Spasm

A simple assembler/disassembler for the Motorola 6809 8-bit microprocessor

Latest Release: 1.2.0

 

About Spasm

spasm is an assembler/disassembler for the Motorola 6809 microprocessor written in Python 3.

It was written to generate 6809 machine code for a separate processor emulation project. As such, it does not output files into a standard format, but in the form of JSON intended to be read by the emulator. The output file’s extension is .6809, but it is a text file containing a JSON object:

{ "address" : <start_address_of_code>,
  "code"    : <string_of_assembled_code_bytes> }

A sample file, sample01.6809, is included with the repository.

Input is in the form of one or more .asm files which are text files containing the source code. For example:

; All comments go after a semi-colon

@tab_len    EQU %11111111       ; Table length in binary
@object     EQU $EE01           ; Object address in hex
@entlen     EQU $08             ; Table entry length
@search     LDB #@tab_len
            BEQ @exit           ; Exit if the table length is zero
            LDY #@object
            LDX #@store
@loop       PSHS B              ; start loop
            LDA #$2
@nextch     LDB A,Y
            CMPA B,X
            LEAX 3,X
            CMPA $2A,Y
            BNE @nexten         ; break out of loop
            DECB
            BPL @nextch
            PULS B
            LDA #$FF
            RTS
@nexten     PULS B
            DECB
            BEQ @exit
            LEAX @entlen,X       ; Interesting problem: this could add 0, 1 or 2
                                 ; bytes depending on the value of @entlen, which
                                 ; might not have been specified on the first pass
@exit       CLRA
@store      RMB @entlen
            RTS                  ; return

Assembler Conventions

Literals

Various literal types are supported. Literals are assumed to be decimal, but you can change this by using the following prefixes:

  • % — a binary value, eg. %10001111 (equals 0x8F, 143).
  • ' — an 8-bit Ascii value, eg. 'A (equals 0x41, 65).
  • $ — a hexadecimal value, eg, $FF00 (equals 65280).
    Note spasm can also read hexadecimal values prefixed with 0x for modern users, but $ is the classic Motorola prefix.

Labels

As the above example shows, spasm supports the use of labels to represent values and memory locations (eg. for jumps and branches). All labels must be prefixed with @.

Comments

Comments can be entered by prefixing them with a ; or * (for DREAM fans). At this time, multi-line comment indicators have not yet been implemented.

Directives

spasm makes use of the following assembler directives (aka pseudo-ops):

  • EQU — assign a value to a label, eg. @label EQU 255.
  • END — optional end-of-code marker.
  • RMB — reserve n memory bytes at this address, eg. @label RMB 8 ; add 8 bytes for data storage.
  • FCB — store the following 8-bit value or values at this address, eg.
    • @label FCB $FF ; poke 255 to this address.
    • @label FCB $FF,$01,$02 ; poke 255, 0, 2 to sequential addresses from this.
  • FCC — store the following string at this address, eg.
    • @label FCC "Message" ; poke 77,101,115,115,97,103,101 to sequential addresses.
  • FDB — store the following 16-bit value or values at this address, eg.
    • @label FDB $FF00 ; poke 65280 to this address.
    • @label FDB $FF00,$FF01 ; poke 65280, 65281 to sequential addresses.
    • Note The 6809 expects the most-significant byte at the lowest address.
  • ORG — continue assembly at the supplied address, eg. @label ORG $3FFF ; continue assembly at address 16383.

Endianism

Motorola microprocessors are big endian, ie. the most-significant byte is written at the lowest memory address and the least-significant byte is written at the highest address.

For example: store the 16-bit value 0x1A2B from the address 0xFF00:

0xFF00    0x1A
0xFF01    0x2B

Sample Code

There are sample 6809 assembler programs and assembled .6809 files in the samples folder.

Disassembly

spasm will disassemble .6809 files, using the start address included in the file. It can also disassemble .rom files. Since these do not include address information, you can use the -s switch to set the effective address of the first byte in the .rom file. Because you may not wish to disassemble the entire file, you can use the -b switch to set the address from which disassembly will begin, and -n to set the number of bytes you want to disassemble.

