Vic 20 Replace BASIC and Kernal ROM’s with 27128 EPROM I had a Vic 20 with a faulty BASIC ROM and thought it would be nice to combine the BASIC and Kernal ROM’s into one. This gives you the ability to modify the standard ROM's. The conversion is pretty simple. The only issues are the two chip select lines that need to be combined into one, and the Address 13 line to select the upper and lower 8k blocks of the EPROM. An AND gate will combine the chip selects and connecting A13 to the BASIC ROM chip select will ensure that the correct ROM is available when required. Vic 20 Memory Map HEX DEC USAGE 8k 1k 0000-03FF 0000-1023 System variables BLK 0 RAM 0 0400-0FFF 1024-4095 3K expansion RAM area B0 RAM 1,2,3 1000-1DFF 4096-7679 User Basic area B0 RAM 4-7 1E00-1FFF 7680-8191 Screen memory BLK 0 RAM 7 2000-3FFF 8192-16383 8K expansion RAM/ROM BLK 1 4000-5FFF 16384-24575 8K expansion RAM/ROM BLK 2 6000-7FFF 24576-32767 8K expansion RAM/ROM BLK 3 8000-8FFF 32768-36863 4K Character ROM BLK 4 9000-93FF 36864-37887 I/O block 0 BLK 4 9400-95FF 37888-38399 COL RAM,BLK 1 filled BLK 4 9600-97FF 38400-38911 Normal COLOR RAM BLK 4 9800-9BFF 38912-39935 I/O block 2 BLK 4 9C00-9FFF 39936-40959 I/O block 3 BLK 4 A000-BFFF 40960-49152 8K Expansion ROM BLK 5 C000-DFFF 49152-57343 8K BASIC ROM BLK 6 E000-FFFF 57344-65535 8K Kernal ROM BLK 7
UD5 or UD11 (901486-01)
Acronyms /OE Output Enable (set low to enable data output from chip) /CE Chip Enable (set low to enable data output from chip) /CS Chip Select (same as Chip Enable) /PGM Program Mode (set low to program EPROM) Vpp Voltage used to program EPROM (usually 12.5 or 21V) BASIC ROM socket to 27128 A0-A12 D0-D7 Vcc to Vcc GND to GND 27128 Pin 1 (Vpp) to Vcc Pin 20 (/CE) to GND Pin 27 (/PGM) to Vcc 74LS08 Pin 7 to GND PIN 14 to Vcc Pin 1 to pin 26 (A13) of 27128 Pin 1 to pin 20 (/CE) of BASIC ROM socket (BLK 6) UD5 or UD11 Pin 2 to pin 20 (/CE) of Kernal Rom Socket (BLK 7) UD6 or UE12 Pin 3 to pin 22 (/OE) of 27128 The chip select lines could also be taken from the 74LS138 (UB1 or UC5) or various other points on the Vic 20 motherboard.