Machine Code Monitor

A screenshot of the TEDMON machine code monitor of the C128

A machine code monitor (aka machine language monitor or computer code monitor, short monitor) is a software program that allows a user to observe the status of a computer from a low-level language perspective. In addition to monitoring, users are able to manipulate memory locations using machine code commands, as well as load/save data from/to a secondary storage device. Some Commodore computers (C128, C16, C116, Plus/4) have built-in machine code monitors, while others (VIC-20, C64) require additional programs.

History

Early computers could only use line-printers to display the status of a computer. Later, when the computer monitor became a viable option for dynamically displaying the status of a computer, software was used to allow the user to monitor this status. Machine code monitors therefore became very popular from the start of the home computer era of the 1970s. Some of these machine code monitors were available as firmware, some as third-party hardware and some as software. The first machine code monitors had no symbolic assembly language capability, and all translation between machine code and assembly language was achieved by the programmer. Even once multi-feature assemblers became more readily available, a machine code monitor remained indispensable for debugging programs. The general decline of assembly language software written from scratch has made the use of a machine code monitor somewhat of a lost art. Nevertheless, the use of machine code monitors persists; especially in the area of hobby-built computers.

Technique

Long data strings of numbers and letters could be difficult to visualise as a program and inevitably some debugging would be required. A common technique when writing machine code programs was to set a halt in the machine code program. Generally this was using the break instruction 00 (which was later represented as BRK in three-letter mnemonic assembly language). When the CPU encountered this breakpoint, the program would be interrupted and control would be transferred to the machine code monitor. Typically this would trigger a register dump and then the monitor would await programmer input. Activities at this point might include examining memory contents, changing code and/or perhaps altering the processor registers prior to restarting the program.

Features

General Features

• View and change CPU status (such as registers and flags).
• Program run at a desired address.
• View and modify memory areas, typically in different formats (such as hex dump, PETSCII, and Sprite data)
• Save/Load into a memory area to/from a secondary storage device.
• Value/pattern/string fill.
• Value/pattern/string search.

Advanced Features

• Disassemble/assemble.
• Set breakpoints.
• Tracing/Single Stepping.
• Search using address ranges.
• Number conversions.
• Calculation of arithmetic and logical expressions.
• Moving memory areas.
• Moving machine code with adjustment of the absolute references.
• Insert/delete machine code instructions in existing program with adjustment of the absolute references.
• Input/output functions for printers/mass storage/floppy disk drive.
• Repair BASIC vectors or BASIC programs.
• Sprite functions.

Limitations

Graphics Modes

Monitors need to run in text mode and not in a GUI. Although this does not matter for an emulated environment, on original hardware the memory use is so restrictive that the most basic display mode of the CPU is required.

Debugging

This can be problematic when programs use custom timing routines. Notably, the debugging of grid line interrupt programming or fast loader routines is almost impossible due to timing confusion.

Commodore Specific Monitors

Name	Type	Size	Command	Source	Remarks
Supermon64	Software re-load	<4KB	SYS 49152 (for version at $C000)	Jim Butterfield, Compute! Magazine, January 1983, Issue 32, Page 162
Supermon+64	Software pre-load	<3KB	RUN (originally) SYS 8 (later)	Jim Butterfield, Transactor, Volume 6, Issue 4, Disk 9	- Compatible with TEDMON - Sits below $A000
RAT MON 64	Software re-load	4KB	SYS 49152 (for version at $C000)	CHIP Magazine, January 1984
SMON	Software re-load	4KB	SYS 49152 (for version at $C000)	64er Magazine, February 1985
MADS Monitor	Software re-load	4KB	SYS 32768 (for version at $8000) SYS 49152 (for version at $C000)	Commodore, 1982	part of the Macro Assembler Development System
MLM64 Plus	Software re-load	4KB	SYS 49152 (for version at $C000)	INPUT 64 Magazine, March 1985
MON-64	Software re-load	1KB	SYS 49187 (for version at $C000)	MC Magazine, October 1983	TIM clone
TEDMON	ROM		MONITOR	Commodore	BASIC 3.5, BASIC 7.0
TIM	ROM re-load	1KB	SYS xxxxx	Commodore	SYS call depends upon model
Final Cartridge 3	Plug-in module		MON or F2	RISKA B.V. Home & Personal Computers
Action Replay	Plug-in module		MON or F8	Datel Electronics
Help	Plug-in module re-load		]  and RETURN
Help Plus	Plug-in module re-load		]  and RETURN
EMON	Plug-in module re-load	4KB	SYS 49152 (for version at $C000)	Print Technik
SpeedDOS	EPROM		F2	Elektronik Service
SpeedDOS plus	EPROM		F2	Elektronik Service
VICE	Emulator		Alt  and M  or Alt  and H	GNU General Public License	PC-based software

Links

• Compute! Issue 32, Jan. 1983, p. 162: Supermon64
• Transactor Disk 9 includes Supermon+ for C64, VIC-20 and PET.