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Is there a DOS memory documentation available? (Users)

posted by ecm Homepage E-mail, Düsseldorf, Germany, 09.01.2024, 00:00

> Basically "extended memory" is what we now consider memory. So simms or
> dimms plugged into the motherboard, directly accessible/addressable memory
> to the CPU. It is the same memory as the first 1 MB, but simply the part
> that is isn't accessible directly in 16-bit CPU modes. (*)
>
> Expanded memory is basically _any_ memory, anywhere (typically a plugin
> card in some slot), , made available over a driver. In XT times this was a
> way to expand memory beyond 1MB while the CPU and mobo couldn't handle it.
>
>
> But even while the 286+ (or better AT, so 286 on a suitable AT mobo, some
> very early 286's excluded) could address more (than 1MB+64kb), it couldn't
> in 16-bit mode.
>
> To make these kinds of physical memory (extended mobo, or off it)
> accessible to then still dominant 16-bit apps, APIs were defined.
It isn't technically accurate to refer to "16-bit CPU modes" when the modes you are pointing at are specifically "Real Address Mode" or Real/Virtual 86 Mode. (Which I call 86 Mode for short.) In these modes only 1024 KiB (with A20 wraparound) or 1088 KiB minus 16 B (without A20 wraparound) are accessible. It is true that the segment limits are generally 65_535 Bytes in 86 Mode, and that the Code Segment D bit (Default size) and the Stack Segment B bit (Big) are both zero, indicating a default 16-bit CS and a 16-bit SS.

However, 86 Mode isn't the only 16-bit mode. There is 16-bit Protected Mode since the 286, which allows to access the entire 16 MiB of the 286 address space, albeit only in 64 KiB chunks.

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l

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