You are using a lot of bits to select I/O devices. As an example, in my POC unit, I/O starts at $00D000, which is the first device. The next device is $00D100, the next is $00D200, the next is $00D300, etc. This address progression is easily decoded using only three bits at the 74AC138 decoder's A, B and C inputs, plus a chip select that is true when A15-A12 is %1101 (a NAND gate driving /E1 on the '138 with A13 driving /E2 does it). The '138 A-B-C inputs are wired to A10-A8, which takes care of telling the '138 which device to enable when the I/O block is being addressed. Splitting up the I/O space into pages is efficient and demands minimal logic.

Generally speaking, as the "distance" between devices gets smaller and smaller, the logic required to decode the address space gets bulkier and bulkier. This is why most of the eight bit micros built around the 6502 (e.g., the Commodore 64) had paged I/O decoding.

My recommendation would be to use the proper symbology. Once you get your unit running you may want to adapt a machine language monitor to it for debugging purposes. Just about every 6502 M/L monitor I have encountered uses the MOS Technology symbology for number radices. In other words, if you are going to live in France you need to learn to speak French.

Hi Billy - good to see some ideas being bounced around here. Bear in mind that you'll get almost as many opinions as respondents - to some extent you have to construct your own wisdom from what you see around you. For me, you can draw a memory map either way up. But zero is always described as low memory, and the vectors as at the top of memory. I think I tend to place them on the page accordingly.

As for hex, we see various conventions, and in almost all cases everyone will understand what you mean. 0xffff, $ffff, &ffff or just plain ffff. The thing to watch out for is that any given assembler (or other software) will have a strict convention. Many a time someone has written 10 when they mean $10, or vice versa, and the program is not going to behave the same way.

(We're all on first name terms here, unless someone has determinedly not used their first name, in which case their forum name or a derivative of it is fine.)

Cheers
Ed

(I don't have any idea how to convert a truth table to a boolean expression and I am not at my computer at the moment but here is a truth table)

(I don't have any idea how to convert a truth table to a boolean expression and I am not at my computer at the moment but here is a truth table)

BDD,

Out of curiosity, what program did you use to draw that decode table? Are you just using your schematic editor?

It's just a schematic editor (Express PCB). I prefer to work with simple tools, my theory being that complex tools consume too much of one's time with their often-steep learning curves, often demanding more than they give in return. I'd rather use my bench time being productive.

My approach extends to the actual hands-on aspect of building and troubleshooting circuits. Most of the time, all I use is a basic logic probe to troubleshoot, even though I have other test equipment at my disposal.

I downloaded ExpressPCB and have found it much more intuitive (I especially like that the help page can be read in 15 minutes instead of 15 hours for Eagle) yet does not feel like it has been dumbed down (like Fritzing).

The only real downside that I can see is having to make custom components for all the WDC parts, but most other schematic / PCB packages suffer from the same issue.