Process Management in CP/M

                   By now, we've seen the basics of I/O, and memory management of CP/M, it is time for process management. As CP/M is a single process operating system, the process management is quite limited. Still, even with this limited functionality, there are several issues regarding processes that the operating system must handle. Generally, when a program is being executed, it is called as a process. So, first we will take a brief look at program execution, and then we will move to process management.

Executing an application program:

                   To execute a program, its executable code must be loaded into memory. Usually, this loading is done by CCP (through loader). CCP accomplishes its work by making BDOS calls only. As CCP is loyal friend of BDOS; CCP never calls BIOS, or hardware directly. CCP always calls them them through BDOS. When a set of characters is entered in CCP, first of all, CCP checks if this set of characters is a built-in command or not. If it is command, then it is executed. If it isn't a built-in command then CCP looks for an executable program file on the disk by that name. In both the cases, the executable code of that file/program is loaded into the memory, and the execution of the program begins.

Command Processing Via CCP:
                        In CP/M, CCP is a program, pretty much like any other program. The CCP is better structured than most of the programs, as it only uses OS system calls, and never bypasses the operating system/BDOS. In other operating systems a component similar to CCP, is sometimes called as a shell, command interpreter, or command line interface. In CP/M a user can directly invoke CCP command by typing the command, or by typing the name of an executable file. The CCP is configured to load in high memory location, just below the BDOS. When a program starts executing, and if the memory is insufficient, then CCP exits the memory creating free space for the program. When a program finishes the execution, it exits with returning control to the BDOS. Then the BDOS checks if the CCP is still in the memory, or not. If it is in the memory, then BDOS returns control to CCP. But if CCP is not in the memory(exited from memory, for making memory available for the program, as mentioned previously),  then the CCP is reloaded into the memory, and given control.

Basic “MULTITASKING” In CP/M:
                        Even in the early days of computing, users wanted to do some work in parallel. Most common request was to print data, and edit another file on the computer system at the same time. Because in these days, printers were very slow. It required 30 to 60 minutes to print out few pages. So this time was obviously wasted, while computer did nothing but printing. In these days, most of the users used to start the printing process, and then they used to do something else, like eating reading, or visit a friend to discuss how many balloons would it take to hover a cat? (come on, what are the odds? There must be atleast one user who did this) But if something went wrong, and user returned after a long time, to find that printer needs some attention, and most of the printing hasn’t been done. Well, this is obviously keyboard pounding frustrating! And this used to happen quite sometimes.
                        Gary’s solution was a background printing process. In CP/M, when a user gives command to print a file, then a small program is loaded into the memory. Let us call that program as “print handler”. This print handler is loaded at the highest location in the memory, just underneath the OS. This program initialises itself, and gives control to CCP, allowing another program to be run. This background printing process usually gets control when the CPU is idle, or if the foreground process makes system call, or by setting a timer which causes an interrupt to the foreground process. This background process prints a little bit of a file, a line or two, whenever it gets control.
                        Even though the computer did not do it, this background processing gave appearance of computer doing two things at the same time, something called as multi-tasking. But still, users liked this idea of doing two things at the same time.