Division of Memory under DOS?

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Hi
I am reading an A+ course and in one slide which has a title of
"Division of Memory under DOS"
Momery address----------------- Type of memory
0- 640k ----------------- Conventional or base momory
640k - 1024k ----------------- Upper memory (A through F ranges)
Above 1024k ----------------- Extended Memory

My questions:

1) Why DOS divides momory like that?? I mean if I have my PC with RAM
only 256k then how can DOS go above that range??

2) What does upper memory, and extended memory mean??

Thanks a lot for the helo in advanced.

 

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Archived from groups: alt.comp.hardware.pc-homebuilt (More info?)

On Fri, 30 Apr 2004 12:47:08 -0700, esara wrote:

> Hi
> I am reading an A+ course and in one slide which has a title of
> "Division of Memory under DOS"
> Momery address----------------- Type of memory
> 0- 640k ----------------- Conventional or base momory
> 640k - 1024k ----------------- Upper memory (A through F ranges)
> Above 1024k ----------------- Extended Memory
>
> My questions:
>
> 1) Why DOS divides momory like that?? I mean if I have my PC with RAM
> only 256k then how can DOS go above that range??

It doesn't go above 256k, if that's all that is available that is all
PC/DR/ms-Dos will use.

> 2) What does upper memory, and extended memory mean??

The upper memory region tends to be used by the BIOS, including video,
possibly disk controllers, network interface cards, ect.

Memory managers can use unused spaces in the upper memory to store things
freeing up some of the available conventional memory.

Extended memory uses a DOS-Extender to switch the CPU into protected mode
and access it, this will address up to 16M.

There is also Expanded memory (formerly known as LIM), that takes extended
memory swaps it into a 64k page frame down in the lower part (could be
conventional or upper memory) one 64k blocka at a time where it can be
accessed.