MOS 6502 Microprocessor Emulator

Have fun playing with your own virtual 6502 based computer!


mos6502 is an emulator for the venerable MOS Technology 6502 microprocessor written in Scala. The emulator is fully functional. It includes a basic assembler and disassembler. Programs (in 6502) can be executed from the command line, or the REPL can be used. There are a few working examples under the code folder that can either be executed directly from the command line or loaded into the REPL.

It's true, there are many 6502 emulators out there, but this one is written in Scala and is therefore easy and fun to extend should you wish to add some more hardware to your virtual computer.

All the examples in the code folder with a file name ending with .asm will assemble using the built-in assembler. Of course, the emulator's built-in assembler will have slightly different syntax than other assemblers for assembly directives. If a more powerful assembler is needed, there is one called crasm which is available under Ubuntu/Mint that generates SREC (Motorola S-Record) files using it's -o option. Those files can then be executed using the emlulator's -le option or loaded into the REPL using the l command. The cc65 tools may also be used in conjunction with a program called srecord to get an SREC file that can be loaded.

Try it out

The following code (assembled using the built-in assembler)

; install required i/o devices
_stdioChar_ = "8000" ; add a character i/o "device" and memory map it to address $8000
_stdioInt_  = "8001" ; add an integer i/o "device" and memory map it to address $8001

; memory mapped i/o
CHIO    EQU $8000    ; character i/o port
INTIO   EQU $8001    ; integer i/o port

; zero page variables
COUNTER RB           ; counter variable

; program ROM
        ORG $9000    ; program starts at $9000
        LDA #0       ; start counter off with 0
.1      INC COUNTER  ; bump the counter
        CMP #6       ; is counter less than 6
        BNE .2       ; if so, print
        BRK          ; otherwise, stop
.2      STA INTIO    ; send counter value to integer i/o port
        LDA #'\n'    ; now print a line feed
        STA CHIO
        JMP .1

        ORG $FFFC    ; reset vector
        DW  START    ; CPU will start executing at 9000

will print


The easiest way to try this out is to follow the build instructions below. Then, once inside the REPL, type

a code/count.asm

Of course, when doing this in the REPL you will also see

A:06 X:00 Y:00 SP:01FD PC:900D
N:0 V:0 B:0 D:0 I:0 Z:1 C:1
900D  8D 01 80   .2              STA INTIO

as well, showing you the state of the CPU after execution.

If you now type the command u 9000 to disassemble the program from memory, you should see

9000  A9 00      START           LDA #0
9002  85 00                      STA COUNTER
9004  E6 00      .1              INC COUNTER
9006  A5 00                      LDA COUNTER
9008  C9 06                      CMP #6
900A  D0 01                      BNE .2
900C  00                         BRK 
900D  8D 01 80   .2              STA INTIO
9010  A9 0A                      LDA #$0A
9012  8D 00 80                   STA CHIO
9015  4C 04 90                   JMP .1

Notice that the REPL uses the symbol information (if available) from the assembler to provide a more readable disassembly.


mos6502 is distributed under the MIT License, meaning that you are free to use it in your free or proprietary software.




If you just want to download the executable so that you can run 6502 programs or use the REPL, you can download it from here. You do not need the Scala library for it to work because the JAR already contains all dependencies. You just need Java 7+ installed.

Run it as a normal Java executable JAR with the command java -jar mos6502-0.3.jar in the folder where you downloaded the file.


Use the following definition to use mos6502 in your Maven project:



Add the following to your build.sbt file to use mos6502 in your SBT project:

resolvers += "Hyperreal Repository" at ""

libraryDependencies += "xyz.hyperreal" %% "mos6502" % "0.3"



  • Java 7+
  • SBT 0.13.12+
  • Scala 2.11.8+

Build and Run the REPL

git clone git://
cd mos6502
sbt run