I'd like to upgrade my Macintosh II to FDHD, I have a SWIM chip (to replace the WIM chip), but don't know how to burn the ROMS.

The upgrade process I'm trying to do is described in the book: "Macintosh If Repair and Upgrade Secrets"

Installing an FDHD/ROM Upgrade into a Macintosh II

The official Apple FDHD Macintosh II Upgrade Kit, pan number M0244, provides the five chips you need to get a 1.4 MB super drive working in an original Macintosh II. Table 8-2 provides a parts list with board references for various revisions of the Macintosh 11 logic board. Figure 8-3 shows the general area where the five chips are located.

ROM/SWIM Locations.

The current chip part numbers and board locations are

Location Apple Part Number
U3 342-0108-B
U4 342-0107-B
U5 342-0106-B
U6 342-0105-B

I'm not sure what type of EEPROMS are compatible, and how the image needs to be split across the 4 chips. The ROM chips I'm attempting to replace are

I've tried looking around for previous attempts at this online, but haven't found any documentation about compatible EEPROMS or how to map the ROM file onto the 4 chips.

  • 1
    It might be a good idea to name (and specify) the ROMs you want to have replaced, as there are several. Don't forget to include the existing version(s).
    – Raffzahn
    Jan 23, 2021 at 0:45
  • 1
    Macintosh II ROM Type
    – user9041
    Jan 23, 2021 at 4:30
  • 1
    Macintosh II Repair and Upgrade Secrets (PDF, 47 MB).
    – user9041
    Jan 23, 2021 at 4:41

1 Answer 1


I'm not very familiar with Macintosh, so my answer is based on my general knowledge of the Motorola 68K architecture (like the 68020 that the Mac-II is based on).

ROM content

In the Repair and Upgrade guide (page 197), the ROMs are refered to as

  • U3: Low ROM
  • U4: Medium Low ROM
  • U5: Medium High ROM
  • U6: High ROM

As the Mac-II is a 32-bit "big-endian" machine this would mean U3 contains bits 0-7 of every 32-bit word, U4 contains bits 8-15, U5 contains bits 16-23 and U6 contains bits 24-31.

I'm not familiar with the way that Mac ROMs are distributed, but if you have a single ROM image file you should split it into 4 parts, with every first 8-bit byte going to the "low" ROM, every second byte going to the "medium low" ROM, every third byte going to the "medium high" ROM and every fourth byte going to the "high" ROM.

Original EPROMS

The first link in the comments together with the images of the motherboard indicate the original ROM chips are not EEPROMs but UV Erasable EPROMs of the 27XXX type. As the Mac-II ROM is 256KB in total, the ROMs should be 64KB each, indicating the EPROMs should be 27512s.

Replacement (E)EPROMs

If you buy replacement 27C512s, make sure you get the "windowed" type as they can be erased and reprogrammed. UV Erasable EPROMS can only be reprogrammed after having been exposed to strong UV (-C) light.

If you don't have a UV eraser that means that you get one shot at programming them correctly.

In this link it's suggested you could replace the 27XXXs with 28CXXX or 29CXXX with some simple modifications.

However, replacement with Flash or EEPROM might not be easy, as any suitable replacement Flash (29C512) or EEPROMs (28C512) I've found have 32 pins and are not a direct replacement for a 27512, which has 28 pins.

Replacement with electrically erasable 28/29 Flash/EEPROMs is technically certainly possible as most of the pins match with the "old" 27 type EEPROM, but you might have to build a separate custom PCB for this.

  • 1
    I ended up buying some STM27C512's off of ebay, wrote a script to put every 4th byte in one of four files like you described, burned the chips plugged them in and was rewarded with a mac that boots. For anyone following the same steps the locations ended up being: U6-Low, U5-Med Low, U4-MedHigh, U3-High Jun 25, 2021 at 19:02
  • 1
    Nice you got your Mac to boot! The ROM desginations seem to be exactly the reverse of what is in my answer.
    – StarCat
    Jun 25, 2021 at 19:34
  • Yeah when I first put them in I ordered the roms with the labels on the board, it didnt work, so I looked at the apple part numbers on the original chips and found that they were descending so I reordered the chips and it booted right up. Jun 25, 2021 at 19:37
  • Now I'm trying to make an adapter for a PLCC28 to PLCC44 so I can get a SWIM into it. The 44 pin ones can still be found on utsource.net but I havent managed to find anyone selling a 28 pin one Jun 25, 2021 at 19:38

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