Dirty tricks 6502 programmers use

https://nurpax.github.io/posts/2019-08-18-dirty-tricks-6502-programmers-use.html

by nurpax • 7 years ago

305 60 7 years ago

nurpax.github.io

fortran772• 7 years ago

Thanks!

6502 is still my favorite architecture, even though I've done assembly language programming (professionally!) on many platforms in the past 35 years.

bcook• 7 years ago

Why is it your favorite?

eej712• 7 years ago

I'm not the OP, but I always liked its simplicity. There's just enough space to do something interesting without getting bogged down in too much complexity.

ddingus5• 7 years ago

I am not the OP either, but 6502 and 6809 are my faves.

6502 was first. It is simple. And that makes it a lot of fun.

6809 is beautiful. I think it is the most powerful and elegant of the 8 bit CPUs. But that spoils a person too.

6502 is like whittling computing down to some useful nubs. There are enough subtleties to make it interesting too.

Zelphyr• 7 years ago

What, if any, modern products still use the 6502?

EDIT: That’s not a dig against the 6502. I still fondly remember leaning BASIC in my C64 and wish now I had ventured into Assembly with it. By today’s standards it seems to have a simpler and more approachable instruction set so I’m wondering if there aren’t products I could hack on to learn Assembly with it. Or maybe I should just break out my old Commie.

LarryMade22• 7 years ago

Toys - and some of those have minimal RAM.

reaperducer2• 7 years ago

Specifically the 6502, or its successors like the 65C816?

The 65C816 is still being made, so someone must use it for something.

eej712• 7 years ago

I believe the Tamagotchi toys used the 6502.

https://hackaday.com/2013/05/24/tamagotchi-rom-dump-and-reve...

forinti• 7 years ago

They were used a lot in cars. I'm not sure if that is still the case.

Gibbon1• 7 years ago

I'm unsure but I think 6502's are available as cores for semi and full custom IC's. Where the the processor core and memory is fully laidout. Bonus runs with a GHZ clock which gives you the ability to twiddle bits like mad.

So outside of retro computing you won't see a 6502 IC in the wild. But they likely are buried deep in nondescript IC's

fulafel• 7 years ago

http://www.6502.org/commercial lists some of these.

kgwxd2• 7 years ago

C64 and many Atari, Apple, and Nintendo products used it, including arcade machines. People still make new stuff, with new interesting hacks, for all of them, every day. I suspect anything new wouldn't be as interesting as playing with the old systems because if anything needed more interesting features these days, they'd use something much more modern. Back then they had to squeeze all the "interesting" they could out of the 6502 specifically. You can easily play with it in the browser [1].

[1] https://8bitworkshop.com/v3.4.0/?=&file=examples%2Fbrickgame...

shagie• 7 years ago

You can still buy them at https://www.westerndesigncenter.com/wdc/chips.cfm / https://wdc65xx.com/chips/ ( https://en.wikipedia.org/wiki/WDC_65C02 )

From the wdc65xx link:

The W65C02S is a low power 8–bit microprocessor utilized in a vast array of products for the Automotive, Consumer, Industrial, and Medical markets. This chip features a full external data (8–bit) and address (16–bit) bus for easy integration with 8–bit peripherals and memory.

Digging in the about page:

Through the last 30+ years as one of the most popular microprocessor architectures of all time the 65xx brand is estimated to have over six billion embedded 65xx processors shipped and is growing by hundreds of millions of units per year, provided by WDC and its licensees. The following is a partial list of high volume applications that have been successful in using 65xx processors:
...
· Toys
· Automobile dashboard
· Appliance controllers
· Industrial controllers
· Embedded heart defibrillator’s
· Pacemakers

userbinator2• 7 years ago

They are still found as embedded cores in computer mouses and keyboards, monitors (OSD processor/scaler), digital picture frames (https://spritesmods.com/?art=picframe&page=1), MP3 players, Furby (https://news.ycombinator.com/item?id=17751599 , actually a 6502-subset) etc.

cbm-vic-20• 7 years ago

There is still a very active 6502 hacking forum at

http://forum.6502.org

(No SSL/TLS, unfortunately).

Lots of people build homebrew computers out of these things, there are a few open source OS and build chains available, etc.

teh_klev• 7 years ago

Not to belittle the article, because it's definitely interesting. But as an ex-BBC 6502 programmer, my nitpick here would be that the title should really be named "Dirty tricks C64 6502 programmers use".

