So here’s where the next video stands.
First, as I noted in a previous blog post, I’ve settled on an 8080-like MPU with 6 8-bit registers and an internal 8-bit bus.
Now to make an internal 8-bit bus work, you nee a tri-state gate, a logic circuit which can be used to disconnect our logic gate from a common bus. The idea here is that you may have six things that can write their output to a single set of lines (the bus), but only one thing actually writes out to the bus at the same time.
Which I now have a successful design for.
I’ve also figured out the timing issues for the accumulator. Basically the accumulator register is always tied to the output of our ALU–but because you’re reading the results into the accumulator at the same time you’re writing the previous value as the output. So ideally we want an accumulator which can latch the results internally when a clock signal changes state–but doesn’t actually output the results until the clock state changes back.
An interesting side effect is that if you want to copy a value into the accumulator, it is easier to simply write the results through the ALU. Meaning we need a control state on the ALU that simply copies the input across. (On the other hand, it means instructions like the 8080 instruction
SUB (IX+index) relatively easy, since once the address is loaded, the memory value at address IX+index passes through the ALU on its way to the accumulator.)
And that means the test ALU board I just sent off for fabrication (and which will probably have a one-month turnaround, because you can have it fast or have it cheap–but you can’t have both fast and cheap) is now out of date.
Don’t worry, I’ll explain all of this in an upcoming video.
I do plan to build separate boards for the regular registers (the B/C/D/E and F registers), and for the A register (which does not have a write-enable line, but does contain multiple latches to shift values on both the clock signal going high and going low).
The only other circuit I need is to develop 90 degree out of phase signal to properly time the loading of all of the registers and the ALU.
Once I have that tested, I can start putting together the next video.