Again, a question that came up while reading through old forum posts. When I first found out about the 65816, I was really, really excited. A 16-bit version of the 6502 with up to 16 Mb of RAM? Awesome! However, it seems that there is not much love for the CPU. Obviously, there is BigDumbDinosaur's POC V1.1 (http://sbc.bcstechnology.net/), and Daryl's SBC-4P (http://sbc.rictor.org/pinfo4.html), for example. But there are fewer projects, assemblers, and simulators than I would have expected. What gives?

So far, I would guess that there are three reasons:

1. The 65816 is not a true 16-bit processor, but a 8/16-bit hybrid (which is what WDC says on their website). As such, it suffers from the worst of both worlds: Segmented memory (some commands wrap to the beginning of their banks), but with important stuff limited to Bank 0 (zero pages, boot vectors, stacks). Even if you tried to use the chip for a bunch of virtual 6502s, the "ZP in Bank 0" situation would prevent ever using the full 16 Mb (max of 256 instances of 64 Kb each with 256 byte ZP would eat up the complete Bank 0, if my math is correct).

2. The 16-bit code seems bolted on. Whereas the 6502 instruction set is elegant, parts of the 65816 feel like, well, downright hacks. Switching from 8-bit to 16-bit mode requires two instructions and manipulating the carry bit, and you have to keep telling the assembler what mode you are in (LONGI ON). REP and SEP makes your head hurt because you have to remember which bits to set. Nothing that a bunch of assembler macros couldn't make easier to tolerate, but not exactly pretty, either.

3. The memory addressing is harder for the hobbyist to handle, mostly because data and bank lines are shared. I see the data sheet for the 65816 has some special glue logic they recommend for this. After reading about BigDumbDinosaur's problems with a hardware bug ("Unlike the 65C02, the 65C816 can generate false address bus states during the intermediate steps of processing certain instructions."), my enthusiasm is dampened as well.

Now, am I being unfair to the 65816 and missing something? Or is this just a chip that could have been designed a whole lot better? And while we are at it, it doesn't seem that there is a good 16-bit CPU out there for hobbyists at all, at least for assembly programming fun.

Thanks,
Y, Scot

For me, the 65816 holds no nostalgic value, unlike the 6502. My first two computers were based on a 6502, and that's where I learned programming. Many popular computers from that time were based on a 6502, while the 65816 was not nearly as popular.

For modern day stuff, I see no reason to use a 65816 or any other 16 bit controller. I'd rather use a modern ARM Cortex. These are cheap, come in hundreds of different packages and configurations, including tons of peripherals, can be programmed in C with free compilers, and are much more powerful than the 65816.

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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?

Hi scottw
I went through the same reactions: the 816 seemed like a great step forward to someone rooted in 6502 systems. With richarde doing most of the work, we made an 816 upgrade for the beeb, which we also tried on the oric (with less success.) See https://sites.google.com/site/beeb816/

But it does seem - to me - much less attractive to program. Perhaps it's just too big, perhaps it seems untidy. From a commercial and implementation perspective, it's a good idea, no doubt about that: compatibility and low pin count are both valuable.

I'd urge you to ignore BDD's hardware bug, or if not, then understand exactly what happened. It was a real bug, but he was interfacing to an unpleasant chip and happened to use a troublesome addressing mode. I'm sure it was frustrating and of course it had to be fixed, but I think he over-reacted.

As Garth says, there have been several efforts here (and probably elsewhere) to dream up a better CPU. Sometimes they even get implemented. The 65Org16 has run successfully on FPGA, including with external memory - which is not so easy, because single-cycle asynchronous SRAM isn't normally what you have. The 65org32 remains unimplemented - it does have the advantages Garth spells out, and the 65Org16 has a subset of those advantages. You might be interested to look through the summary of threads at viewtopic.php?f=1&t=1982

Cheers
Ed

The biggest issue is what Arlet brought up. It didn't have big commercial success in home computers, like the 6502, Z80, 68k, and x86 did. The only real presence there was was the Apple IIgs, which wasn't that much of a hit as 68k home computers were already on the market at that time. Of course, it was in the wildly popular SNES, but that's not user-programmable on its own.

So it didn't entrench itself with the hobbyists, and in shopping for features it makes more sense to get a "real" 16-bit or larger processor when you need oomph beyond the basic 8-bit. I think it has very little to do with the programming model.

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WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

There's a fair amount of interest and activity still in SNES land - as you say, it's not a computer you program, but then cross-development is common anyway now that we all have bigger faster computers to work on.

The other point in favour of '816 is that there are not a lot of attractive alternatives at 16bit: x86... and arguably 68k, although that's somewhat like a 32bit system with a 16bit bus.

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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?

The 65CE02 went into the never-released Commodore 65, of which there are a few (very pricey) prototypes floating around, but those were only clocked at 3.54MHz. And yeah, it was a "Let's not compete with the Amiga" sort of thing, and C= going to Bermuda instead of caring about finishing & marketing tech.

I do have a SuperCPU, but I don't know all too many technical details about it. It's got a CPLD on board as well to handle all the glue to the C64's bus.

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WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

White Flame, if you could take some photos of the SuperCPU circuit board I'd love to see them! Especially if we can make out the component numbers.
Cheers
Ed

It's unfortunately packed away right now, but GIS seems to know about it:

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WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

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

All processors could be designed better.

The 65816 was of course designed to stay close to the 6502 in hardware and software. I like having the 16 bit instructions when needed but I wouldn't want to be in 16 bit mode all the time. ARM has thumb instructions as well.

It just takes a little time to get used to the 816, macros make 8bit-16bit switching a non-issue.

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

Thanks for the photo WF!
It led me to some info: I've started this new thread.
Cheers
Ed

The shared data and bank address lines now have a thread of their own, here.

Yes, the 65C816 can generate false addresses. That's a problem that existed on the NMOS '02 and was fixed on the 'C02. I guess maybe it's easy to get blind-sided by this, but the fix on the '816 takes a different form, namely the VDA signal (which is unique to the '816). The data sheet states,
So they do tell ya, but you really need to use your eagle eyes when reading the doc! So it's good to mention this issue. But, as BDD says,
cheers.
Jeff
ps- thanks, White Flame, for posting the SuperCPU photo!

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