Re: commodore 64 in a small package
Date: 1999-11-10 21:13:58

Hallo John, Sam,

> Sam Laur writes:

> And I wasn't going to waste
> any time by using the simplest chips (74HC00), although that would have
> a point too. It's just that then, a lot of time goes wasted,
> constructing dozens of flipflops from NAND gates.

I meant using TTL-chips from the whole ALS/LS/F-range as long as it is normally 
available. For example, I intend to use four 74LS191's as Program Counter.

> Also the PLA should be easy-ish to make in discrete logic.

How many people have an EPROM-programmer? And how many do have a PLA-programmer?

> >Instruction decode using an EPROM will be much easier, yes - but how
> >complex would it be to make in hardcoded logic?

Just give me the Boolean-equations and I'll build it. Some figures: every 
opcode can need up to 7 cycles. This means 8 plus 3 plus 1 bit for the clock. 
Then the needed cycles for some opcodes can vary: think for example of the 
branches. I reduced it to 1 bit. We need 1 bit to tell there is an Interrupt or 
Reset going on. This means that the Booelan equation of every outputline is 
calculated out of 14 inputs. 
Maybe as a little exercise: I'll use a 74ALS573 as Accu. Please calculate the 
boolean equation for outputting its data to the internal bus for all steps of 
every opcode :)

> I think the main advantage of using an EPROM for complex logic is that
> it can be changed very easily.  No cutting tracks, soldering, and so on.
> And it's very unlikely you'll get it perfectly right the first time.

This was one of my reasons to choose for EPROMs as well. Another problem with 
PAL's are they are not re-usable. GAL's are but are expensive and need their 
own programmer as well.

Groetjes, Ruud

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