6,144 [LWN.net]

6,144

Posted Aug 9, 2014 15:03 UTC (Sat) by jzbiciak (guest, #5246)
In reply to: 6,144 by pr1268
Parent article: 3.17 merge window part 1

That's 5 ✕ 2ⁿ (for n = 17). A PC with five SIMM slots (or whatever the onboard memory expansion slots were called back then) would be interesting...

IIRC, on the PC XT, it was actually 4 rows of 9 DIPs (36 discrete chips, including parity). 2 rows of 256Kx1 chips, and 2 rows of 64Kx1 chips, as I recall. Some were socketed and some were soldered in.

Why not three rows of 256Kx1? Not sure. If I had to venture a guess, it was probably due to the memory map they established with the original IBM PC. The MDA and CGA adaptors started at B:0000 and B:8000, which prevents you from going to 768K of contiguous memory without moving them and breaking compatibility with the original 5150. Given that the 5150 came with only 16K or 64K of RAM originally, I guess it wasn't a pressing concern at the time?

I don't miss the 640K era.

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6,144

Posted Aug 9, 2014 17:10 UTC (Sat) by dlang (guest, #313) [Link]

> it was probably due to the memory map they established with the original IBM PC. The MDA and CGA adaptors started at B:0000 and B:8000, which prevents you from going to 768K of contiguous memory without moving them and breaking compatibility with the original 5150

exactly, it wasn't until the memory got cheap enough in larger sizes that manufacturers started installing 1M of RAM on the boards even though the system could only use 640K (because it was cheaper for them to do so then to deal with two sizes of chips) that high memory was born.

and when the XT was shipped (at a price of ~$5K IIRC with two floppies and 16K of RAM, imagining that 640K would be limiting was hard.

I don't miss those days either.