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A python based 6502 assembler, disassembler and simulator
David Johnston

The Philosophy

I enjoy old-school electronics and programming. I've learned over time that there is a benefit to combining old-school methods with modern tools.

For example:
1) My day job is a chip designer, focusing on cryptgraphic circuits. For a particular project I either needed a big state machine or a small CPU. An old school 8 bit processor took me 4 hours to implement in synthesizable RTL and on a modern silicon process it runs blindingly fast in a tiny bit of silicon area. It came with lots of tools already since the instruction was standard.

2) I wrote a Point-of-Sale system for a family store, since the off-the-shelf solutions were expensive and none of them matched the needs of the store. Instead of going for some windows interface or touch driven app on a tablet, I wrote the user interface using the old-school curses library for a text based interface that was designed to minimize keystrokes at the checkout. The staff loved it since the key strokes took a couple of minutes to learn and I wrote it in Python, bringing the simplicity and power of modern programming tools.

In both cases, the benefits of the efficiencies of old school methods with the power of modern methods yielded something better than either.

This project is a proof-of-concept for this idea. I use it for hacking my Apple //e.  It's a 6502 assembler, disassembler and simulator written in Python. Many old 6502 assemblers exist, but they suffer from inconsistent formats, directives and macro processors. In particular the macro processors tended to be horrible.

The thing that makes it a little different is that instead of offering a 'better assembler language' or 'better macro language' I've stripped down the programs to the very basic functions but written them such that they are intended to be called from a python program that feeds it assembler and gets object code back. This then makes python the macro language. So you get the ability to write assembly code normally, or you can write python to automate the code generation or generate parameterized code, or unroll loops or any number of other things, but using a nice language that makes it easy rather than a set of confusing macro directive written in 1978. 

If you want to instrument and test of a bit of code, it's easy to assemble it and then write a program in python to iterate over your chosen input states and check the outputs and simulate the code repeatedly, calling the simulation of each instruction directly from python, coding in whatever analysis meets your needs.

The simulator and disassembler works directly with the object code and symbol table from the assembler.

An Simple Example: Sending Assembly to the Assembler From Python
This python assembles a few instructions

from asm6502 import asm6502

        thecode = """
                ORG  $100
                LDA #$10
                LDX #$00
                STA $1000,x
                SBC #$01
                BPL loop
        lines = thecode.splitlines()
        a = asm6502() 

The output looks like this:

        65C02 Assembler
        1    0000 :                                
        2    0100 :                  ORG $100      
        3    0100 : start:                         
        4    0100 :         A9 10    LDA #$10      
        5    0102 :         A2 00    LDX #$00      
        6    0104 : loop:                          
        7    0104 :         9D 00 10 STA $1000,x   
        8    0107 :         E8       INX           
        9    0108 :         E9 01    SBC #$01      
        10   010A :         10 FA    BPL loop      
        11   010C :         60       RTS           
        start      = $0100
        loop       = $0104
        0100: A9 10 A2 00 9D 00 10 E8 E9 01 10 FA 60


The Object Code Map

If after running that, you typed this:
        >>> a.object_code[256:268]

You would see the list of object code values, but in decimal, since that's how python displays by default:

        [169, 16, 162, 0, 157, 0, 16, 232, 233, 1, 16, 250, 96]

What's going on is the assembler keeps a complete map of the 64K memory space of the 6502 and populates the code and values into that map. The 'object_code' class variable is a list containing the map. Each untouched location is set to -1. Other values indicate the 8 bit value at that location.

So after assembling the code into the map, it is possible to add in other things to the map by assigning to the object_code list. E.G.

        a.object_code[0xfffd] = 0x00
        a.object_code[0xfffc] = 0x10
Which would set the reset vector to 0x1000.

The Symbol Table

You can also see the symbol table as a dictionary after assembling:
    >>> a.symbols
    {'start': 256, 'loop': 260}


There are a small number of directives:

; Comment
ORG address ; Sets the current aseembly location
STR some_text ; Include text as ascii bytes 
DB comma_separated_list_of_bytes ; $ prefix for hex
DW comma_separated_list_of_16_bit_numbers ; $ prefix for hex
DDW comma_separated_list_of_32_bit_numbers ; $ prefix for hex
DQW comma_separated_list_of_64_bit_numbers ; $ prefix for hex
LE ; For multi word data (DW, DDW and DQW) sets the encoding to little endian
BE ; For multi word data (DW, DDW and DQW) sets the encoding to big endian
The assembler defaults to little endian.


$ for hex. $10 = 16
@ for octal. @10 = 8
& for a label pointer. &labelname = the 16 bit address of the label, only works with DW.


