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Note: All links are to the 6502, HP-41, and related things.  Please do not ask me to add a link to your stop-smoking program or anything else unrelated to my website.

Major features on this website: (and three more very major features are planned, plus several smaller ones, but it all takes so much time!)

list of 77 of just my own articles, on this site and others

Link categories on this page:

65-family processors, history, HDL, emulators (which are hardware):

Why use the 6502? (2-page forum topic, with links to more)  It has great strengths that don't initially meet the eye!
6502 origins
65816 origins, 6516, 65032, 65832
65CE02 improvements over the 65c02  Original Commodore 65CE02 data sheet is here (.pdf)
6516 (Synertek) 16-bit pseudo-6502 for Atari 400/800 computers, never made it to market
65020 double-wide 6502 proposal
6502EX (6502 extended to 32 bits)
65Org32 developments of ideas for an all-32-bit 65816 extension (forum topic)
65GZ032, a 6502-compatible 32-bit VHDL core from Gideon Zweijtzer and his team.  They had hardware working, but never finished.
65m32, a 65-oriented 32-bit processor concept from Michael Barry.  Operands up to 16 bits wide are merged with op codes so the entire instruction can be fetched in a single clock cycle.

Silicon Genesis interviews: Bill Mensch, inventor of the 65c02 and 65816 (about 40% of the way down the page)  Click on his name, the title above that paragraph, for the video interview.  If you click on the "here" which is the last word in the paragraph, you'll get the text trascript instead.
Bill Mensch interview Aug 2015 on 6502 and '816, and comparing them to ARM, 68000, x86, 6800, 6501, etc. and his business model, and his goals.&nbps; He obviously has a very clear vision, and he's accomplishing what he wants. Still. Today.
David Cramer interview (VP of business dev at WDC) Mar 2015.  He says there are hundreds of different products being made today with 65xx processors in them.
Chuck Peddle gives reasons for design decisions on the 6502

6502 emulation with an ARM
6502 emulation with an AVR by Daryl Rictor
6502 emulation with an ATMega16 by Klaus Dormann  (See also this forum topic.)
Daryl Rictor's 65802 module, a hybrid using a 65816 and other ICs to plug into a 6502 socket like WDC's no-longer-available 65802.  The forum topic that goes with it is here.

How 6502 illegal op codes really work
More on illegal op codes
Yet more on illegal op codes
x7 and xF illegal op codes article from Byte magazine, Dec '77
visual6502 wiki page on illegal op codes
No More Secrets v0.9 [2015] A very complete, 67-page indexed .pdf illegal op code reference, very nicely laid out, with good explanations of every instrucion.  From The C-64 Scene Database.

Rob Finch's bc6502 verilog 6502 source  (Note: This finitron website is his newer one, but the file dates are older than what's in the archive.org URL linked in the 65xx HDL page linked above.)
links to 18 free HDL 65xx cores
comparison of many HDL 6502 cores available (forum topic, with links)
BMOW tiny 6502-like CPU in CPLD
programmable-logic basics: free online course
links to more help understanding the innards of the 6502 at the gate and transistor level
photo of decapped 6502
For 65-family peripherals made with programmable logic, see the next section below, "65-family peripheral ICs."

KimKlone 6502 w/ pointer-arithmetic-friendly extended address space and 9-cycle ITC Forth NEXT
Build your own 6502 with TTL by Ruud Baltissen
6502 ALU using 74xx logic (minus BCD), by someone called "latebit"

Western Design Center (WDC) is the main 65c02/816/134/265 manufacturer today.
Jeff Laughton's improved to-scale timing diagrams for 14MHz 65816, including for understanding to help prevent bus contention at changes in data direction which can happen twice per '816 clock cycle.
Jeff Laughton's animated visualizations of timing margins  These excellent, drawn-to-scale (unlike most in data sheets) .gif's help understand what timings are constant and what varies with clock speed.  Discussion about them is in the forum topic here

