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Memory Structuring in Vintage Computers: Where does Row 1 begin?

T-Squared

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In my recent adventures in getting my old systems to work with increased memory (My Sanyo MBC-775, and my Macintosh 128K), I've been having a bit of a problem trying to figure out where the start of memory begins: Which is row/column 1?

Does it start closest to the CPU, farthest from the CPU, or am I completely wrong, and it's a different kind of organization?
 

Eudimorphodon

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I'm a little confused, when you mean "farthest from the CPU"do you mean, like, physically? In which case, yeah, there's no convention, you need to understand the addressing circuitry and see which CAS line is active at which address range. (I mean, I guess I would probably assume that in a machine that has CAS0-CASx lines the "0" would be the lowest one, but that's just an assumption.)

In a machine that has 16 bit wide memory like the Macintosh the CAS lines can also indicate "even/odd"; since CAS is also effectively the chip select when these machines are writing only 8 bits of a 16 bit word (when reading memory both CAS lines would be active simultaneously) the CAS line for only that byte will be active. But both will be active simultaneously for word-wide transaction. So, again, to use the Mac as as example, since it only has one "bank" of memory (measured in words) the same CAS/RAS addresses are always in effect for both rows of chips, it's just a question whether the CPU is only accessing the LSB, MSB, or both. (The control circuitry sorts this out.)
 
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