On the beeb we had our own set of tricks specific to the memory layout and ROM of our beloved beige and black machines.

reaperducer2• 7 years ago

title should really be named "Dirty tricks C64 6502 programmers use".

In that case, probably Dirty Tricks 6510 Programmers Use would be even better.

vidarh4• 7 years ago

While that may be technically correct, the programmer-visibile difference between the MOS 6502 and the MOS 6510 is totally incidental in this case - the 6510 has a a built in 6/8-pin IO port (partially used for bank switching the ROMs in the C64). Unless you touch the IO ports, they should behave identically, down to the cycle timings of instructions and the same behavior of undocumented opcodes.

In this case, the real C64 specific tricks are not 6510 specific, but depending on the specific initialization done by the C64 ROM and calling C64 ROM routines.

joezydeco2• 7 years ago

Yeah the moment they introduced a ROM call to optimize the routine, I kind of lost interest.

scarface743• 7 years ago

Why? The memory layout of the text page would have also been c64 specific.

LarryMade22• 7 years ago

There can be a significant trade-off on size vs speed, the more tricks you do to shave down bytes usually adds to the complexity of the iterations.

So assembly programmers may go for the more kludgy looking code as the execution far outpaces the optimized byte count version. Ive heard of such things in video timing and game loops.

jwr• 7 years ago

This really depends on the specific architecture and the application. In some cases, you will want to optimize mostly for size, so that your hotspots fit entirely into I-cache. Modern CPUs spend most of their time waiting for data (or instructions) to become available, so often computations are essentially free.

gpderetta• 7 years ago

the average IPC over a variety of loads is, IIRC, estimated to be ~1. So no, most modern cpu do not spend most of their time waiting for data.

ddingus5• 7 years ago

Screen blitting is a frequent unroll for speed case, often combined with self modifying code to make compiled sprites.

rusk3• 7 years ago

Entries were posted as Twitter replies and DMs, containing only the PRG byte-length and an MD5 hash of the PRG file.

This is clever. So basically rather than getting bogged down reviewing submissions you just pick a winner and then validate post-hoc! (because when you win the hash of your code has to match the one you submitted)

saagarjha4• 7 years ago

I wonder if you could brute force a particular solution with that information.

stjo• 7 years ago

I would say no. 34 bytes is 256^32=7588550360256754183279148073529370729071901715047420004889892225542594864082845696 combinations, and even if you could easily narrow it down to only valid programs you would still need to simulate it, which is way slower than computing a hash.

NieDzejkob• 7 years ago

I guess one should use something more expensive to compute, like a KDF.

FartyMcFarter2• 7 years ago

One way to work around that is to allow authors to include comments in the code that gets hashed.

(I'm not sure if this particular competition did that)

kazinator• 7 years ago

My first 6502 program was self-modifying; I wrote it just before reading the book chapter on using registers for indexing relative to a base address. That book was Programming the 6502 by Rodney Zaks.

I have some 1986-dated 6502 assembly code of mine in hard copy (on dot matrix paper with the "holes" intact). I'm going to scan it one day and post.

vidarh4• 7 years ago

A lot more 6502 code was self modifying than necessary - I know a lot of people (myself included) did not pick up zero-page indexed indirect/indirect indexed address modes and instead kept using absolute x/y indexed and modified the absolute part for larger loops. A large part of the reason why I didn't learn about it until fairly late was that I mostly saw absolute x/y indexing in the code I looked at to learn. It's interesting how many bad habits you'd see in code like that, given e.g. the C64 ROMs were extensively dissected and documented and published, and they used zero page all over the place.

ddingus5• 7 years ago

Raises hand. Yeah, me too.

When I first got zero page, I thought it looked like up to 128 address registers, with only a couple cycle penalty.

But, like you, a lot of code self modified the easier a solute indexed address mode instructions.

And it was right there, easy to see.

js2• 7 years ago

Sorta related, some 6502-based demos:

https://hackaday.com/2018/12/05/apple-ii-megademo-is-countin...

Apple II, but in the comments are links to a C64 demo, a IIGS demo, and a ZX spectrum demo (which is Z80, not 6502, but same era).

Dirty tricks 6502 programmers use

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