Resident monitor

In computing, a resident monitor is a type of system software program that was used in many early computers from the 1950s to 1970s. It can be considered a precursor to the operating system. The name is derived from a program which is always present in the computer's memory, thus being "resident". Because memory was very limited on those systems, the resident monitor was often little more than a stub that would gain control at the end of a job and load a non-resident portion to perform required job cleanup and setup tasks. On a general-use computer using punched card input, the resident monitor governed the machine before and after each job control card was executed, loaded and interpreted each control card, and acted as a job sequencer for batch processing operations. The resident monitor could clear memory from the last used program (with the exception of itself), load programs, search for program data and maintain standard input-output routines in memory. Similar system software layers were typically in use in the early days of the later minicomputers and microcomputers before they gained the power to support full operating systems. Resident monitor functionality is present in many embedded systems, boot loaders, and various embedded command lines. The original functions present in all resident monitors are augmented with present-day functions dealing with boot time hardware, disks, ethernet, wireless controllers, etc. Typically, these functions are accessed using a serial terminal or a physical keyboard and display, if attached. Such a resident monitor is frequently called a debugger, boot loader, command-line interface (CLI), etc. The original meaning of serial-accessed or terminal-accessed resident monitor is not frequently used, although the functionality remained the same, and was augmented. Typical functions of a resident monitor include examining and editing ram and/or ROM (including flash EEPROM) and sometimes special function registers, the ability to jump into code at a specified address, the ability to call code at a given address, the ability to fill an address range with a constant such as 0x00, and several others.