SMON is a machine language monitor and direct assembler for the Commodore 64, published in 1984 in "64'er" magazine (for more info see the credit section below).

In a nutshell, SMON provides the following functionality:

• View and edit data in memory
• Disassemble machine code
• Write assembly code directly into memory (direkt assembler with support for labels)
• Powerful search features
• Moving memory, optionally with translation of absolute addresses
• Trace (single-step) through code
• Set breakpoint or run to a specific address and continue in single-step mode

The best description of SMON's commands and capabilities is the article in the 64'er magazine (in German) available here. For English speakers, C64Wiki has a brief overview of SMON commands.

The version published here is an adaptation of SMON for a simple MOS6502-based computer, such as the one built by Ben Eater in his YouTube video series. The following original SMON functions are not available in this version:

• Loading and saving programs/data to disk or tape (L/S/I commands)
• Sending output to a printer (P command)
• Producing BASIC DATA statements for memory content (B command)
• Disk monitor mode and other extensions

The following new commands have been added in this version

• H - show a help screen with a brief overview of available commands
• L - load files in Intel HEX format into the 6502 by pasting them into the terminal
• MS - check and print size of installed memory
• MT - test memory

If you are using Ben Eater's standard setup (16k RAM at $0-$3FFF, ACIA at $5000, VIA at $6000, ROM at $8000-$FFFF, 1MHz clock) you can just download the smon.bin file from this repository and burn it to the EEROM.

Connect your terminal or USB-to-serial converter to the 65C51N ACIA as described by Ben in his videos.

Configure your terminal (program) for 9600 baud, 8 data bits, 1 stop bit and no parity. After turning on the 6502 you should see SMON showing the 6502 register contents and command prompt.

If you are using a non-standard setup, SMON can easily be adapted by changing the settings in the config.asm file (see below).

At startup, SMON shows the current 6502 processor status, followed by a "." command prompt

  PC  SR AC XR YR SP  NV-BDIZC
;E00B B4 E7 00 FF FF  10110100
.                             

Where "PC" is the program counter, "SR" is the status register, "AC" is the accumulator, "XR" and "YR" are the X and Y registers and "SP" is the stack pointer. the "NV-BDIZC" column shows the individual bits in the status register.

At the command prompt you can enter commands. For example, entering "m 1000 1020" will show the memory content from $1000-$1020:

.m 1000 1030                                                                    
:1000 00 01 02 03 04 05 06 07  08 09 0A 0B 0C 0D 0E 0F         ........ ........
:1010 10 11 12 13 14 15 16 17  18 19 1A 1B 1C 1D 1E 1F         ........ ........
:1020 20 21 22 23 24 25 26 27  28 29 2A 2B 2C 2D 2E 2F          !"#$%&' ()*+,-./

The column on the right shows the (printable) ASCII characters corresponding to the data bytes.

If your terminal supports the VT100 cursor movement sequences, you can modify the memory content by just moving the cursor into the displayed lines, editing data and pressing ENTER on each line where data was modified. If your terminal does not support cursor keys you can modify memory by typing (for example) :1015 AA BB and pressing ENTER. The example here will set $1015 to AA and $1016 to BB.

If you supply only one argument to the "m" command, SMON will show the memory content line-by-line, stopping after each line. Press SPACE to advance to the next line, ESC to go back to the command prompt or any other key to keep displaying memory without pausing (press SPACE to pause the scrolling display).

The "d" (disassemble) command will disassemble code in memory, for example:

.d f000
,F009  A9 FF     LDA #FF
,F00B  A2 04     LDX #04
,F00D  95 FA     STA   FA,X
,F00F  CA        DEX
,F010  D0 FB     BNE F00D

You can use the cursor keys to move over the displayed assembly statements and their arguments and modify them (assuming the code is in RAM).

You can use the "a" (assemble) command to assemble code directly into memory. SMON will show the current address as a prompt and you can enter an assembly statement (e.g. LDX #12). Press ENTER and SMON will assemble it, place it directly in memory, and advance the address to the next location according to the previous opcode's size. To exit assembly mode, type "f" as the opcode. SMON will then show you the full disassembly of the code you entered, in which you can edit again. For example:

.a 2000                  
 2000  ldx #00 
 2002  inx     
 2003  bne 2002
 2005  brk     
 2006 f                  
,2000  A2 00     LDX #00 
,2002  E8        INX     
,2003  D0 FD     BNE 2002
,2005  00        BRK     

To run your code just enter g 2000. Note that to jump back into SMON after your code finishes, it should end with a BRK instruction.

SMON also allows you to single-step through code using the tw (trace walk) command. For example:

  PC  SR AC XR YR SP  NV-BDIZC
;2002 23 E7 00 FF FF  00100011
.tw 2000                      
 2002 23 E7 00 FF FF  INX     
 2003 21 E7 01 FF FF  BNE 2002
 2002 21 E7 01 FF FF  INX     
 2003 21 E7 02 FF FF  BNE 2002
 2002 21 E7 02 FF FF  INX     
 2003 21 E7 03 FF FF  BNE 2002

After entering the tw command, SMON executes the first opcode and stops after finishing it and displays the next opcode (the first opcode is not shown). It also shows you the processor registers in the same order as they appear in the register display line. Press any key to advance one step or ESC to stop. If the next command is a JSR, press 'j' to "jump" over the subroutine and continue after it finishes (this only works if the JSR command is located in RAM).

