How Much Memory Does My Vintage Mac Really Accept?
In the early days of Macs, Apple stated the theoretical maximum memory a Mac could use. But with the Macintosh II, a mixture of issues meant that in reality, you really struggled to reach the theoretical 128MB. There were ROM bugs, you needed a Memory Management Unit upgrade, there was no 32bit addressing and the standards for RAM changed meaning the Mac II and IIx accidentally trigger a test mode in normal 4MB SIMMs and crash.
They got sued.
As a result, there after, Apple generally only quoted the maximum RAM a computer had been tested with during development, which was usually, but not always less than the actual maximum.
Note that maxing out the RAM on a Mac will make the built in RAM test take as long as 2 minutes in some cases. During the test the computer will sit with a black, empty screen. On later OSes, possibly from about Mac OS 8.5 onwards, it is possible to disable the Memory test with a hidden option in the Memory control panel. If I remember correctly, it appears when you open the control panel with the Command and Option keys held down.
For Apple’s official recommended maximum RAM for each machine, please see the Service Source document “Apple Memory Guide” from 1998. Where I haven’t included a machine on this page, either Apple’s maximum actually applied, or I’m unfamiliar with the model. Apples maximums apply to machines including the… Plus, SE, Classic, Classic II, Colour Classic, LC, LC II, LC III, IIvx, IIvi, IIci, IIfx, Quadra 660av, Quadra 840av, Quadra 900, Quadra 950, Performa 6200, Performa 6300, Performa 6400, Performa 6500, as well as other machines that are very similar (such as the 5*** series equivalents to 6*** machines, and Performa equivalents to LCs).
I’m not very familiar with PowerBooks, so have mostly excluded them.
Actual Capacities and Observations
The following does not consider hacks or modifications to ROM or hardware, but only what can be easily done by any user.
Macintosh II / IIx
128MB, but you’ll need IIx ROMs and a MMU in the Mac II, plus both need PAL SIMMs and either a 32bit clean ROM, or MODE32.
See Apple Memory Guide for how to correctly fill the two banks of RAM.
Macintosh IIcx & SE/30
128MB, but your’ll need a 32bit clean ROM or MODE32.
See Apple Memory Guide for how to correctly fill the two banks of RAM.
Macintosh IIsi
The IIsi will take four 16MB SIMMs plus the 1MB soldered, for a total of 65MB. Set your hard disk cache to 1MB for improved CPU performance due to a quirk of the video circuit design.
Performa / LC / Quadra 630 & LC / Performa 580
There are two logic board variants. A one RAM slot machine will take up to 128MB plus the onboard 4MB for a total of 132MB. A two slot machine will take up to 196MB. Note the second slot only works with 4, 16, or 64MB SIMMs.
The LC / Performa 580 uses the same board as the two slot 630.
LC / Performa 475, LC / Performa 575 / 577 & Quadra 605
These machines will accept a 128MB SIMM, for a total of 132MB including 4MB onboard.
Centris / Quadra 610
A quirk of the ROM in these machines means that even though the hardware is capable of seeing banks of up to 64MB, the ROM only checks for 32MB banks at most. There are up to two banks per SIMM, but not all SIMMs have two banks. Weirdly, this means that the most RAM you can fit is 128MB + onboard RAM (4MB) for 132MB total, but because 64MB SIMMs are usually single bank, you need to fit two 128MB SIMMs. The computer recognises each 128MB as two 32MB banks instead of two 64MB banks, but otherwise works fine.
Quadra 700
The Quadra 700 accepts up to four 16MB SIMMs plus 4MB of soldered RAM, for a total of 68MB.
Upgrade with a matched set of four.
Quadra / Centris 650 and Quadra 800
A quirk of the ROM in these machines means that even though the hardware is capable of seeing banks of up to 64MB, the ROM only checks for 32MB banks at most. There are up to two banks per SIMM, but not all SIMMs have two banks. Weirdly, this means that the most RAM you can fit is 256MB + onboard RAM (4 or 8MB) for a total of 260 or 264MB, but because 64MB SIMMs are usually single bank, you need to fit four 128MB SIMMs. The computer recognises each 128MB as two 32MB banks instead of two 64MB banks, but otherwise works fine.
Upgrade in matched pairs for improved performance due to memory interleaving.
Power Macintosh 4400
I don’t have one of these so I’m struggling to remember off the top of my head but I believe that you can fit more RAM that Apple’s documentation states… I think you can fit two 64MB SIMM and one 32MB DIMMs for a maximum of 160MB.
Warning : This machine requires 3.3V 168 pin DIMMs. Very unusual for this era of Macs.
Power Macintosh 6100
The Power Macintosh 6100 can actually recognise two 128MB SIMMs, plus 8MB onboard for a total of 264MB of RAM.
Upgrade with matched pairs.
Power Macintosh 7200 / 8200
Technically, the 7200 could recognise 768MB if you could find some 5V, FPM, 256MB SIMMs, short enough to fit in the chassis. I’ve never seen any. That means you’re likely looking at 384MB, which will be expensive as 168 pin 128MB 5V FPM DIMMs are not cheap.
Power Macintosh 7500, 7600, 7300, 8500 and 8600
Technically, if you could find 256MB 5V FPM or EDO DIMMs that fit the chassis, the maximum would be 1.5GB of RAM (limited by the OS). I’ve never seen any that aren’t too tall. That means you’re likely looking at 1GB, which will be expensive as 168 pin 128MB 5V FPM or EDO DIMMs are not cheap.
Can be upgraded individually, but works better with matched pairs.
Power Macintosh 9500 & 9600
With 12 slots, these will take up to 1.5GB using 128MB 168 pin, 5V FPM or EDO DIMMs.
Can be upgraded individually, but works better with matched pairs.
Power Macintosh G3 (Beige)
These machines will accept three 256MB PC66, PC100 or PC133 DIMMs for a total of 768MB. The DIMMs need to be physically short to fit in the desktop case, and additionally, “high density” DIMMs do not work. Practically, this means you need physically short DIMMs with 16 chips (eight on each side).
PowerBook G3 Pismo
Some (but not all) 512MB PC100 and PC133 SODIMMs work in the Pismo, for a maximum of 1GB of RAM.