Hi,

First let me point out that all these questions and many more are answered in the "IA-32 Intel ® Architecture Software Developer’s Manual Volume 1 : Basic Architecture" which can be downloaded for free from developer.intel.com

There is 6 segment registers: cs, ds, ss, es, fs, gs. And yes in real-mode (and virtual mode) each is 16-bit large and therefore 64kb each. You can change these segments at run-time to access 1MB.

In protected mode (or the management mode) each segment is 32-bit and therefore 4GB large. In protected mode the IA-32 supports two memory models - the flat memory model and the segmented memory model. The flat - which is the one used by the compilers I know (do they support the segmented one?) uses the same value for all the segment registers so you only get one segment. This is convenient since you avoid segment hell - but it *is* possible to use segmented mode and get access to a total of 64GB virtual memory.

Check out AWE (Address Windowing Extensions).

quote:Original post by Leadorn
Yeaaaa, now i only have to stick some more memory modules in my pc.......... 64 gb

One note, don''t confuse *virtual* adress space with *physical* memory. A virtual adress is the adress that the application sees. This is mapped by the processor to a physical adress that is entirely different (using paging) - or it might even map to the harddisk by the OS is the physical memory is low.

So the amount of virtual memory is (in the segmented model) limtied to 36-bit (64GB) but the physical processor limits might be much lower - and be even lower due to limitations on the motherboards. (Few PC motherboards today supports that much RAM!)

quote:
The compiler must support whatever the operating system it compiles for supports. AFAIK there are no operating systems that use the segmented model (not any major/widespread ones at least, there are probably some small hobby OS'' that do).

I didn''t know this. Thanks for the info.