65-family peripheral ICs:

my "Tip of the Day" column for 6502 development has many tips on using the 6522 VIA and 6551 ACIA
Daryl Rictor's 65SPI IC he sells.  It is a 65-family I/O IC to multiple SPI devices.
6581 SID emulator in Atmega8
SwinSID 6581 emulator in 32-bit ARM Cortex running at 80MHz  (There are different versions there.)
Papilio Duo C64SID SID in FPGA
6502.org's datasheet index
data sheets to lots of less-common 65-family peripheral ICs (Rich Cini's site)
Western Design Center (WDC) is the main 65c21/22/51 manufacturer today.
Jeff Laughton's unconventional circuit tricks for ultra-fast (single-cycle!) I/O using 65c02 illegal op codes in the _3 and _B columns, plus re-mapping op codes  (This is in the circuit potpourri page of my 6502 primer.)

interfaces and buses (also see the descriptions on en.wikipedia.org):

digital audio plug-in modules for Commodore 64/128
Wishbone computer bus
concise comparison of synchronous-serial interface types (forum post)
I2C overview from Philips Semiconductor (.pdf)
I2CChip.com I2C, SPI, 1-Wire interfacing made easy
guide to reliable 1-Wire® networks
Microwire/Plus description from National Semi (.pdf)
B&B Electronics: serial converters
65SIB spec. (serial interface bus, hobbyist-friendly, compatible with SPI, Microwire, and dumb shift registers, but more flexible)
SS22 spec. (6522 synchronous-serial data link between computers)
I2C-6: proposed connector standard for I²C, hobbyist-friendly, suitable for breadboarding (6502.org forum)
SPI-10: proposed hobbyist-friendly connector standard for small SPI modules, suitable for common perfboard
My RS-232 primer
SD-card SPI Maxim ap note (.pdf)
SD/MMC cards, using in SPI mode
SD/MMC/SDHC card library
USB/Bluetooth for Apple II
USB modules (Hobby Engineering)  Note, fall 2016: (hopefully) temporarily closed due to a bad accident.
MAX3421E USB peripheral/host controller IC with SPI
running the bus off the board, and interfacing external boards and devices (forum posts)

65 software: math (including coprocessors), algorithms, simulators, OSs, programming languages, programmers' info:

    (See also the assemblers and Forth sections further down)
Large look-up tables for super fast, accurate, 16-Bit scaled-integer math (like having a coprocessor)
6502.org wiki (math, utility routines, datasheet errata, etc.)
source code on 6502.org, in various categories
tutorials and primers on 6502.org
Omegamatrix's fast, short division routines to divide an 8-bit number by any number up to 32
Lots of 6502 books, including for programming, scanned and available for download.  See especially under "Programming", starting about 70% of the way down the page and going to the end.  I have a few of those in paper.
André Fachat's context Switching and thread synchronization on a 6502
André Fachat's GeckOS/A65 V2.0, a full-featured Multitasking/Multithreading OS for the 6502.  It is preemptive and implements some Unix-like features, like signals, semaphores, relocatable file format, standard library, internet support via simplified sockets, and virtual consoles.
Jon's preemptive multitasking GUI OS for Atari 6502 computers
DOS/65 CP/M-like OS for 6502
operating-system wiki (OSDev.org)
Taygeta C64 Forths
Forth processors (stack computers)
EhBASIC by Lee Davison.  An outstanding interpreted BASIC for 6502.  Note: If that link doesn't work, try this one.  Unfortunately Lee passed away, in 2013.
Create your own version of MS BASIC for 6502
Microsoft BASIC for 6502 Original Source Code (1978)
Steve Wozniak's Sweet16
cc65 wiki.  cc65 is a 6502 C compiler and supporting software.
high-speed function approximator using Minimax quadratic interpolator
HAKMEM programming tricks (math, algorithms)
many math routines in 6502 assembly on the Codebase 64 wiki
Codebase 64: Commodore 64 source code, articles, tutorials
CORDIC algorithms for FPGA-based computers (.pdf)  (this link has the same article)

