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 listReceived on 2017-10-26 18:00:57
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