The Mac II was Apple’s first modular Macintosh. Using a 68020 CPU instead of the older 68000, it can address far more memory by using 32-bit addressing. Although it took System 7 to provide the 32-bit option and Mode32 to let the Mac II operate in 32-bit mode, this allowed users to use more than 8 MB in their systems.

However, there were other limitations: Without a PMMU or 68030 upgrade, the Mac II can’t use 4 MB SIMMs, and without the SuperDrive ROM upgrade, high density SIMMs can’t be used in Bank A.

The greatest drawback in upgrading RAM in the Mac II is the location of the SIMM sockets. You need to remove the entire drive support assembly (it holds the floppy and hard drives) to access the memory sockets.

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

Mac II 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 the Mac II shipped from Apple, 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, has 1 MB SIMMs in all 8 sockets. This is the most memory a Mac II can work with unless you have a PMMU or a 68030 accelerator.

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 Mac II.

Remember, a Mac II must have either a PMMU or 68030 accelerator and the SuperDrive upgrade before you can use 4 MB or 16 MB SIMMs in Bank A.

Special instructions for a Mac II with PMMU or 68030 but no SuperDrive: You can use 4 MB SIMMs in Bank B, leaving 256 KB or 1 MB SIMMs in Bank A. This provides either 17 MB or 20 MB total RAM. This is the most you can use without the SuperDrive upgrade.

For 16 MB, 17 MB, or 20 MB, 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 II 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 II 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. After upgrading your system memory, I suggest you increase the size of the disk cache for better overall performance.

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