I wrote this up for a forum post and thought I’d share it here. It’s not 100% accurate but is “good enough” to help in a lot of cases.
The discussion started with a poster trying to understand how we knew what the capacity of a 41256 RAM chip was.
It’s actually there in the chip name. There’s a standard (Gotta love standards. There’s so many to pick from)
The first digit is whether it’s SRAM or DRAM. 4 for DRAM, 6 for SRAM, so this is a DRAM chip.
The second digit is how many “bits wide” the chip is. Ie how many data pins are there exposed. Commonly it’ll be 1 or 4. I suspect there’s 8 but I’ve never seen it. So 41 series is 1 bit wide DRAM. You can only read / write 1 pit per chip. I have systems using 44 series RAM (Commodore 64 later models) so they’re 4 bits wide and you can read or write a 4 bit nibble per chip.
The rest of the numbers to the hyphen is the number of “cells” of memory there is. Now this is tricky, because you need to multiply the number of “bits wide” the RAM is by this number to get the actual bits, and then by 1024, because it’s a measurement of kilocells. (Confused yet? Took me YEARS to be able to read these). So this chip is 256k x 1 bit wide, thus 32 kilobytes of potential storage.
Now remember that second digit? This is where that becomes significant.
The RAM on late model C64s was 4464 RAM. Thus DRAM, 4 bits wide, 64k cells of 4 bits wide. Thus each of these was also 32 kilobyes wide. The difference? Whereas 41256 needs eight chips to make an 8 bit datapath, 4464 only needed 2 chips to to make the full 8 bit datapath.
Now, not all systems have 8 bit datapaths. I have an Archimedes (Sadly, currently not working as well as it should) that has a 32 bit wide data bus. It has an absolute sea of 411024 RAM on the motherboard (The nomenclature is slightly different because it was sourced by a different manufacturer, but it’s equivelent).
So, DRAM, 1 bit wide, 1megabit. But it needs to read it 32 bits at a time, so there are 32 of them, for a grand total of 4MB of RAM.
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