One of the things that I loved about the Apple ][ was that it was possible for one person to completely understand everything about that computer from the hardware to the software. I've never had that level of complete understanding of any system I've used since.
Yep, similar experience here. My first computer was a Tandy / Radio Shack Color Computer. It had a 6809 processor (8/16-bit precursor to the 68000) @1.8MHz, 4k of RAM (upgradable to 64k), 16k or 24k ROM memory with a quite expansive MSFT Extended Basic Interpreter (supposedly the last ROM OS & BASIC that had assembler written by BillG himself).
I taught myself BASIC, assembler, graphics programming and game programming on that machine over a period of about four years of hacking around on it (including hand-commenting some significant chunks of the ROM). By the time I retired it for a shiny new Amiga 1000 in 1986 I'd upgraded it to 256k of bank switched RAM with a soldered-in hack board, added four floppy drives, various I/O boards and learned OS/9 (a UNIX-inspired multi-tasking, multi-user OS) and hacked in my own extensions to the ROM OS (including adding my own new commands and graphics modes to the BASIC interpreter).
It started out as a lot of trial and error but, on later reflection, ended up being a surprisingly thorough grounding in computer science from which to launch my career. That 6809 machine was also the last time I really felt like I was aware of everything happening in a computer from interrupts to registers to memory mapping down to the metal.
Yes, that was the beauty of the 8 bit era, and many people lost it without even knowing that they lost something very precious. The total control is a very nice feeling.
I'm not sure why "simple, understandable system design" would have to be synonimous with 8-bit computing. One of the most appealing things about new open hardware initiatives is how they bring this simplicity and surveyability together in what's otherwise a very modern design and context.
Seems every time someone applies that to hardware with a wider compute path, other complexity creeps in.
Would be interesting to make a 32 bit Apple 2 style computer. Include a ROM for a means to boot, and leave everything else simple, with some nice slots. Could be a great development / learning machine.
I've "built" such machines in FPGA; PicoRV32 core, hand made display processor, a bit of RAM, and a couple UARTs. It was fun and not that hard for me, a newbie to FPGA.
One of the bigger challenges is integrating peripherals. I got bogged down trying to do SD Card interfacing. There are off the shelf bits of IP from Xilinx, etc. you can use to do this, but that sort of defeats the purpose of the exercise.
I think modern machines started their slide into mind boggling complexity when bus speed and CPU speed outstripped RAM speed. So much complexity and unpredictability is in the all the infrastructure built around cache.
Something like an Amiga or Atari ST was still not hard to understand all/most of, despite being 16/32 bits.
Because once the clock speed gets past a handful of MHz, maintaining good clock distribution and good signal integrity become more painful very, very rapidly.
It's unclear what you're asking for here? The Raspberry Pi Pico and Raspberry Pi Zero are existing and very well documented ARM based single board computers.
Fair enough. I suppose what I'm asking is whether it's possible to purchase a more modern CPU independently of a SOC and design my own single board computer built around that. Can you buy a naked Cortex M0 in a PDIP package? Are there data sheets for it?
A search of the interned didn't turn up what I was looking for, but I'm very new at hardware work. Perhaps newer chips have such tight timing requirements that you can't work with them without using a SOC?
Nearly everything is going to be a SOC because that's what commercial applications need to minimize part count and cost; there's negligible demand for a standalone processor.
If you're just looking to breadboard up a computer but don't want to go back to 8-bit processors, the Motorola MC68000 / MC68008 used in the original Apple Macintosh is a 32-bit processor in a DIP package running at a manageably low frequency and can be found on eBay inexpensively.
You may like Project Oberon [1] designed by Niklaus Wirth [2] then. His guiding principle was to make a powerful but simple system that could be understood from top to bottom by a single person, from the RISC hardware to the OS to the compiler used to compile and run the OS.
It's quite a bit above the Apple ][ in terms of power.
