Interesting Video about 6502 "broken" ROR

Discussion of development of software for any "obsolete" computer or video game system. See the WSdev wiki and ObscureDev wiki for more information on certain platforms.
Post Reply
User avatar
gravelstudios
Posts: 159
Joined: Mon Mar 13, 2017 5:21 pm
Contact:

Interesting Video about 6502 "broken" ROR

Post by gravelstudios »

An interesting YouTube video in which the narrator claims the ROR instruction on the first version of the 6502 was not broken as has long been assumed, but was left intentionally unimplemented as a design choice:

https://www.youtube.com/watch?v=Uk_QC1eU0Fg
Drag
Posts: 1615
Joined: Mon Sep 27, 2004 2:57 pm
Contact:

Re: Interesting Video about 6502 "broken" ROR

Post by Drag »

I watched that too. I suspect the reason everyone thought it was a bug was because there isn't a clear revision marker between the original 6502 and the revision which adds the ROR instruction (i.e., you'd have to rely on the date code), and the unimplemented instruction which takes ROR's place in the original design ends up doing a shifting operation anyway. Still, it's interesting to have proof that the ROR is completely physically omitted in the original design, and not just "bugged". Myth busted!

Something else that bothered me about the 6502 is, the asymmetry of their decision to call it Logical Shift Right, but Arithmetic Shift Left. They call it "arithmetic" where it doesn't matter (ASL and LSL would do the same thing), and there's no accompanying "arithmetic" right shift (which duplicates bit 7 as it shifts). This clearly didn't hold the 6502 back, but it's a completely harmless yet pedantic thing that bothers me nonetheless. :P
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: Interesting Video about 6502 "broken" ROR

Post by lidnariq »

In the interview that TubeTime had with Bil Herd and Bill Mensch (where in Bill Mensch described the missing ROR as a "brain fart"), Bil Herd said that he'd been told that the mnemonics all came from the PDP-11
User avatar
Movax12
Posts: 541
Joined: Sun Jan 02, 2011 11:50 am

Re: Interesting Video about 6502 "broken" ROR

Post by Movax12 »

I have "The Best of BYTE Volume 1" and remember reading this article: https://www.tech-insider.org/microproce ... 7511-a.pdf
I couldn't remember what they said about ROR, but on the bottom of the second page, they clearly refer to ROR as being absent and nothing about a bug. Perhaps a bit more evidence that it wasn't called a bug until after it was added to later revisions.
Pokun
Posts: 2681
Joined: Tue May 28, 2013 5:49 am
Location: Hokkaido, Japan

Re: Interesting Video about 6502 "broken" ROR

Post by Pokun »

Drag wrote: Fri Mar 03, 2023 12:27 pm Something else that bothered me about the 6502 is, the asymmetry of their decision to call it Logical Shift Right, but Arithmetic Shift Left. They call it "arithmetic" where it doesn't matter (ASL and LSL would do the same thing), and there's no accompanying "arithmetic" right shift (which duplicates bit 7 as it shifts). This clearly didn't hold the 6502 back, but it's a completely harmless yet pedantic thing that bothers me nonetheless. :P
This always bothered me too and basically just makes the mnemonics harder to memorize. An LSL and LSR pair would have been simpler and easier to remember than the ASL and LSR pair.
I suppose they wanted to show that ASL, being fully arithmetic, can be used with both unsigned and signed values while LSR, only being logic, only works with unsigned values.
Post Reply