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FreeDOS on SBC188 (Announce)

posted by GizMo79, 15.09.2020, 13:40

> > By the way, it might be possible to have a special driver offer some
> > additional memory (between 640 KiB and 1024 KiB) to DOS as upper memory.
> If
> > so then your DOS=UMB setting could take effect and DOS could link such
> > memory as UMBs into its memory chain.
> >
> > Perhaps such a driver already exists for the SBC188. I created
> >
> such
> > a driver for the 8086tiny environment. However, it would have to
> be
> > adjusted to work with the SBC188. And it would be crucial to know
> exactly

I asked the creator of sbc188 he replied:

RAM mapping:

All RAM maps to address space 00000..FFFFF, except

1. Upper memory block maps to ROM, and is excluded from the above. ROM is currently 64K.

2. Middle memory block maps to off-board (VGA) memory and Expanded Memory (4MEM board): A0000..BFFFF. It likewise is excluded from the RAM area.

Thus C0000..EFFFF regions are available as Upper Memory Blocks.

B0000.BFFFF are the MDA and VGA a/n regions. A0000..AFFFF would normally be Graphic memory. But with no graphics cards for the ECB bus, this is the area for use by EMM. This is a somewhat odd arrangement, but is dictated by the 80C188 on-board memory mapping hardware. Consult the datasheet for the chip, and note especially the restrictions on use of the Middle Memory Map: its location must be an exact multiple of its size.

Making use of the UMB's is a good idea.

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