For example, if you have a 16KB ROM that is expected to be placed at 0x8000 in the 6809 memory map, you set the start address (with -s) to 0x8000. However, you only want to disassemble from 0x9000, so you use -b to set the base address to 0x9000. You only want to disassemble the 128 bytes at 0x9000, so you use -n 128:

./spasm.py my_rom.rom -s 0x8000 -b 0x9000 -n 128

This might output:

Address   Operation       Bytes
-------------------------------
0x9000    BNE    $9008    2606
0x9002    CMPB   #84      C184
0x9004    BNE    $900C    2606
0x9006    LDA    #3A      863A
0x9008    STD    ,U++     EDC1
0x900A    BRA    $8FA1    2094
0x900C    STB    ,U+      E7C0
0x900E    CMPB   #86      C186
0x9010    BNE    $9014    2602
0x9012    INC    >44      0C44
0x9014    CMPB   #82      C182
0x9016    BEQ    $8FC3    27AA
0x9018    BRA    $8FA1    2086
0x901A    LDU    #011B    CE011B
0x901D    COM    >41      0341
0x901F    BNE    $8FE2    26C0
0x9021    PULS   X,U      3550
0x9023    LDA    ,X+      A680
0x9025    STA    ,U+      A7C0
0x9027    JSR    8ADF     BD8ADF
0x902A    BLO    $9019    25EC
0x902C    COM    >43      0343
0x902E    BRA    $9019    20E8
0x9030    INC    >42      0C42
0x9032    DECA            4A
0x9033    BEQ    $8FE4    27AE
0x9035    LEAY   $0F,Y    313F
0x9037    LDB    ,Y+      E6A0
0x9039    BPL    $9038    2AFC
0x903B    BRA    $8FED    20AF
0x903D    BEQ    $90A1    2762
0x903F    BSR    $9044    8D03
0x9041    CLR    >6F      0F6F
0x9043    RTS             39
0x9044    CMPA   #40      8140
0x9046    BNE    $904D    2605
0x9048    JSR    B786     BDB786
0x904B    BRA    $9057    200A
0x904D    CMPA   #23      8123
0x904F    BNE    $905E    260D
0x9051    JSR    B7D7     BDB7D7
0x9054    JSR    B63C     BDB63C
0x9057    JSR    >A5      9DA5
0x9059    BEQ    $90A1    2746
0x905B    JSR    89AA     BD89AA
0x905E    CMPA   #CD      81CD
0x9060    LBEQ   $A224    102711C1
0x9063    BEQ    $90AD    2748
0x9065    CMPA   #BB      81BB
0x9067    BEQ    $90C6    275D
0x9069    CMPA   #2C      812C
0x906B    BEQ    $90AE    2741
0x906D    CMPA   #3B      813B
0x906F    BEQ    $90DF    276E
0x9071    JSR    8887     BD8887
0x9074    LDA    >06      9606
0x9076    PSHS   A        3402
0x9078    BNE    $9080    2606
0x907A    JSR    9587     BD9587
0x907D    JSR    8C59     BD8C59

The -b and -n switches can be used when you are disassembling .6809 files, but the code’s start address will always be taken from the file, not an address set with -s.

See below for a full list of spasm switches.

Command Line

spasm is a command line tool. It supports the following switches:

Option Alternative Action
-h --help Print help information
-v --version Display spasm version information
-q --quiet Display no extra information during assembly. This overrides verbose mode,
which is the default
-s --start Set the start address of the assembled code, specified as a hex or decimal value.
Note You can use $ as a prefix for a hex value, but you will need to place
the address in single quotes, eg. spasm.py zzz.asm -s '$FF00' to avoid confusing Bash
-b --baseaddress Set the base address for disassembled code, specified as a hex or decimal value.
Ignored during assembly
-n --numbytes Set the number of bytes to disassemble, specified as a hex or decimal value.
Ignored during assembly
-o --output Cause the 6809 output file to be written and, optionally, name it. If you pass no name,
the output file name will match that of the input file but with a .6809 extension
-l --lower Display opcodes in lowercase
-u --upper Display opcodes in uppercase.
Note This and the above switch will overwrite each other; if both are called:
the last one wins. If neither is used, the output matches the input

Release Notes

  • 1.2.0 29 May 2019
    • Improvements
      • Fully support ORG directive: assemble code into multiple chunks.
      • Add support for FCC directive: assemble code Ascii strings to bytes.
      • Improve operation of FCB and FDB directives.
      • .6809 files' code field now contains a string of two-character hex values.
      • Change -v switch to present version info (as verbose mode is default).
    • Bug Fixes
      • Handle negative operands correctly.
      • Handle indirect extended addressing correctly.
      • Check ops that expect an 8-bit value don’t get a 16-bit value.
      • Correct address increments during disassembly of extended opcodes.
  • 1.1.0 30 April 2019
    • Improvements
      • Add disassembly of .rom files.
      • Add -n switch to set number of bytes of code to be disassembled.
      • Add -b switch to set base address of disassembly.
  • 1.0.0 12 April 2019
    • Initial public release.

Source Code

You can view Spasm’s source code at GitHub.


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Site Last Updated: 23 October 2020