Batch processing - is execution of a series of programs ("jobs") on a computer without human interaction.

Batch jobs are set up so they can be run to completion without human interaction, so all input data is preselected through scripts or command line - parameters. This is in contrast to "online" or interactive programs which prompt the user for such input. A program takes a set of data files as input, process the data, and produces a set of output data files. This operating environment is termed as "batch processing" because the input data are collected into batches on files and are processed in batches by the program.

Multiprogrammed Systems - a number of programs were resident in memory and the CPU could choose which one to run. One way to choose is to just keep executing the current program until an I/O delay is pending - instead of just waiting, the CPU would move onto the next program ready to be run.

• Keep multiple jobs resident in memory
• OS chooses which job to run
• When job waits for I/O switch to another resident job

Result: Job scheduling policies, Memory management and protection, improves throughput and utilization, still not interactive.

Time-Sharing Systems Interactive Computing - while multiprogrammed systems used resources more efficiently i.e. minimized CPU idle time, a user could not interact with a program. By having the CPU switch between jobs at relatively short intervals, we can obtain an interactive system.That is, a system in which a number of users are sharing the CPU (or other critical resource) with a timing interval small enough not to be noticed e.g. no more than 1 second. We say that a time-sharing system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared computer.

• The CPU multiplexed among several jobs in memory and on disk (CPU allocated only to jobs in memory). The CPU switches to the next job that can be run whenever the current job enters a wait state or after the current job has used a standard unit of time.When viewed over a relatively long time frame, we obtain the appearance that the CPU is simultaneously running multiple programs.
• Job swapped in and out of memory to disk. If the time-sharing computer does not have enough semiconductor memory installed to hold all of the desired programs, then a backing store must be used to temporarily hold the contents relating to some programs when other programs are present in semiconductor memory. In effect, we are now ``memory sharing'' between competing users (programs). This idea leads to a mechanism called virtual memory.
• On-line communication between user and system provided; when OS finishes execution of a command, it awaits next ``control statement'' from user.

The sensible sharing of resources such as CPU time and memory must be handled by the operating system, which is just another program running on the computer. For this control program to always be in control, we require that it never be blocked from running. The operating system, which might in fact be organized like a small number of cooperating programs, will lock itself into memory and then control CPU allocation priority in order that it never be blocked from running.