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ROM updates & working with EPROMs

The ROM's in the C64 and the original 1541 models are 24 pin and are not directly pin compatible with burnable 2732 or 2764 EPROMS. In order to upgrade to a newer ROM revision or change the ROM to something else and use either a 2732 (for the character set) or 2764 (for kernal/DOS) EPROM, a socket adapter must be built. See here for detailed instructions on how to build one.

The speed of the EPROM is not so critical. The Commodore chip specs for the 2332 and 2364 PROM's show them as slow as 450ns so almost any EPROM faster than 450ns will so. I generally go with 200-250ns as they were very common.

To erase an EPROM you will need a UV eraser. Typically EPROMs need about 3 minutes to erase. You can check them on the burner to see if they are indeed erased and if not, re-erase them for 1 minute increments, checking after each erasure session until they show erased.

To burn an EPROM requires an EPROM programmer. I will not attempt to document how to burn an EPROM as each programmer is unique and the instructions vary greatly. The one I use supports the Motorola S19 format, a text-only format with checksums.

Some notes about EPROMs and burners:

ROM Chart

The ROMs below are the latest versions for the C64's and drives. The BINARY files are raw format images (without load addresses) and the S19 files are also provided if you want to burn them and you have an EPROM programmer that supports the S19 format.

Model Type Size Replacement
ROM Number Download Format
2732 + socket adapter
2764 + socket adapter
2764 + socket adapter
Binary or S19
Binary or S19
Binary or S19
C64C BASIC & KERNAL 16Kb 27128 251913-01 Binary or S19
1541 DOS
2764 325302-01
Binary or S19
Binary or S19
2764 + socket adapter
2764 + socket adapter
for the 1541
for the 1541
Binary or S19
Binary or S19
Binary or S19
1541C DOS & KERNAL 16Kb 27128 251968-02 Binary or S19
1571 DOS & KERNAL 32Kb 27256 310654-05 Binary or S19

* The 8Kb split versions of the 1541-II ROM's are for use in an older 1541. Apparently, you can use these in the older drives and it makes the drive more reliable as most of the save-@ and REL bugs are gone. I have not verified this yet.

ROM & EPROM Pinouts

These charts show the pinouts of the various EPROMs that are common on Commodore equipment.

2316 PROM pinout

2716 EPROM pinout

2316, 2Kb PROM
2716, 2Kb EPROM

2332 PROM pinout

2732 EPROM pinout

2332, 4Kb PROM
2732, 4Kb PROM

2364 PROM pinout

2764 EPROM pinout
2364, 8Kb PROM
2764, 8Kb EPROM

27128 EPROM pinout
27256 EPROM pinout
27512 EPROM pinout
27010 EPROM pinout
27128, 16Kb EPROM
27256, 32Kb EPROM
27512, 64Kb EPROM
27010, 128Kb EPROM

The 2332 is a 4Kb PROM and is the same pinout at the 24-pin ROM used for the character generator in the C64. The 2316 and 2716 are 2Kb ROM's but are not common on Commodore equipment. I've included them for completeness.

The 2364 is an 8Kb PROM and is the same pinout as the 24-pin ROMS used in the original C64 and 1541, like 901227-0x (KERNAL in the C64) or 901229-0x (KERNAL in the 1541). In order to use a 2764 in place of one of these ROMS, an adapter socket is needed. The instructions to build one are here.

The pinout differences between the 2764 to 27512 are not extensive, mainly extra address lines that need to be dealt with. This makes installing larger EPROMs easy to do as you only need to deal with switching in/out the extra address lines.

The 27010 is a 128Mb EPROM which is in a 32 pin package. This makes it a little more difficult to deal with as you need a larger socket and there are more pins/wires to jumper.

Email the author: Peter Schepers | Last updated: July 9, 2020