8088 card and bank 15

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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