IEEE floating-point standard (.pdf)
IEEE-754 floating-point standard for arithmetic (Wikipedia)
IEEE floating-point conversion page
6502 BCD floating-point scientific math package, 12-digit plus 3-digit exp, up to hyperbolic functions
128-bit floating-point 65816 implementation, up to log, trig, and hyperbolic functions, by Marco granati
AWC Electronics math coprocessors, 32-bit floating-point, serial-interfaced
Micromega Corporation math coprocessors, 32- and 64-bit floating-point, serial-interfaced

sorting algorithms, as dances
BDD's 65816 string-manipulation library
BDD's Supermon 816 machine-language monitor for the 65816  (Also at this forum topic.)
Program-Writing: Where Do I Start?     (section 18 of the 6502 primer)
Tips For Programming the 65(c)02 (sometimes affecting hardware design optimization)    (section 20 of the 6502 primer)
my program structures in 65c02 assembly, through macros (for PIC16 also)
simple methods for multitasking without a multitasking OS, for systems that lack the resources for a multitasking OS, or where hard realtime requirements would rule one out anyway
programmers' articles about memory
6502 v. 65816 code length comparison
6502 v. PIC16 code length comparisons
"Programming the 65816—Including the 6502, 65C02 and 65802" 6502/65816 programmer's manual by David Eyes and Ron Lichty.  The best!  From Western Design Center.  (.pdf)  Note: There were many problems with the earlier .pdf version that were not in the original paper manual; but in late March 2015, WDC scanned and OCR'ed the paper manual and posted the new, repaired .pdf.


65816 free assemblers
lots of 6502/816 assemblers, disassemblers, C compilers, simulators, & related tools
Andrew Jacobs' As65 assembler which has program-structure capability built in
Anton Treuenfels' HXA 6502 assembler which also has program-structure capability built in
P65 portable 6502 assembler
64tass powerful macro assembler and linker for 6502, 65C02, R65C02, W65C02, 65CE02, 65816, DTV, & 65EL02, from soci
Cross-32 (C32) assembler (which I use) formerly from Universal Cross Assemblers, now sold by Data Sync Engineering.  Also available at http://www.mpeforth.com/cross32.htm.
Merlin 32 modern cross-assembler, linker, disassembler, simulator, and more, for 6502, 65c02, 65816, for Apple IIgs and others, by Antoine Vignau and Olivier Zardini
SB-Assembler 3 (free) which now runs on Linux, MAC and Windows machines, and assembles for 6502, 65c02, and many other microprocessors and microcontrollers, announced Dec 7, 2016.  There are also earlier versions available that run on DOS and Apple II and even Apple I.
MADS multi-pass crossassembler for 6502 and 65816.  (This page is mostly in Polish, but important stuff is available in English through the links in the top-right corner.)  Binaries are generated mainly for Atari 8-bit systems (supported are AtariDosII and SpartaDOS X formats).
Assemblers And Loaders 1993 book by David Salomon on how to write them.  294 pages.  Available for free download.   (Not 6502-specific.)
Michal Kowalski's 6502 IDE and simualator (.zip download, with executable for DOS/Windows, but can be used with Mac & Linux under WINE)  More info here
BDD's macros for assembling 65816 code on Kowalski's 6502 assembler
Kick Assembler for NMOS 6502 is the main (but not the only) one discussed in the Retro Assembler facebook group, at the link.  It is written and well maintained by Mads Nielsen on that page.  You can have direct contact with him there and get quick answers.  You can download the Kick Assembler here.

for individual computer brands:

Acorn BBC Micro (beeb)  Lots of links to beeb resources (forum post)

Apple II original ROM info, ROM listing, Sweet 16, monitor, floating point, articles, plus SYM and AIM monitors, and AIM BASIC ref manual.  257 pages.
eBook: "What's Where in the Apple" 400-page Apple II resource which had been out of print since '84
SD card "drive" for Apple II on Hack-a-Day
Apple IIgs programmers' home
loads and loads of scanned Apple II books
"Apple Assembly Line" archived issues, 1980-88

Atari Archives
APE Atari pages
Trevin Beattie's Atari Technical Information page
loads and loads of scanned Atari books
Atari Wiki

