I'm following the "very basic whole-computer schematic" from the 6502 primer, but other than what's on there, is there anything you lot recommend buying (useful logic ICs, tools, etc)?

I'm planning to build it on a breadboard to save having to design a PCB or learn wire wrap (yes, I know there are issues with breadboards but my dad built a 6502 computer on breadboards and it works fine). One thing I was wondering about is putting LEDs on the buses. Do I need latches or something for that?

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probably the youngest person on this forum

Okay, so what tools, if any, do you currently have?

In general, you should have a multimeter at the very least. Also, a way to program the EPROM or EEPROM. Beyond that, things get more expensive.... a logic analyzer is nice, as is an oscilloscope.

LEDS, how do you plan on hooking them up? You can connect them to the 6522 port lines, but you'll need some resistors so you don't overdrive them.

Other bits to think about... once you get some success with the basic setup, then what? More I/O is typical... perhaps a UART and USB-to-Serial adapter so you can setup a terminal program on a PC and interact directly with the system.

Whatever you do, have fun with it.

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Regards, KM
https://github.com/floobydust

Breadboard is fine to get started - as 1000s of Ben Eater fans will tell you. (And me - I started my Ruby project on a breadboard) Personally I'd never suggest wire-wrap in this century - others have different ideas though, but it's quite expensive to get into. Here in the UK (and possibly Europe) stripboard is still very popular if you want to hold off a PCB for a bit longer. See e.g. https://projects.drogon.net/ruby-6502-on-stripboard/

If you want to be able to control an LED then yes, you'll need a latch of some sort. I started with a 74als573 for 8 output bits. A 65C22 VIA is more flexible but also bigger (physically) and needs a bit more care & time to get going (but not much).

Have fun,

-Gordon

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Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

Machine pin sockets for all of your ICs. If you're experimenting / troubleshooting you'll be putting things in and pulling them back out pretty frequently (especially your ROM). All those insertion cycles can be pretty hard on your chips' poor little legs. You might even want to go as far as getting a breadboard-compatible (caution: not all of them are!) ZIF socket for the ROM.

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"The key is not to let the hardware sense any fear." - Radical Brad

I have a multimeter with voltage, current and resistance settings. My dad has a scope that I could probably borrow if I asked nicely. I have access to a wire cutter/stripper, a soldering iron, and a solder sucker as well (I'm using a breadboard to avoid soldering but who knows, something might come up).

As for programming ROMs, I have an arduino and a raspberry pi. Googling for "arduino eeprom programmer" turns up a lot of results, so I think we're fine there.




I was more thinking "LEDs on the buses for debugging", because I've heard of that being done before and I think it would be cool. I've done bare metal raspberry pi GPIO before so I can probably figure out how to make a 6522 go.



Yeah, I was looking at the MAX3100 for a UART, because I'll probably want SPI set up anyway. I can (and have in the past) use the aforementioned arduino as a USB-serial adaptor. Another thing I could do is add some keypads. I have one or two Adafruit 4*4 keypads lying around that I've never really used.

I'll get some sockets for the ROM, if I can get some ROM. Having had a quick peruse of Mouser, they appear to be completely out of 28c256 AND 28c64 ICs. Is there an alternative that still works with 6502?

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probably the youngest person on this forum

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Ok, I have a partial schematic (see attached PDF). I haven't done the clock generation, reset button, or interrupts yet but I'll hopefully get those done soon.

I feel like the address decoding could be done a lot cleaner but I'm not sure how. I'm aiming to have RAM from 0000 to 7fff, IO from 8000 to bfff, and ROM from c000 to ffff.

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probably the youngest person on this forum

Like you, I have a bare-metal OS for the Pi to support my BASIC, but unlike the Pi, you've got to (a) wire it all up here and (b) write most of the code in assembler rather than C/Python/Forth/etc. Nice to read the stuff on your blog though.

However, you can boot a 6502 directly into C with a small hand-written "crt0.s" file using e.g. cc65 (I've done it as a proof of concept), you will need to patch the ROM with the reset vector.