A word followed by a colon makes a label. It can be on it's own line, or in front of an instruction or directive.

alabel: ; A label on it's own
anotherlabel: STA #$10 ; A label with an instruction
Any address or 16 bit data field can be replaced with a declared label and the label address will be inserted there.
In a DW declaration you need to prefix a label with & to tell the assembler it's a label. This may change. I.E.:

        dw  $1000, @2000, 123  ; Implicit numbers have a base prefix. 
ttable:  dw  &l1, &l2, &l3      ; labels in a DW prefixed with & 
        org $1000
l1:     lda #$20
        jmp skip
l2:     lda #$30
        jmp skip
l3:     lda #$40
skip:   sta $10

Assembling Into the Same Map

The assembler instance clears it's state before assembling, except for the object_code map. This enables you to assemble multiple pieces of code into different locations and they will be added to the map.
The print_object_code() class method displays the current object code map
E.G. The following code assembles a sequence, then modifies its origin, then reassembles it:
        from asm6502 import asm6502
        a = asm6502()
        lines = [' ORG $1000', ' NOP', ' LDA #$20', 'here: NOP', ' DB 10,11,12,13', ' RTS']
        lines[0] = ' ORG $2000'

This yields this memory map with the same code in two places.
        >>> a.print_object_code()
        1000: EA A9 20 EA 0A 0B 0C 0D 60
        2000: EA A9 20 EA 0A 0B 0C 0D 60

Getting IntelHex format data out
After assembling you can output the object code in intelhex format.
calling the intelhex() method returns lines of intelhex as a list.

        >>> a.intelhex()
        [':10010000A000B90000990020B90001990021B900', ':1001100002990022B90003990023B90004990024', ':10012000B90005990025B90006B90026B9000799', ':0E0130000027B90008990028C8D0D04C59FF', ':10100000A000B90000990020C8D0D0A900850085', ':1010100002A9018501A9218503B2009202E602E6', ':1010200000D0F8E603E601A501C909D0EE4C59FF', ':00000001FF']
Calling the print_intelhex() method outputs intelhex format object code to stdout.        
        >>> a.print_intelhex()

Getting SRecord format data out
After assembling you can output the object code in S19 Srecord format.
calling the srecord() method returns lines of S19 S Record as a list.

The parameters are (int:version, int:revision, str:module name, str:comment)

>>> a.srecords(10,20,"Module Name,","Comment")
['S02e00000a144d6f64756c65204e616d652c436f6d6d656e74ad', 'S12e1000600D0C0B0AEA20A9EA96', 'S12e2000600D0C0B0AEA20A9EA86', 'S5030003f9', 'S9032008d4']

Calling the print_srecords() method outputs S19 format object code to stdout.        
>>> a.print_srecords(10,20,"Module Name,","Comment")

The Simulator and Disassembler

#The simulator is in, the disassembler is in
#So start with all three..
import asm6502
import sim6502
import dis6502

# The assembler code
src = """
<assembly goes here>
lines = src.splitlines()

#The simulator must be given an object code map. You can get it from the assembler:

# Assemble the code in a list called lines.
a = asm6502.asm6502(debug=0)
object_code = a.object_code[:]

# Then instantiate the simulator.
# Also pass it the symbol table so it can know addresses
s = sim6502.sim6502(object_code, symbols=a.symbols)

# And instantiate the disassembler:
d = dis6502.dis6502(object_code, symbols=a.symbols)

# Now
# s.reset() will reset the simulation
# s.execute() will execute the current instruction
# The 6502 state will be in
#   s.pc  Program Counter
#   s.a   Accumulator
#   s.x   X registers
#   s.y   Y register
#   s.sp  stack pointer
#  Flags
# d.disassemble(address) will disassemble the instruction as the address and
#                        will return a text string of the disassembly
# E.G.


# Print a header for the simulator/disassembler output
print ("LABEL      " + "ADDR HEX      INSTR").ljust(status_indent)+" PC   A  X  Y  SP   Status"

# Print the initial state
print " ".ljust(status_indent) + " %04x %02x %02x %02x %04x %02x" % (s.pc,s.a,s.x,s.y,s.sp,

# Execute 200 instructions
for i in xrange(200):
    # Disassemble the current instruction
    distxt = d.disassemble_line(s.pc)

    # Execute that instruction

    # Print out the disassembled instruction followed by the simulator state
    print distxt.ljust(status_indent) + " %04x %02x %02x %02x %04x %02x" % (s.pc,s.a,s.x,s.y,s.sp,

# Each output line will then show the address, the hex, the instruction executed and the state of the 6502 after the execution.

Comments to

TBD 1: Write a 65C02 simulator that runs from the object_code state generated by the assembler

TBD 2: Write an output generator for more of the flash/prom/eeprom programming formats
  Added Srecords. Should also add ascii hex and binary.

TBD 3: Give it decent error handling

TBD 4: Set up a unit test bench to fuzz it with code and do directed tests.


A Python 6502 Assembler/Disassembler/simulator in which python serves in place of the a macro language.







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