DLH's Commodore Archive  Tons of Commodore documents
Zimmer's Commodore pages
Source code of GEOS 2.0 for the Commodore 64
The Fridge source code programmers' resources for C64 and 6502
64 Bites C64 programming tutorial videos.  They say it's "The easiest way to start programming Commodore 64," and Michal Taszycki says, "I am working hard to make it the #1 best place to learn about Commodore 64 in the world."  (Although most of the resources in this links page are free, the videos on 65bites.com are not.)
Commodore PET index
Commodore.ca  All things Commodore
The C-64 Scene Database, dedicated to gathering as much information as possible about the productions, groups, sceners, events, and BBS's in the Commodore 64 scene
Dan's 8-bit Commodore Info  Lots of Commodore links
Commodore disc images on CD  Over 1100 disc images (D64 and ADF) containing over 10,000 programs for C64, VIC20, Amiga, C128, and PET, many of them originals published by TPUG, plus historical photos, emulators, and the library catalogs in PDF format.
Commodore.software, archived software and reference material for most Commodore computer systems.  There are C64 and Pet BASIC compilers here, and lots more Commodore links here.
Commodore 1541 floppy-disc drive ROM disassembly
The C64 PLA Dissected (.pdf)
modern products for C64 from NKC Electronics
V-1541, a virtual 1541 disc drive that you can use from your Commodore 64 computer
Commodore Wi-Fi modem
Flyer, the combination internet modem and disc-drive emulator for Commodore 8-bit computers
C-One FPGA-based enhance adaptation of the Commodore 64
Codebase 64 wiki C64 source code, articles, tutorials, magazines, books, projects, and links
Creative Micro Designs unofficial site  CMD was a third-party supplier of software and hardware for the C64, including the 20MHz 65816-based SuperCPU accelerator.
SuperCPU Home
loads and loads of scanned Commodore 64 & 128 books
loads and loads of scanned manuals for Commodore 64, accessories, and software
lots of scanned Commodore PET books
Retro Innovations Store facebook page for new boards and accessories for VIC-20 and C64
ACE OS by Craig Bruce for the Commodore 128 and 64 that provides a Unix-like command-shell environment.  All ACE programs will run on either a C128 (in 128 mode) or a C64.  "ACE" stands for "Advanced Computing Environment."

Memblers' resource-rich NES page
Dave's OSI (Ohio Scientific) pages
loads of documents on AIM-65, Apple II, VIC-20, C64, Kim-1, Sym-1, others  (Rich Cini's site)
Briel Computers retro computer kits: Apple, Altair, KIM, others, plus modern accessory boards

65-family current suppliers:

Western Design Center (WDC) makes and sells 65-family hardware (all 14MHz+) but makes most of their money licensing IP.
list of distributors

6502-related discussion groups:

6502.org website: many homebuilt projects, primers, tutorials, books, data books, articles, magazines, dev tools, source code, etc.
6502.org forum (lots of activity, unlike the Yahoo forums below)
6502 assembly-language programming facebook group  (Consider facebook groups to be more of a chat room, since fb records are not very permanent, nor searchable, and they don't allow categories or any formatting options suitable for code or much of anything except chats.)
Retro Assembler facebook group is dedicated to 6502 assemblers, primarily (but not only) the Kick Assembler.
6502 Yahoo forum
Apple II Yahoo forum
C64 Yahoo forum
CSDb C64 forum
OSIweb (Ohio Scientific) forum
MOS6502 on G+  weekly 6502-related posts, curated by André Fachat and Ed Spittles (There's a post index in a 6502.org forum topic here).  See this page for monthly summaries.
See http://6502.org/groups for 6502-related USENET newsgroups and mailing lists.
Vintage Computer Forums, including sections for Apple II, Atari, Commodore, vintage computer programming, etc.

others' 6502 web pages (definitely non-trivial!)

