Re: 8088 card and bank 15

From: william degnan <billdegnan_at_gmail.com>
Date: Thu, 26 Oct 2017 13:40:18 -0400
Message-ID: <CABGJBuez+qJtZzB67hXm+QB7iFwf02A-ntNWhZ-z1LtWu8wcKg@mail.gmail.com>
It's possible that it was expected that one install a RAM cart in the user
port to so something related to the 8088 card within bank 15 ($2000-2FFF).
Many people with the 8088 coprocessor would also have had the RAM cart

I have a newer one that goes all of the way up to 7FFF for use with the
FastBus that allowed for connection to a serial drive like the 1571
http://www.vintagecomputer.net/browse_thread.cfm?id=513


Bill

On Thu, Oct 26, 2017 at 12:31 PM, David Wood <jbevren@gmail.com> wrote:

> 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
>>
>>
>>
>>
>>
>>
>>
>>        Message was sent through the cbm-hackers mailing list
>>
>
>


       Message was sent through the cbm-hackers mailing list
Received on 2017-10-26 18:00:57

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