Here you will find a collection of useful programs and routines (mostly written in assembly language) that can be adapted to any 6502-based computer. Please send your contributions to us via in any format.

Steven Judd runs an excellent site called The Fridge which contains 6502 assembly language routines for the Commodore 64 and other computers.

Scott Alfter hosts the entire archive of Apple Assembly Line, an old Apple II publication with a lot of good general 6502 content.

  Games

• MicroChess  by Peter Jennings, modified by Daryl Rictor

MicroChess is a very famous game written by Peter Jennings in 1976. It was originally written for the MOS Technology KIM-1 but is easily ported and was used on other systems like the Rockwell AIM-65. Daryl Rictor has updated the program for use on projects with a 6551-based serial port. Both the original and the enhanced version are provided.

  Floating Point Math

• Floating Point Routines for the 6502 by Roy Rankin and Steve Wozniak

Originally published in the August 1976 issue of Dr. Dobb's Journal, these floating point routines allow 6502 users to perform most of the more popular and desired floating point and transcendental functions, namely: Natural Log, Common Log, Addition, Subtraction, Multiplication, Division, and conversions between floating and fixed point numbers.

• Errata for Rankin's 6502 Floating Point Routines by Roy Rankin

In the November/December issue of Dr. Dobb's Journal Roy Rankin published three error corrections to the Floating Point Routines presented above.

• Floating Point Implementation in the Apple II by Steve Wozniak

An almost identical set of the above routines appeared in the original manual for the Apple II (the Red Book, January 1978). Documentation for these routines appeared in another book, the Wozpak II, in November 1979.

  General Purpose

• Calculation of Clock Frequency by Leo Nechaev

This program calculates the clock frequency of the 6502-based system it is running on and reports it back via RS-232 through a 6551 ACIA.

• Clearing a Section of Memory from 6502 Software Gourmet Guide & Cookbook, Simplified by Mickael Pointier

Clear up to 256 bytes anywhere in memory using this routine.

• Day of the Week by Paul Guertin

Computes the day of the week from (almost) any given date within the years 1900 and 2155.

• Enhanced Day of the Week by W.J. Brier

This is an alternative to Paul Guertin's program. It is longer but has the advantage of supporting a wider range of dates.

• Practical Memory Move Routines by Bruce Clark

Here are some reasonably fast general-purpose routines for moving blocks of memory. You simply specify the address to move from, the address to move to, and the size of the block.

• Address Line Test by Tony Gonzalez

The typical RAM test consists of reading and writing $AA and $55 to each memory location. This works well for finding bad RAM locations. However, it does not specifically find bad address lines. This simple method does just that.

• Program Structures in 65c02 Assembly, Through Macros by Garth Wilson

Cut the spaghetti code and label usage, and get more clarity, better control, and fewer bugs, with no memory or runtime penalty, using program structures for control of looping, conditionals, CASE statements, etc., mostly nestable, using assembler macros

  Integer Math

• Large Multiply & Divide from 6502 Software Design, Expanded by Greg

Two routines for multiplying and dividing larger integers. The first is a 32-bit multiply routine which computes a 64-bit product and the second is a 64-bit divide routine which gives a 32-bit quotient.

• Fast Multiply by 10 by Leo Nechaev

Multiplying a number by ten is fast and easy with this routine.

• Fast, Table-Driven Multiplication by Martin Arndt

This routine uses tables of squares to quickly multiply two 8-bit numbers. The algorithm is very useful when you want to multiply multiple bytes by the same factor, and could easily be expanded for 16-bit numbers.

• Division (32-bit) by Garth Wilson

Routines for unsigned division of a 32-bit dividend by a 16-bit divisor, resulting with a 16-bit quotient and a 16-bit remainder. Presented for the 65C02 (with changes for use on 6502) and 65C816, with notes of interest to Forth users.

• Large Look-up Tables for Hyperfast, Accurate, 16-Bit Fixed-Point/Scaled-Integer Math by Garth Wilson

So how fast?  How about three microseconds for a 10MHz 6502 to get even a trig or log function, accurate to all 16 bits?  Memory is cheap enough now to do it.  You can probably implement them even if your computer is already built up, the address space is full, and your I/O is almost all taken.  See how.  Plus: Introduction to the commonly unfamiliar and misunderstood world of fixed-point/scaled-integer math (and how to make it far more powerful than you thought possible, and take advantage of its efficiency)

• BCD to Binary Conversion (32-bit) from 6502 Software Design, Expanded by Greg

Two routines for converting between larger binary and BCD numbers. The first routine converts a 32-bit packed BCD number to its binary equivalent and the second performs the reverse operation.

• Hexadecimal to Decimal Conversion by Garth Wilson

Routines for converting 8- and 16-bit hexadecimal numbers to their decimal (BCD) equivalents.

• More Hexadecimal to Decimal Conversion by Andrew Jacobs

Alternate versions of Garth's conversion routines, slower but eliminating the lookup tables.

• CRC Calculations by Paul Guertin

Three routines for computing 32-, 16-, and 8-bit CRC (Cyclic Redundancy Check) error detecting codes.

• More CRC Calculations by Greg Cook

Alternate versions of Paul's CRC calculation routines that execute in constant cycle time and eliminate the lookup tables.

• Permutation Generator by Paul Guertin

Generating permutations is usually done with a recursive procedure, but here is a cute iterative routine that does it simply and efficiently.

• Square Calculator by Lee Davison

Calculates the 16-bit unsigned integer square of a signed 16-bit integer.

• Root Calculator by Lee Davison

Calculates the 8-bit root and 9-bit remainder of a 16-bit unsigned integer.

• Linear Congruential Pseudo-Random Number Generator Routines by Bruce Clark

A set of routines for generating pseudo-random numbers, using a linear congruential type generator.

