Ben wrote on Tue, 6 Aug 2019 13:47:59 -0600 > It was too bad the 6809 did not have a pin to indicate Instruction or > Data memory bank in use. That would of given a real unix system in the > 8 bit world, as by then (late 70s) 64kb was proving just to small for > any real use.
I added a circuit to generate such a signal in my 1983 children's computer: http://www.smalltalk.org.br/fotos/pegasus1.jpg If I remember correctly, it only used half of a 74LS74. The notebook with the circuit is a bit hard for me to get to right now. It didn't work perfectly since it depended on there being a non memory cycle between fetching the instruction bytes and the data access. And that doesn't happen if you use the zero offset addressing mode. So I just wrote my assembler to never generate that. Instead it used a five bit offset with a value of zero. This wastes a byte and a clock cycle but I thought it was worth it. The extra signal was used to select between ROM and DRAM. This meant I couldn't fetch data from ROM nor execute code from DRAM but since the only program I was interested in was the Logo interpreter it seemed like a reasonable design. I did have a Logo compiler planned, which would not have worked with this. > The 68000 was the only real 16/32 bit cpu out at time, but nobody could > afford it. The 6809, and specially the 6809E were hardly cheap themselves. And their prices stayed stable while the 68000's dropped quite a lot. By late 1985 the difference was rather small. And the 6847/6883 chipset was expensive too, at around $40 for these two chips. I regretted using them as you can get a lot of TTLs for $40 and have far nicer video than they gave you, specially if you didn't care about the built in text modes. -- Jecel