Q&A With Bill Mensch, co-creator of the 6502

Same here, Atari 400.

I did get to see some 6809 and 68000 assembly around the time of seeing x86 (and before I wrote any) that I accepted it--but still disliked it. Don't know what I disliked more: direction flag, all registers are special, segments.

The weird instruction/register usage is likely one reason why x86 was faster/cheaper than programmer-friendly processors like M68k--which also made the 6502 a success.

I learned 8080/8085 assembly before looking at x86, and while I can figure out x86 well enough it's still a mess! I don't know that prior experience necessarily makes x86 look any less ugly :P

I agree with both of those points.

I learned 6502 and 68000 assembly language. Then I bought an 80286 assembly language book, started reading it, become disgusted, and returned the book to the store.

Hi there, I'm the alternate timeline you, the one who did start learning assembly with an x86 (and a copy of Microsoft Macro assembler for my 12th birthday). My parents, specifically my father, clearly meant well by trying to get me into a computer that they perceived would be more relevant to career skills than the 8-Bit micros that a couple friends already had. But what I got was not an actual IBM PC or even a close match like a Compaq, but a relatively obscure clone (panasonic, iirc) which, unbeknownst to me at the time, had a slightly different method for switching between video modes than the default CGA adapter/BIOS combo that was presumed by the meagre and not-written-with-middle-schoolers-in-mind materials available in my rural environment. this a decade before the internet would become widely accessible. reaching a worthwhile BBS meant furtive and risky calls to long distance numbers at strange hours of the night, which would certainly be discovered when the next bill came, and have to be covered by whatever allowance I had coming.

Since the main motivating interest was trying to get usable sprite-like animation working for a game I wanted to create, not being able to switch out of character mode without hanging the machine and having to start all over again was a leaden and demotivating drag. Especially because the thing did not instantly reboot but rather, loaded MS-DOS two-point something from floppy at a couple minutes a pop, and then loading up masm with much disc swapping involved the whole time due to the machine having only 256k.

I felt very bad and didn't want to tell my parents about this because they had spent a great deal of their at the time squarely middle class disposable income to get me the machine, and on masm itself, which wasn't particularly easy to get ahold of in the middle of farmland nowhere. And because I knew they meant well, trying to get me on track with a machine that appeared to be the dominant platform at the time and thus worth the extra cash to give me a head start. almost crushed all interest I had in programming, the obscene frustration of having to trial and error calling an interrupt or something, guessing at what the parameter could be and having no idea which out of the 220 (or more) possibilities would be the one that worked. This is not even getting into the perplexing and arcane yak shaving involved in dealing with the stupid x86 segmented memory model ( which I tried to avoid as much as possible, and just fit everything inside a single segment). I didn't understand why it was set up the way it was, so it was purely mechanical "how" ritual memorization without any context or insight.

Even as a 12 year old with nothing to do all summer, I did not have enough time ro sit there cycling the power and to waiting 7 or 8 minutes between reboots. And God forbid the thing broke due to all the power cycling, or chewed up one of the expensive, (for a 12 year old), system disks.

I had no idea how much I was missing by having a supposedly higher end "real computer" . Other me, indeed be thankful you dodged that bullet.

For me it was:

BASIC→Z80→6502→6802→6805→68000→8086→Pascal→...

And honestly, 6502 was the odd one out that felt like programming in mud.

Thanks for sharing this - it's a great interview.

I do think there is a subtlety about the 'wasn't intended to compete with the 6800' line though.

Chuck Peddle recognised the market for a much cheaper version of the 6800 which Motorola refused to make, possibly because they couldn't make it that cheaply with the contact lithography process they were using.

So Peddle went off to MOS Technology (taking lots of Motorola engineers with him including Mensch) to make the cheaper version. To achieve that goal, they had to cut down on some of the 6800's features. Reducing the size of the stack pointer, only one accumulator, and so on.

As it turns out, the end result - the 6501/2 - was actually very competitive with the 6800. So whilst the original intention wasn't to compete with the 6800, in the end it did compete with it very successfully in some important markets.

Of course, in addition MOS Technology didn't have the resources of Motorola and so also couldn't compete with them in terms of support, peripherals and so on.

Note: I wrote the original post on the 6800 that led to the discussion [1] and I'm now deep in the writing of Part 2 of the 6800 story. (So I've now provided spoilers for Part 2!)

[1] https://news.ycombinator.com/item?id=38616591

>not intended to compete with the 8080 and 6800

Clearly, as evidenced by 6501 drop in pin compatibility followed by Motorola lawsuit settled for $200K and pin configuration change.

x86 has a similar regular instruction pattern except that it's in octal:

https://gist.github.com/seanjensengrey/f971c20d05d4d0efc0781...

AIUI, the main difference is that memory locations $0000 and $0001 are used for the 6510 i/o port. So dropping a 6510 in a system that makes use of those perfectly valid memory locations won't work either.

Yeah there's a bunch of 6500 series CPUs with various changes! I'm hacking on a SBC that uses the 6507 at the moment, this particular SBC was a replacement for some industrial control board using random logic sequencing (no CPU). The cut down 6507 saved space and let the designer get everything into the allotted space on a two-layer board.

> If carmaker puts a cup holder in their car & customers like it, competitors' next cars 'must' include a cup holder too. Rinse & repeat ad nauseum.

I think this doesn't reinforce your argument in the way you intended. A car manufacturer isn't going to look at a competitor and see what items need to be removed to lower cost when that item, as you've stated was a success since customers like it, it would be dumb to not include it. You don't want to not include liked items. You want to not include the items the customers do not like.

Ultimately these decisions come down to value judgements. Which is better, more simplicity and lower cost, or more robustness and longer service life? Because often it's one or the other.