Memory Map

The VIC-20's 6502 Microprocessor can directly address 64KB of memory.

A stock VIC-20 has 5K RAM, and 20K ROM built-in. Furthermore 4K of the address range access I/O devices, including the VIC chip. The I/O area also contains 1K of 4-bit values serving as Color RAM.

A range of 256 bytes, with the base address also divisible by 256 is called a page.

The memory map is divided in 8 blocks, each 8K in length, and numbered 0 to 7.

	Onboard RAM		Expansion port		Onboard ROM
	I/O chips		Color RAM		Expansion Port I/O

Memory map

Block 0

1 KB low Memory RAM (built-in)

• $0000-$00FF: Zeropage
• $0100-$01FF: CPU Stack
• $0200-$03FF: KERNAL and BASIC working areas

3 KB open area in Block 0

• $0400-$07FF: 1 KB, accessed by RAM1 line
• $0800-$0BFF: 1 KB, accessed by RAM2 line
• $0C00-$0FFF: 1 KB, accessed by RAM3 line

4 KB Main RAM (built-in) in Block 0

• $1000-$1FFF: 4 KB, Main RAM

Blocks 1-3: 8 KB Expansion Blocks

• $2000-$3FFF: 8 KB, Expansion Block 1, accessed by BLK1 line
• $4000-$5FFF: 8 KB, Expansion Block 2, accessed by BLK2 line
• $6000-$7FFF: 8 KB, Expansion Block 3, accessed by BLK3 line

Block 4

4 KB Character ROMs

• $8000-$83FF: 1 KB uppercase/glyphs
• $8400-$87FF: 1 KB uppercase/lowercase
• $8800-$8BFF: 1 KB inverse uppercase/glyphs
• $8C00-$8FFF: 1 KB inverse uppercase/lowercase

4 KB I/O Blocks

I/O Block 0: VIC/VIA chips

• $9000-$900F: VIC Registers
• $9110-$911F: VIA #1 Registers
• $9120-$912F: VIA #2 Registers
• $9130-$93FF: Unused^[A]

I/O Block 1: Color RAM

• $9400-$97FF: Color RAM (1K of 4 bit nibbles)^[B]

I/O Blocks 2-3: Expansion port

• $9800-$9BFF: 1 KB, I/O Expansion 2, accessed by I/O2 line
• $9C00-$9FFF: 1 KB, I/O Expansion 3, accessed by I/O3 line

Block 5: 8 KB Expansion Area

• $A000-$BFFF: 8 KB, Expansion Block 5, accessed by BLK5 line. Often used for ROM cartridges. Allows autostart sequence.

Blocks 6 and 7: System ROMs

• $C000-$DFFF: 8 KB, Block 6, BASIC Interpreter ROM
• $E000-$FFFF: 8 KB, Block 7, KERNAL ROM
  • The top six bytes of the entire memory space are special to the 6502. They form three pairs which contain vectors:
    • 65530–65531/$FFFA–$FFFB: Non-maskable interrupt
    • 65532–65533/$FFFC–$FFFD: Reset vector; contains the address where the processor will start running on boot
    • 65534–65535/$FFFE–$FFFF: Interrupt request

Screen memory map

Following is the memory map for the default screen on an unexpanded Vic 20:

Screen Character Codes

Screen Colour Codes

Notes

A.

This memory range actually repeats the memory windows of the VIC and VIA chips. This is due to the VIC-20's physical design. The entire 1k region of $9000-$9400 sits on a chip select line, but when this line is active, only address lines A0-A3 are connected to the chips, and lines A4-A5 are decoded to select from the three chips. Ergo, $9130-$913F map to nothing (decoder output is unused), $9140-$914F maps to the VIC, $9150-$195F to VIA #1, etc.

B.

Color is mapped to $9400-95FF or $9600-$97FF depending upon a setting in the VIC registers. Typically, it is $9600-$97FF on an unexpanded or 3k expanded VIC, otherwise it is at $9400-95FF.