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Is there a DOS memory documentation available? (Users)

posted by ecm Homepage E-mail, Düsseldorf, Germany, 10.01.2024, 21:14

> Because EBDA is supposed to be data, the kernel can move it to other
> places, possibly in UMB (or HMA?) simply by copying it around and updating
> the EBDA segment value somewhere at 40:xx similar to updating a pointer.
The HMA is not addressable using an offset of zero as the EBDA requires. The highest segment value is 0FFFFh which still does not point into the HMA at offset zero, rather it points to the last 16 Bytes of the ROM-BIOS image usually placed below 1 MiB. So you cannot move the EBDA into the HMA.

> If you see far less than 640k of RAM, you may have some code or data
> buffers at the end of your 640k. The kernel can NOT move those around.
> Think about MBR -based drivers for supporting harddisks larger than 504 MB
> or 8 GB if your BIOS is too old. Also think about viruses and maybe buffers
> of exotic network or disk controller ROMs.
Bootable debuggers such as lDebug may also decrease the size of the Low Memory Area as indicated by interrupt 12h in order to protect their allocation. (The int 12h simply returns the word value at 413h in AX. You modify the variable, not the interrupt handler.) As int 12h specifies the memory size in KiB, the debugger must round up the size to allocate for itself to happen upon a KiB boundary.

Barely related ad: For lDebug, the first paragraph of its bootloaded mode allocation starts with a small header that reads either NSYM or NDEB. The NDEB image identification paragraph is described a bit in https://hg.pushbx.org/ecm/ldebug/file/bbf39043e416/source/init.asm#l2114

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l

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