But looks like good progress so-far.

I'd suggest is a slightly different memory map - 32KB RAM then 32KB ROM (with a small hole for IO) then you'll be able to run e.g. BBC Basic natively on it - but it's also easy for me to suggest that as that's what I did in my systems... However the 16KB from $C000 up is more than enough for e.g. EhBASIC and a small 'monitor'.

If you have a PiTubeDirect interface (ie. a big level shifter) then you can hook the VIA up to a Pi and use an 8-bit parallel transfer mode to use the Pi as a terminal - or bit-bang serial through the VIA. Or have a look at how the "slow bus" works on the BBC Micro - it's like an 8-bit interface driven through a VIA.

Cheers,

-Gordon

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Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

How do you recommend mapping the hole? And would it be worth putting IO in zero page? One of the original 6502 manuals actually mentions doing it like that.



Arduino runs at 5v, so no need for a level shifter there.

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probably the youngest person on this forum

No idea. I used a GAL in my designs so I didn't have to bother with all that TTL. It was easy in the GAL to map $FExx to a 256 byte page for IO and I have just one device - a 65C22 VIA.

The zero page thing was supposedly to save code (ZP addressing needing 2 bytes not 3) and make accesses go a cycle faster. Personally I think it's bogus today but in 1976 when speeds were low and memory expensive, then it might have been a thing.





I only mentioned this as you said you'd done Pi stuff before and wondered if you might be using one. I use an ATmega on my Ruby boards too, but it's somewhat different in that it can "see" a small part of RAM ($FFxx) in a mutually exclusive sort of way and I don't have any ROM - the ATmega copies the bootloader over.

-Gordon

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--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

I also agree with Gordon on a smaller I/O window. For years, I've used a single memory page (256 bytes) for an I/O page. I've always located mine at $FE00. I don't recommend putting I/O in page zero.

I designed a small CPU board back in the 80's that use 3 logic chips:

- 7400
- 7430
- 74138

Using the above, you can have a memory map as:

RAM - $0000 - $7FFF
ROM - $8000 - $FDFF / $FE00 - $FFFF

I/O - $FE00 - $FEFF
- I/O is broken into 8- I/O selects, each at either 16- or 32-bytes wide.

Also, the write line for memory is qualified using phase 2 CPU clock, which is required.

I've attached a section of a schematic for this, albeit the I/O selects are done at 16-bytes wide, so only half of the I/O page is used.



Note that in the setup, the chip select for the 32KB RAM is tied directly to A15 from the CPU, which eliminates decoding it separately. You can also use a jumper for the write line on the 28C256, as these devices can be programmed insitu if needed.

You can also find the entire system with an I/O adapter on my github page.

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Regards, KM
https://github.com/floobydust

To be clear, the IC goes into the socket and never (or rarely) comes back out of it. It is the socket that is inserted / removed from the breadboard. The round legs on machine pin sockets are MUCH more durable for this use than the legs of dual-wipe sockets.

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"The key is not to let the hardware sense any fear." - Radical Brad

A perennially divisive question on this forum! And, a classic case of Your Mileage May Vary.

It's true as Gordon says that speeds are higher nowadays, and it's also worth mentioning that many typical applications are compute bound rather than IO bound, which means a substantial boost in IO speed will probably yield no appreciable gain. [see addendum]

So, the question becomes this: What sort of work will you be doing, and what priorities are important to you? IO in zero page does undeniably provide a substantial boost in IO speed... even if your speed was already pretty fast to begin with.

There's more on the subject here. My own 65xx machines all have IO in zero page because bit twiddling is an important focus for me. There are pros and cons but my own priorities are such that I have never regretted the decision for an instant.

ETA: bit-bang serial IO is the sort of code that can really benefit. And if you're shooting for a certain specific speed goal (say, blab-blah kilobaud on your RS232) then IO in zero page can make the difference between possible and impossible. Note that this applies even if the associated application is something compute bound like a BASIC interpreter.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html