Re: 8088 card and bank 15

Do you suppose the adder in the 8088 card is partly the reason RAM starts
in bank 1 instead of bank 0 on the CBM2 series then?  I had always wondered
about this.

On Thu, Oct 26, 2017 at 12:24 PM, <Ruud@baltissen.org> wrote:

> Hallo alemaal,
>
>
> When studying the schematic I noticed a 283 adder. It genererates
> the lines BP0..3 that are used to select the needed 64 KB DRAM bank
> using the PLA.
> The adder adds one to the original address. Why? When the 8088
> addresses the the 0Fxxxxh area, say bank 15, it addresses the ROM.
> But when addressing 0Fxxxxh, BP0..3 are all zero. But what happens
> if the 8088 addresses the 0Exxxxh area, BP0..3 are all one. Or in
> other words: bank 15!
>
> I studied the schematic and so far I don't see any reason why it
> wouldn't work. So those who have a working CBM-II/8088 card
> combination, please try it out.
>
>
>
>
> How did I get to the above:
> Michał and I are researching if it is possible to create an improved
> 8088 card. Why? There won't be that many customers for a remake of
> the orignal one. And even that remake will be improved a bit at
> least. For example: it will use the better available 27xxx series as
> ROM. So the idea was that if we had to offer a card at all, why not
> one with extra features?
>
> Before anything else : the improved card will be 100% compatible
> with the old one.
>
> One idea was adding an ISA slot. So far it looks that I can do it
> with adding just one extra IC, 74LS00.
>
> One of my ideas was adding 512 KB of SRAM to the board. It can be
> used as addition to the original RAM of the CBM-II. Having a 256 KB
> machine it would mean we have a surplus of 128 KB of RAM. This can
> be used as UMB RAM in the 0Dxxxh and 0Exxxxh area. And maybe better,
> 64 KB in the 0Axxxxh area so the machine can have 704 KB of RAM for
> DOS. The other 64 KB can be used in one of areas mentioned before.
> At this point I started studying the schematics and noticed the 283
> for what it was, raising the question: why?
> A 74LS138 plus some jumpers could do the trick.
>
> As said the improved will be 100% compatible with the original one.
> But that means we have to give in on some things. One example: IR7
> of the 8259 Programmable Interrupt Controller (PIC) is already in
> use. So I decided to connect IRQ7 (for the LPT port) to IR3. Having
> no IR3 anymore, I decided to connect IRQ3 and IRQ4 together to IR4.
> IRQ5 has been connected to IR5 and IRQ6 to IR6.
> IRQ6 is used by the floppy disk. A 360 KB floppy can be read not
> using DMA. And maybe a 720 KB one as well. (for Michał, I think I
> was wrong in a previous email)
> I have seen a Z80 and a 6809 system using the 765 W/O DMA so why
> not?
>
>
> --
>
> Kind regards / Met vriendelijke groet, Ruud Baltissen
> www.Baltissen.org
>
>
>
>
>
>
>
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