Re: BASIC for the CBM-II/8088

Mia Magnusson wrote:

> That would probably be easiest, or rather the most pcb space efficient,
> to implement using programmable logic.

Probably so :-)

> Otherwise something like 74LS279 contain four set/reset flip flops. A
> 74LS138 could detect which 1/8 part of video ram is written, and it's
> output could go to the set inputs of two of theese. It would also
> require some three state thing to read this,

Not necessarily. The value of this latch should go to the 6509 anyway,
so you can connect them (using some additional connector) to the user
port, where they would be read by a CIA port.

> Btw how fast is the 8088 at comparing two memory blocks, compared to
> how fast the 6509 is at copying the same size of memory? Would there be
> any benefit using the 8088 to search for what has been changed since
> last time the video memory were examined, and build a list for single
> bytes (or smaller blocks) for the 6509 to copy?
> I recall a discussion about the REP instructions of the 8088, but can't
> remember exactly what we were discussing. Probably video ram :)

That's a good suggestion. If my 8086 instruction set reference is
correct, you should be able to do a REP SCASW of the whole screen in 8ms.

However, it would also mean that you must keep an additional copy of the
screen on the 8088 side to compare what's changed. The cost of updating
this copy will add to the solution. I guess it depends on what kind of
application is being run; if the application updates large small
portions of the sceen at a time, the SCASW solution will be faster; but
if it redraws the whole screen a lot, it will be slower.

> Using BIOS means that there is one place less where the program has do
> do things different with monochrome v.s. color display, so it might had
> made sense to use those INT's even for programs using the video memory
> directly.

Yes, that's what I think too.

Regards,
Michau.