SMON has a number of other "trace" related commands, a range of "find" commands to examine memory and several other commands. To get a quick overview of commands type "h" at the command line. For a bit more information on each command, refer to the C64Wiki page or for the full description read the 64er article (in German).

This version of SMON provides the "l" (load Intel HEX) command to help test 6502 programs written on your PC and compiled there using a compiler such as VASM:

1. Tell your compiler to produce Intel HEX output (in VASM, use the "-Fihex" command line parameter).
2. In SMON, type "l" followed by ENTER on the command line
3. Copy-and-paste the content of the (plain text ASCII) .hex file produced by your compiler into the terminal.

SMON will show a "+" for each HEX record processed. If a transmission error occurs, SMON shows a one-character error code followed by "?". Possible error codes are:

• I?: Input character error - an unexpected character was received in the input
• C?: Checksum error - the checksum at the end of a record did not match the expected value
• M?: Memory error - After writing a byte to memory it did not read back properly (most likely attempting to write to ROM)
• B?: Break - Either ESC or CTRL-C was received before the end of the transmission

If no "?" is shown and SMON goes back to the command prompt then the transmission succeeded.

The new "MS" (memory size) command checks memory starting at address $0100 and upwards until it finds and address where a read after write does not result in the same data. It then shows that address as the memory size.

The "MT xxxx yyyy [nn]" command tests memory between $xxxx and $yyyy by writing different patterns of data to it and checking whether the data reads back the same. Each time a difference is found the corresponding address is printed. The optional "nn" parameter specifies a repetition count (defaults to 1). At the end of each test, a "+" is printed.

There are three basic settings that can be changed by modifying the config.asm file:

• RAM size (default: 16k). RAM is assumed to occupy the address space from $0 to the RAMTOP setting. For example, if you have 32K of RAM then set RAMTOP to $7FFF
• VIA location (default: $6000). Change this if the location of the VIA differs from the default setting.
• Clock speed (default: 1000000). Change this if your system's clock is running at a different rate than the standart 1MHz. This setting is used for UART timing.
• UART driver. Communication with SMON works via RS232 protocol. The following UARTs are supported at this point:
  • WCS 65C51N ACIA (default). This is the UART Ben Eater is using in his project. The serial parameters are set to 9600 baud, 8 data bits, 1 stop bit and no parity. You can change the serial parameters and base address for the ACIA at the top of the uart_6551.asm file.
  • Pseudo-UART using 6522 VIA. This emulates a UART using the 6522 VIA present in Ben Eater's design. The serial parameters can be modified at the top of uart_6522.asm and default to 1200 baud 8N1. Note that on a 1MHz system baud rates above 1200 may lead to corruption of received data. Connect your terminal (or serial-to-usb adapter) to the VIA as follows:
    • Receive (RX) pin of the terminal goes to pin 39 (CA2) of the VIA.
    • Transmit (TX) pin of the terminal goes to pin 40 (CA1) and pin 2 (PA0) of the VIA.
    • Make sure the VIA's pin 21 (IRQ) is connected to the 6502 CPU's pin 4 (IRQ) The RX and TX pins can also be configured at the top of uart_6522.asm.
  • Motorola MC6850. If you choose this UART in the config.asm file you can configure it in the uart_6850.asm file, most importantly the base address (default is $8100) and the serial parameters.

To produce a binary file that can be programmed into an EEPROM for the 6502 computer, do the following:

1. Download the *.asm files from this repository (there are only 7)
2. Download the VASM compiler (vasm6502_oldstyle_Win64.zip).
3. Extract vasm6502_oldstyle.exe from the archive and put it into the same directory as the .asm files
4. Issue the following command: vasm6502_oldstyle.exe -dotdir -Fbin -o smon.bin smon.asm

Then just burn the generated smon.bin file to the EEPROM using whichever programmer you have been using.

After implementing the C64 kernal functions necessary to get SMON to work I realized that the same functions are enough to run Commodore BASIC. Installing and/or compiling BASIC follows the same rules as SMON (just use basic.bin or basic.asm).

Note that this is more of a toy example since only very simple BASIC programs will work (nothing with graphics or sound). Also saving or loading programs is obviously not supported.

The SMON machine language monitor was originally published in three parts in the November / December / January 1984/85 issues of German magazine "64er".

SMON was written for the Commodore 64 by Norfried Mann and Dietrich Weineck.

The code here is based on a (partially) commented disassembly of SMON by GitHub user Michael (cbmuser).

The code for handling RS232 communication via the 6522 VIA chip was taken and (heavily) adapted from the VIC-20 kernal, using Lee Davidson's commented disassembly.

The code for handling RS232 communication via the 65C51N ACIA chip was put together and tested by Chris McBrien, based on the ACIA code from Adrien Kohlbecker.