Commodore VIC-20 Memory Expansion

44 pin female edge at the computer. The pins function as described below:

Pin	Name	Description	Pin	Name	Description
1	GND	System ground	A	GND	System ground
2	CD0	Data bus bit 0	B	CA0	Address bus bit 0
3	CD1	Data bus bit 1	C	CA1	Address bus bit 1
4	CD2	Data bus bit 2	D	CA2	Address bus bit 2
5	CD3	Data bus bit 3	E	CA3	Address bus bit 3
6	CD4	Data bus bit 4	F	CA4	Address bus bit 4
7	CD5	Data bus bit 5	H	CA5	Address bus bit 5
8	CD6	Data bus bit 6	J	CA6	Address bus bit 6
9	CD7	Data bus bit 7	K	CA7	Address bus bit 7
10	/BLK1	8K decoded RAM/ROM block 1 ($2000 - $3FFF)	L	CA8	Address bus bit 8
11	/BLK2	8K decoded RAM/ROM block 2 ($4000 - $5FFF)	M	CA9	Address bus bit 9
12	/BLK3	8K decoded RAM/ROM block 3 ($6000 - $7FFF)	N	CA10	Address bus bit 10
13	/BLK5	8K decoded RAM/ROM block 5 ($A000 - $BFFF)	P	CA11	Address bus bit 11
14	/RAM1	1K decoded RAM block ($0400 - $07FF)	R	CA12	Address bus bit 12
15	/RAM2	1K decoded RAM block ($0800 - $0BFF)	S	CA13	Address bus bit 13
16	/RAM3	1K decoded RAM block ($0C00 - $0FFF)	T	/I/O2	I/O block 2 (located at $9600)
17	VR/W	Read/Write line from VIC chip (high-read, low-write)	U	/I/O3	I/O block 3 (located at $9C00)
18	CR/W	Read/Write line from CPU (high-read, low-write)	V	S02	Phase 2 System Clock
19	/IRQ	Interrupt request line to 6502	W	/NMI	6502 Non maskable interrupt
20	n/c	Not connected	X	/RESET	6502 Reset pin
21	+5V	+5 VDC	Y	n/c	Not connected
22	GND	System ground	Z	GND	System ground

Source: VIC-20 User Manual, VIC-20 Programmer's Reference Guide