André Fachat, HW & OS architecture including a multitasking, multithreading 6502 OS
Daryl Rictor's 6502/816 boards, modules, and parts he sells
Sprow's MiniB BBC-Micro-based SBC
Douglas Beattie's 6502 pages
excellent visual transistor-level simulation of 6502
BCS Technology, "BigDumbDinosaur's" site, with his POC V1 SBC


techniques for construction of reliable high-speed digital circuits (forum topic, with lots of good resources)
wire-wrap Q&A (section 12 of the 6502 primer)
raster graphics on an analog oscilloscope (involving a 6522 VIA's shift register port)
stacking ICs to get more on a board (in the lower third of the page)
VGA primer
my "Tip of the Day" column for 6502 development
my article on interrupts.  6502 has outstanding interrupt performance compared to other processors.  (Enjoy my out-of-date cartoons! :) )  It's also on 6502.org, here.
BDD's 65816 follow-up on my 6502 article on interrupts (also posted on 6502.org, at http://6502.org/tutorials/65c816interrupts.html)
my project pages on 6502.org.  The workbench computer portion is outdated, but I want to eventually get it all on my own site here, up to date, more complete, and with more pictures and better scans.  I use my workbench computer as kind of a Swiss army knife for the workbench.
many 6502 books and magazine articles for free download
thousands of scanned magazines, C64, Apple, other
thousands of scanned books on 6502, C64, PET, Apple, Atari, others
my slide rules I used in my early years (also slide-rule links)


Software survivalism, by Samuel Falvo  (In spite of the name, it's about hardware too.)  I would like to see this way of thinking become more popular and organized.  (If that link ever goes down, it's cached here on this site.)
Neo-Retro Computing, by Samuel Falvo.  (If that link ever goes down, it's cached here on this site.)
Low Fat Computing (A politically incorrect essay by Jeff Fox)  He and Chuck Moore (inventor of Forth), taking an entirely different programming philosophy, plus Forth hardware and software, have improved the compactness and speed of code by factors of 100 to 1000.
ANSI Forth is anti-Forth, by Jeff Fox, agreed by Charles Moore, inventor of Forth.  See why.
Assembly Language:  Still Relevant Today
Why I do this web site
Why we do 6502 (forum topic)
Things I do and don't like, relating to the computer hobbyist  (You don't have to agree.)


my 32-bit DO LOOP & related words in 6502 ITC Forth
Taygeta C64 Forths
Forth processors (stack computers)
Stack Computers: The New Wave The entire book is available to read online or to download.
Many links for stack computers and Forth material
UM* (multiplication) bug in common 6502 Forths (and my fix)  Also shows some faster variations, with code size and speed comparisons.
UM/MOD (32-bit division) bug in common 6502 Forths (and my fix)
D< bug in common Forths, plus my fix (on forum)
65c02 interrupt service in high-level Forth, with zero overhead!
KimKlone 6502 w/ pointer-arithmetic-friendly extended address space and 9-cycle ITC Forth NEXT
6502.org forum Forth topic index
Bruce Clark's 2-instruction 65816 NEXT in ITC Forth
Bruce Clark's single-instruction, 6-clock 65816 NEXT in DTC Forth
Bruce Clark explains how the faster-running STC Forth avoids the expected memory penalties.  He gives 9 reasons, starting in the middle of his long post in the middle of the page.  STC of course eliminates the need for NEXT, nest, and unnest, thus improving speed.
"Starting Forth" book by Leo Brodie, available online, somewhat modernized, but with the original cartoons.
explanation of five different Forth threading methods, by Brad Rodriguez.  There's a list of more of his related articles at http://www.bradrodriguez.com/papers/.
Walter Rottenkolber's explanation of the A081 in figForth's 6502 Forth, for forming new vocabularies
Atari Wiki Forth index
See also the links in the philosophy section above.

