The Mac IIx was Apple’s first 68030-based computer. You need to run some version of System 7 to have access to the 32-bit option and use the free Mode32 utility to let the IIx run in 32-bit mode. This allows users to use more than 8 MB in the IIx.

The Mac IIx is essentially a Mac II motherboard revised to use a 68030 CPU. As a newer design, the IIx does not require any special upgrades before it can use high density RAM.

The greatest drawback in upgrading RAM in the IIx is the location of the SIMM sockets. You need to remove the entire drive support assembly, which holds the floppy and hard drives, to access the memory sockets.

When upgrading, be careful not to damage the SIMM sockets, especially if you are removing memory. Although the IIx shipped with 1 MB of memory, odds are pretty good yours has already been upgraded beyond that point.

Mac IIx page from Apple Memory Guide.

Looking at the motherboard with the connectors to the rear and the power supply to the right, 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 30-pin SIMMs 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 all 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. Finally, 4 MB and 16 MB SIMMs must be PAL SIMMs; conventional 30-pin SIMMs will not work in the IIx.

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 IIx 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 IIx will support a total 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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