Another Neutron Update + A Rant on Annoying 16-bit Microprocessor Things

Let us start off with the rant mentioned in the title. Whenever I think about making a retro computer or device of some sort, I immediately jump to 16-bit because that era is the most interesting one to me. Sadly, the 16-bit processors available on the market (or at least Mouser) are extremely annoying to design around.

Let’s start with the W65C816S, which was originally planned to be used in the Neutron. Annoying thing number #1: multiplexed pins. The 65816 has a 24-bit address bus and an 8-bit data bus, with a 16-bit internal architecture. However, the physical package only has 16 address pins. Where do the other 8 pins go? Why, they’re the data bits! Because for some god dang reason, Western Design Center decided that making the data bus sometimes be part of the address bus was a good idea (IT ISN’T). And that’s not taking into account annoying thing number #2: the interrupt vectors and handlers HAVE to be located in bank $00 (specifically, the vectors must be in the $00FF00 area). The stack is also restricted to bank $00. And this causes a dilemma: unless you’re willing to add a chip (or a feature to an existing chip) that copies the interrupt vectors and handlers from ROM into RAM on reset, you’re going to end up with a crazy memory map like the one in the Super Nintendo if you want to have a stack. So, TL;DR: The 65816 is annoying due to multiplexed address/data pins and the location the interrupt vectors and handlers have to be in.

Up next on the chopping block: The Z8000, or specifically the Z16C02 (being the only version that’s still on the market). For some reason, instead of just having SOME of the data pins act as address pins, THE ENTIRE ADDRESS BUS AND DATA BUS USE THE SAME GOD DANG PINS, WHAT WERE YOU THINKING ZILOG????? WHY DID YOU THINK THAT WAS A GOOD IDEA? AND WHY IS THE ONLY VERSION THAT’S STILL AVAILABLE THE ONE THAT CAN ONLY ACCESS 64K????

Finally we have the Motorola 68000. While Motorola isn’t producing it anymore, the semiconductor company that they spun off during the split, Freescale, does. Or should I say, DID, because NXP ate them. Luckily, NXP does make the chips available. Sadly, they’ve got too many variations. It is seriously confusing. I’d give you an image to demonstrate this, but WordPress is an unstable mess and I fear that trying to add an image will brick the site for another 5 minutes…

Okay… finally got that out of my system. Since there aren’t any easy-to-implement 16-bit processors on the market, I’m just going to start out with an 8-bit processor. The 6502 should do nicely. I do plan on returning to 16-bits once I can get my hands on an easy to implement processor, but for now the 6502 will be good enough.

I’ve also revised what exactly I’m making. I still want to make a retro console, but that doesn’t mean I can’t also make a retro computer. Or multiple of both of those. So, here’s my plan:

  • I’ll make 2 8-bit computers:
    • A Desktop running at 8 MHz, called the Neutron Station (Model N8D)
    • A Handheld running at 2 MHz, called the Neutron Pad (Model N8P)
  • I’ll make 2 8-bit consoles:
    • A stationary console running at 8 MHz, called the Neutron Box (Model N8C)
    • A handheld console running at 2 MHz, called the Neutron Mate (Model N8H)

Yep, 4 models. And 2 of those will need built-in screens. How fun :D. The computers will share one memory map (which will be similar to but not compatible with the Commander X16), while the consoles will use a different memory map (because they have to account for the cartridge). All of them will have the same audio and video chip, which is still called the AVC, or Audio/Video Controller. There will be 64K of Video RAM.

The desktop will have 4 expansion card slots. The handheld computer will have 1 expansion card slot.

As for the consoles, they will need cartridge slots. As for the memory map, it is halved, then halved, then halved again. It starts out with a 16 KiB internal RAM area, followed by 8 KiB MMIO, then a banked 8 KiB area for cartridge RAM (this is the same as High RAM in the computers). This is then followed by the cartridge 16 KiB Low ROM/Flash, which is banked similarly to the cartridge RAM (meaning you can have up to 4 MiB in here!). Finally, at the end of the memory map, is the 16 KiB of cartridge High ROM/Flash.

Okay, I said a lot in this post. And it’s the middle of the night, so I’m tired. I’ll see y’all later, I guess. Have an awesome day!

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Ashton Snapp

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