New 6809 system with bus-connector sees the light!

Nick Brok

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Sep 19, 2021, 12:00:35 AM9/19/21

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Hello,

Scrumpel8d is the newest 6809 computer. It has one serial port and one parallel I/O port.

Specs:

48K RAM ($0000-$BFFF)

16K ROM ($C000-$CEFF and $D000-$FFFF)

256 bytes I/O in unused part of ROM ($CF00=-$CFFF)

Serial baudrate is 115200 8N1.

64 pins extension header connector for the extension bus.

Here a picture of the CPU board:

Dylan Hall

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Sep 19, 2021, 12:59:29 AM9/19/21

to Nick Brok, retro-comp

Nice!

I'm not familiar with the 6809 (being more a z80 person). How is it you're able to have the ROM at the top of the address space? Does the 6809 not start execution from $0000 at power up?

Any chance you have a schematic you'd like to share?

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Alan Cox

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Sep 19, 2021, 8:52:04 AM9/19/21

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On Sun, 19 Sept 2021 at 05:59, Dylan Hall <dy...@deedums.com> wrote:

Nice!

I'm not familiar with the 6809 (being more a z80 person). How is it you're able to have the ROM at the top of the address space? Does the 6809 not start execution from $0000 at power up?

A lot of CPUs fetch a start address from a fixed address in memory. The 6800 series CPUs all use addresses at the top of memory for reset and other exceptions, the 6502 likewise. So on these systems you put the ROM at the top. Bizarrely in the 16/32bit world it's the Intel style processors that start at the top of the address space and the 68000 that fetches a start from address 0!

Alan

Nick Brok

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Oct 22, 2021, 12:56:25 AM10/22/21

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