: Port A output register
$9122	VIA2: Data direction register B	DDDDDDDD	Direction 0=read 1=write
$9123	VIA2: Data direction register A	DDDDDDDD	Direction 0=read 1=write
$9124	VIA2: Timer 1 low byte	LLLLLLLL	Set Timer (60 hz) (Timer1=IRQ)
$9125	VIA2: Timer 1 high byte & counter	HHHHHHHH	Set Timer (60 hz)
$9126	VIA2: Timer 1 low byte	LLLLLLLL
$9127	VIA2: Timer 1 high byte	HHHHHHHH
$9128	VIA2: Timer 2 low byte	LLLLLLLL
$9129	VIA2: Timer 2 high byte	HHHHHHHH
$912A	VIA2: Shift register	SSSSSSSS
$912B	VIA2: Auxiliary control register	1O2SSSPP	1=T1 Control PB7 Out / O=One shot free run / 2=T2 control PB6 sense / S=Shift register control / P=PB PA Latch control
$912C	VIA2: Peripheral control register	BBBbAAAa	B=CB2 (Pup) control in/out / b=CB1 in casette #2 / A= CA2 graphics lowecase in out / a=CA1 in polarity .... 12=Graphic
$912D	VIA2: Interrupt flag register	ITtBBSAa	I=IRQ status / T=T1 interrupt / t=T2 interrupt / B=CB1 Casette / S=SR Interrupt / A=CA1 Interrupt / a=CA2 Interrupt ... Write 255 to clear
$912E	VIA2: Interrupt enable register	ETtBbSAa	E=Enable or disable following ints / T=T1 interrupt / t=T2 interrupt / B=CB1 interrupt / b=CB2 Interrupt / S=SR interrupt / A=CA1 Interrupt / a=CA2 interrupt
$912F	VIA2: Port A	AAAAAAAA