A machine code monitor (

machine language In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...

monitor) is

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...

that allows a user to enter commands to view and change memory locations on a

computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...

, with options to load and save memory contents from/to

secondary storage Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer ...

. Some full-featured machine code monitors provide detailed control ("single-stepping") of the execution of machine language programs (much like a

debugger A debugger or debugging tool is a computer program used to software testing, test and debugging, debug other programs (the "target" program). The main use of a debugger is to run the target program under controlled conditions that permit the pr ...

), and include absolute-address code assembly and

disassembly A disassembler is a computer program that translates machine language into assembly language—the inverse operation to that of an assembler. A disassembler differs from a decompiler, which targets a high-level language rather than an assembly lan ...

capabilities. Motorola published the MIKBUG ROM monitor for the 6800 in 1973 and the BUFFALO ROM monitor for the

68HC11 The 68HC11 (6811 or HC11 for short) is an 8-bit microcontroller (µC) family introduced by Motorola in 1984. Now produced by NXP Semiconductors, it descended from the Motorola 6800 microprocessor by way of the 6801. The 68HC11 devices are more p ...

. Machine code monitors became popular during the home computer era of the 1970s and 1980s and were sometimes available as resident

firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...

in some computers (e.g., the built-in monitors in the

Commodore 128 The Commodore 128, also known as the C128, C-128, C= 128,The "C=" represents the graphical part of the logo. is the last 8-bit home computer that was commercially released by Commodore Business Machines (CBM). Introduced in January 1985 at the ...

Heathkit H89 The Z-89 is a personal computer produced by Zenith Data Systems (ZDS) in the early 1980s. Description The Zenith Z-89 is based on the Zilog Z80 microprocessor running at 2.048 MHz, and supports the HDOS and CP/M operating systems. The ...

and Zenith laptops). Often, computer manufacturers rely on their ROM-resident monitors to permit users to reconfigure their computers following installation of upgrade hardware, such as expanded main memory, additional disk drives, or different video displays. It was not unheard of to perform all of one's programming in a monitor in lieu of a full-fledged symbolic assembler. Even after full-featured assemblers became readily available, a machine code monitor was indispensable for

debugging In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems. Debugging tactics can involve in ...

programs. The usual technique was to set break points in the code undergoing testing (e.g., with a

BRK The 65xx family of microprocessors, consisting of the MOS Technology 6502 and its derivatives, the WDC 65C02, WDC 65C802 and WDC 65C816, and CSG 65CE02, all handle interrupts in a similar fashion. There are three hardware interrupt signals comm ...

instruction in

6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...

assembly language) and start the program. When the

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...

encountered a break point, the test 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, patching code and/or perhaps altering the processor registers prior to restarting the test program. The general decline of scratch-written

assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...

has made the use of a machine code monitor somewhat of a lost art. In most systems where higher-level languages are employed,

s are used to present a more abstract and friendly view of what is happening within a program. However, the use of machine code monitors persists, especially in the area of hobby-built computers.{{cn, date=February 2017

References

* Microcomputer software