Thread: 8 meg agnus mod
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Old 05 March 2019, 09:50   #11
Jope
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Join Date: Jul 2003
Location: Helsinki / Finland
Age: 39
Posts: 7,670
Quote:
Originally Posted by AmigaHope View Post
Yeah, general consensus 8M CHIP is basically not possible unless you rolled yourself a whole new Amiga platform including new Lisa and Alice, and a new motherboard. The address lines just aren't there. I too have looked longingly at that 8M CHIP jumper on my A4000, but it's a phantom dream.
It depends on the SIMM geometry that you plug in.

10 address lines are there (1024 bits). Two rows, four columns are controllable by U212.

Each RAS signal can hide behind it a geometry of 1024x4096 bits = 4MB. But you have two RAS signals.

So the SIMM you use must be at least 2048x4096 total and it must respond to two RAS signals. Not impossible, but "not just any" stick will work, as some have a very shallow geometry thanks to the amount of address pins available in the 72 pin SIMM specification.
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