  I/O Techniques

• Counting Push-Button Switch Closures, With Debouncing from 6502 Software Design

Counts closures on a push-button switch until a second push-button is pushed. Uses 6522 VIA Timer #1 to generate a 10-millisecond delay for the debounce routine.

• One-Shot Timer Examples for the 6522 from 6502 Applications

Two small programs demonstrate how to use Timer 1 and 2 in one-shot mode and also a short explanation of how to generate a pulse.

• Identifying 6522 Internal Interrupt Sources from 6502 Applications

A simple program to identify which of the seven possible sources caused a 6522 interrupt.

• Interrupt-Serviced 256-Byte Data Buffer by Lee Davison

Demonstrates how to set up a data buffer suitable for many operations such as sending data to a printer or serial device.

• Print Immediate (PRIMM) from Commodore 128 KERNAL ROM

By cleverly manipulating the stack this useful routine allows you to inline data to be printed with your assembly code, almost like a BASIC PRINT statement.

• Intel HEX File Downloader by Ross Archer

This program allows any 6502 system with a 6551-based serial port to painlessly download programs from a host computer by sending them in the Intel HEX file format.

• X-Modem File Transfer Protocol by Daryl Rictor

This implementation of the X-Modem file transfer protocol will work on any 6502-based computer with a 6551 serial port. Using X-Modem allows PC's to easily exchange files with your project using a terminal program.

  Monitors, Assemblers, and Interpreters

• 2KSA [2KSA.PDF: 4.34MB] by Robert Ford Denison

The 2K Symbolic Assembler is a two-pass assembler for 6502 systems which assembles to less than two kilobytes. This PDF includes all 65 pages of the original documentation which includes commented source code and hex dumps for the program. It is a really nice assembler considering its size and it so well documented that making changes to it are easy.

• Macros to assemble WDC's '816 code on the Kowalski simulator's assembler by BigDumbDinosaur

The Kowalski simulator was designed to simulate the NMOS 6502 and Rockwell 65C02.  It does not recognize the WDC 65C02 STP and WAI instructions.  This set of macros may be used to synthesize these instructions in the simulator's integrated assembler, plus synthesize the new instructions and addressing modes of the WDC 65C816.  (Note, however, that the simulator cannot execute the synthesized instructions and will report an error if it encounters them during simulation.)

• FIG FORTH 6502 [FORTH65.ZIP: 32KB] from the FORTH Interest Group

FORTH Interest Group (FIG) FORTH kernel for the 6502. It is written for the Rockwell System-65 but is easily adaptable to your own systems. Note that this has the "UM/MOD" division bug. An explanation of the bug and a correction can be found in Garth Wilson's 32-bit Division entry, which can also be found here in the Source Code Repository.

• 32-bit DO LOOP & related words in 6502 ITC Forth [text source-code file] by Garth Wilson

All in 32-bit (including the increment value for 2+LOOP): 32-bit equivalents for DO, ?DO, LOOP, +LOOP, I, BOUNDS, LEAVE, ?LEAVE, UNLOOP, and the internals, plus explanation.  These augment, not replace, the normal 16-bit ones, putting "2" in front of the names to tell the double-precision ones apart from the normal single-precision ones.

• Porting Sweet 16 by Carsten Strotmann

Sweet 16 is a metaprocessor or "pseudo microprocessor" implemented in 6502 assembly language. Originally written by Steve Wozniak and used in the Apple II, Sweet 16 can also be ported to other 6502-based systems to provide useful 16-bit functionality. This article includes the source code for Sweet 16, along with a brief history, programming instructions, and notes to help port it.

  Operating System Kernels

• Mini Multitasking Kernel by Joachim Deboy
This mini-kernel uses IRQ interrupts from a free running 6502 timer for task switching. It supports four concurrent tasks, each receiving fixed time slices in a round-robin method.

  Sorting Algorithms

• Bubble Sort (for 8-bit Elements) from 6502 Software Design

A short description of the bubble sort technique followed by a subroutine that arranges the 8-bit elements of a list in ascending order.

• Bubble Sort (for 16-bit Elements) from 6502 Software Design

The sorting routine above expanded to sort a list containing 16-bit elements.

• Combination Sort by Daryl Rictor

The Combination Sort is a modified bubble sort (iterative) that takes much less time to complete.

• Optimal Sort (for 8-bit elements) by Mats Rosengren

The Optimal Sort is another improvement on the bubble sort but uses a different approach than the Combination Sort.

• Optimal Sort (for 16-bit elements) by Mats Rosengren

An expansion on the 8-bit Optimal Sort above, this version allows for sorting of large numbers (over 255) of 16-bit elements.

• Shell Sort (for 16-bit elements) by Fredrik Ramsberg

The Shell Sort is a highly-efficient sorting algorithm. This implementation can sort a large number (>32,000) of 16-bit elements. Sorting a completely unsorted array of 10,000 elements takes just over thirty seconds on a 1 Mhz machine.

  String Manipulations

• Pattern Matcher by Paul Guertin

Matches a string against a pattern and returns with the carry bit set if they match, or clear if they don't. Supports wildcard characters "?" and "*".

• Simple Comparisons by Jonathyn Bet'nct

Compare two strings in memory stored in Pascal string format (length byte followed by string). Case-sensitive and -insensitive versions are provided.

• Convert ASCII Number String to 32-bit Binary by BigDumbDinosaur

This 6502 assembly language program converts a null-terminated ASCII number string into a 32-bit unsigned binary value in little-endian format. It can accept a number in binary, octal, decimal or hexadecimal format.

• Convert 32-bit Binary to ASCII Number String by BigDumbDinosaur

This program converts 32-bit binary back to an ASCII string. It is a companion to "Convert ASCII Number String to 32-bit Binary" above.