robfinch wrote:
I was thinking along the line of the TV typewriter or AppleII graphics, destined for a composite monitor or TV. Text mode may not use much memory.
I thought chip memory like 2102's were being used, but maybe that is too advanced. for 1973 ish.
This is more a hardware emulation, than a true retro design. I am cheating in few things like using a 6850 ACIA and a SD card
but I feel the design has the spirt of a TTL computer. The 21 bit computer is a after thought for for the working 20 bit computer
as it can be hacked into the 20 bit cards.
robfinch wrote:
With 21-bit wide memory, 7 3-bit pixels could be read in a single cycle lowering memory access requirements. The dot clock could be 7x the 1.x base clock rate.
FPGA would make things much easier, but much less authentic.
I disagree, with the right FPGA macros, with schematic entry the data path is well covered.
micro code ROM as the issue, as different venders do things different.
Four phase computers? had 24 bit alu and did it that way about 1975.They were leased rather than bought
so they where not well known. They also had their own custom memory and logic chips.
robfinch wrote:
What is the motherboard bus like?
A 6502 or 6800 style bus for the 6850 ACIA. Right now it is at the get it working stage.
I have a serial terminal for use when a get a stand alone system, so a video display.
using old VGA monitors, because they free was not considered with early 70's time frame.
Also they need a 25 Mhz dot clock, so the bus would need to be revised. Color and inverse video also need
to addressed, so 20 bit machine might be better since you have 8 bits for each character, and 2 bits for color.
