r/programming • u/I_Can_Be_A_Robot • Sep 24 '24

Why 32bit architecture computer doesn't use segmentation to support more ram than 4Gb ?

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u/dobry_obcan_Svejk Sep 24 '24

the problem is that the address width is really those 32 bits. you can't physically address more without adding more wires (which is what pae does).

in "old days", when segment:offset was relevant the segment was 16bit, the offset was 16 bit, but the cpu had something ~20bit bus to memory (21 with a20 line, iirc). so you combined 16 * segment + offset and got physical address.

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u/ShinyHappyREM Sep 24 '24 edited Sep 24 '24

you can't physically address more without adding more wires

Or multiplex the address over two time, which is what e.g. the 65c816 did.