HP-41 calculator/computer:

HP Journal, Mar 1980 (pdf), 26 pages of articles telling about the design of the new HP-41c, card reader, HP82143A printer, LCD, and more
MLDL2000 (Meindert Kuipers)
MLDL2000 pictures
HP41CL (50 times as fast, 200+ KB of RAM, 230 modules built in, retains port compatibility) (.pdf)
Newt: 41 processor emulator (for 41CL) (.pdf)
NEWT processor and 41CL
41CL wiki
Fix That Calc (Randy's repair service.  Note that due to health problems, Randy is not taking new repair jobs at the moment.  He hopes to have good news early in 2017.)
Vintage Calculator Repair  (Bruce Larrabee's repair service)  A death in the family caused a lot of chaos and stress in the family and made him unable to maintain the calculator repair business for the last year and a half, and neck surgery didn't help the situation either; but an update on his site 2/23/17 says he will be getting back to the repairs and has some improvements in his process that should speed things up.  He has a new phone number, 541-731-5130, which you can call 9:00 am to 8:00 pm PST.  Note that he has moved.
hp*classic*, an eBay seller, has a service where he charges $80US for repairing HP-41 card readers.  At the time of this writing, the repair link is http://www.ebay.com/itm/262868410512?ul_noapp=true but I don't know if that will change.  Assuming I keep getting updates, I'll keep the link updated here.
flex-circuit replacement.  There's a video showing how to install the replacement here.
41cv coconut repair
41 service manuals download (Shucks—just gives a 404 now.  Tell me if you know where it got moved to if it still exists.)
41 LCD pictures, for servicing
Post replacement that works! (forum post)
For card-reader repair, see here and here.
French HP-41 forum
3E0_RTN's Corner (French)
French HP-41 website
HPIL files explanation
LIF utilities for Linux
HP82161A HPIL digital cassette drive service manual
HP computer museum (Australia)
HP calculator museum
HP41.org, called TOS (for "that other site") at HP Museum (I am a moderator on the forum
HP-41 bar-code generation
online bar-code generator
Michael Göricke's software for generating HP-41 bar code on a PC
Forth for the HP-41 (.pdf)
here and here are a couple of HP Museum forum topics on Forth for the 41; and linked there are:
this and this beautifully done, very clear .pdf M-code guide (in two 70-page parts, lower page and upper page, respectively) to HP-41 Forth, done in Apr 2014, more excellent work by Ángel Martin!
MCODE wiki
Geir Isene's HP-41 pages
Matthias Werli's HP collection
Jeff's calc page
Jeff's Emu41 page
Jeff's HPIL resource page
over 200 HP-41 programs in Jean-Marc Baillard's collection
MoHPC's HP-41 software library
Math in Silicon (.pdf)
module database
Advantage ROM: Valentin Albillo's write-up (.pdf)
manual for Ángel Martin's 41z module which gives a true 4-level complex stack and tons of complex-arithmetic functions and enhancements written in MCode for maximum performance.  Update, Apr 15, 2016: The manual on the latest version is available here.
manual for Ángel Martin's Sandmath module which gives gobs of math extensions for the HP-41.  Update, Jun 13, 2016: The manual is also available here, in case you don't want to sign up on hp41.org to download it.
manual for Ángel Martin's Total_Rekall 2015 Module (.pdf) which offers RCL math on steroids, and full stack tests, to make programs in any discipline faster and more compact, plus the Unit Management System and constants library
More modules from Ángel Martin
HEPAX quick-reference card (.pdf)
keyboard overlay generator
HP manuals, discussion by Richard Nelson on p.26 (.pdf)
HP calcs wiki
Floppy Days Vintage Computer Podcast with Randy Kindig, #59 (May 2016), "The HP41C Calculator, Part I, History with Richard Nelson and Gene Wright"
Floppy Days Vintage Computer Podcast with Randy Kindig, #60 (June 2016), "The HP41C Calculator, Part II, History with Richard Nelson and Gene Wright"  Randy asked me for a recording on how I got into the HP-41, and he put that at the beginning of the podcast.
How I got into HP-41
My HP-41 alarms and daytimer program

Email me at wilsonmines@dslextreme.com if you have suggestions for additional links, corrections, or to report dead links.  You can use this email address for individual discussion too; but for forum-type discussion, use 6502.org's forum for 6502-related discussions (Note!  We absolutely do not tolerate spam!) and HP Museum's forum or hp41.org's forum for HP-41 discussions.  HP41.org's forum is 41-specific, but it does not have much traffic at all compared to HP Museum's which is for all the HP calculators, including ones being made today. 

last updated Apr 21, 2017