The Mac IIcx was Apple’s first compact model in the Mac II series, essentially a Mac IIx with three NuBus expansion slots instead of six and a smaller power supply. Like the Mac II and IIx, it can only access 8 MB of memory under System 6 and earlier. You need to run some version of System 7 to have access to the 32-bit option and use the free Mode32 utility, which allows the IIcx to run in 32-bit mode so you can use more than 8 MB of memory.

Unlike the Mac II and IIx, the Mac IIcx uses standard off-the-shelf 30-pin SIMMs for memory expansion, and it supports up to 128 MB of RAM.

The modular design of the IIcx provides very easy access to the memory banks on the logic board. SIMM sockets are between the drive mounting assembly and the NuBus expansion cards.

When upgrading, be careful not to damage the SIMM sockets, especially when removing memory. Although the IIcx shipped with 1 MB of memory, odds are that any you buy today have already been upgraded beyond that point.

Mac IIcx page from Apple Memory Guide.

Looking at the motherboard with the connectors to the rear, you will see eight SIMM sockets. The four nearest the right side of the board are Bank A. The other four are Bank B. Memory must be installed in sets of four matching 30-pin SIMMs per bank, with RAM rated at 120ns or faster.

A 1 MB configuration, which is how it shipped from the factory, has four 256 KB SIMMs in Bank A; a 2 MB configuration has 256 KB SIMMs in all eight sockets.

A 4 MB setup has four 1 MB SIMMs in Bank A; Bank B is empty. For 5 MB, Bank A has 1 MB SIMMs and Bank B had 256 KB SIMMs.

The 8 MB configuration, which seems quite common and is the most System 6 and earlier can access, has 1 MB SIMMs in each of the 8 sockets.

Any configuration beyond 8 MB requires some version of System 7 and Mode32. You must select 32-bit addressing in the Memory control panel to address more than 8 MB of RAM.

For 16 MB, 17 MB, or 20 MB configurations, you must place four 4 MB SIMMs in Bank A. Leave Bank B empty for 16 MB, populate it with 256 KB SIMMs for 17 MB, or with 1 MB SIMMs for 20 MB.

To reach 32 MB, insert a 4 MB SIMM in each SIMM socket.

Although the Mac IIcx supports 16 MB SIMMs, the Apple Memory Guide gives no details on installing them. I suggest you install the first bank of 16 MB SIMMs in Bank A, since that follows the pattern established with other configurations. Bank B can then hold 256 KB, 1 MB, 4 MB, or 16 MB SIMMs. The Apple Memory Guide states that the Mac IIcx supports up to 128 MB.

After installing memory and reconnecting your drives, boot your Mac with extensions off (hold down the Shift key) and check “About This Macintosh” under the Apple menu. If it doesn’t give the expected number, you should reseat your memory.

Once you know the upgrade is a success, bolt everything back in place, attach the cover, and enjoy the extra memory. With the extra memory you’ve installed, I suggest you increase the size of the disk cache for better overall performance.

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