Why the 10MiB ceiling in a Classic II?

[gsteemso]

Hi all,

The various threads here about 128MiB 72-pin SIMMs working in certain machines that were never intended to use them got me thinking. What, precisely, about the Classic II prevents it from using SIMMs over 4MiB? I know that 30-pin SIMMs were available in up to 16MiB capacities. Is there some simple hack that can be performed to remove or work around the limitation?

 

[tmtomh]

It's not a SIMM-size limit; it's a limit of the logic board's memory controller. The Color Classic, for example, comes with 4MB RAM soldered onto the logic board, and it has two SIMM slots, in which you can put 4MB SIMMs. That would make 12MB - but the CC can only see 10MB. AFAIK the same basic situation holds with the Classic II (even though it has only 2MB of soldered RAM.)

 

[H3NRY]

Some Macs have hardware memory controller limits - only so many address lines. Many are limited by the boot ROM to a certain ceiling. I think the Classic II is one of these. There are some which will take more memory than Apple states, and those are found by somebody experimenting, putting in bigger compatible chips to see what happens. Usually, the extra memory is ignored. Sometimes the system doesn't boot, sometimes the refresh cycle is wrong and the memory is flaky, and occasionally the supply Voltage is wrong and the memory is destroyed (not a risk with 30-pin RAM, which is all 5V). The Mac II is limited to 256K & 1MB SIMMs by the video circuit, which accidentally puts 4MB & 16 MB SIMMs into factory test mode. There were special 4MB "PAL" SIMMs which were Mac II compatible, but they are very rare.