Re: Software for MS-DOS 1.25

Mia Magnusson wrote:

> A compromise could be to have a character set based on what's actually
> used in real life. Very few if any programs use for example the
> smileys, and block graphics might not need inverse mode, but it would
> be good to have inverse mode for atleast ascii code 32-127 and
> preferable a subset of national character (which each user should be
> able to select when generating a font eprom image with some "generate"
> utility).

I would definitely like to have a character set that at least has the
backlash character (used in MS-DOS as path separator), pipe and curly
brackets. These seem quite important to me.

> As Ruud already stated, a XT class machine can use 1.2MB drives if you
> provide appropriate driver (usually with a small extra rom). But that
> is just a limitation of the XT bios. DOS will happily use whatever it
> gets provided with.

But what does this driver actually do? How does it tell DOS "hey, now
you have a 1.2M drive instead of 360k"?

> True. Maybe a great mod would be to have a daughterboard that both
> moves the CBM ram and also adds more RAM? I don't know if it would be
> even possible to add ram so it would be visible for the CBM too 

No, because there are chips on the CBM-II mainboard which multiplex the
address bus between 6509 and 8088.

But creating an 8088-only RAM extension should be straightforward, if we
tap into the RAM extension connector _and_ the PLA which fortunately
should be socketed.

> But how does disk i/o work? I would have thought that the 8088 would be
> shut down completely giving memory access back to the 6509 while it
> does it's thing, and then starts up the 8088 again. 

That's exactly what happens.

> Or is it done
> sending one sector at a time via the 6526 interfaces while
> both CPU's running at the same time?

Only control bytes (sector number, buffer address) are sent via 6526,
the 6509 places the disk sector contents directly in the DRAM.

> Yeah, as I understand it there is 2k of ram for the 6509 on the 8088
> board, that's a bit tight to have any code not strictly neccesary.

But you also have existing 1k at $0400 which, as I understand, is used
for bootstrap code. If we move more bootstrap code to the 8088, we can
re-use this RAM.

> But I assume that writing a new BIOS on the 8088 side would also mean
> at least changing the 6509 code somehow. Then IRQ0 and IRQ7 can be used
> freely.

No, you don't have to change 6509 code if you don't want to provide new
IO functions to the 8088. The extension could theoretically work with
the 6509 code unmodified.

Regards,
Michau.

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