A Brief History of Operating Systems

• The first computers had no operating systems. Each user (programmer) was locked in a room with the machine and had it all to herself.[*]

• This "hands-on operation" was quickly decided to be an uneconomical waste of very expensive computers. It led to the first operating systems, called job sequencing monitors.

• Job sequencing monitors did three things:
  • Automatically ran a sequence of preprepared programs.
  • Prevented a program from going into an infinite loop or printing out reams of paper by "early termination."
  • Prevented a program from overwriting the memory the job sequencing program was using!

• A modification of job sequencing monitors which used two computers, one for input/output (punch cards, anyone?) and one for processing was very popular and went by the name of batch processing.

• Batch processing was still uneconomical because doing input/output is much slower than doing computations, so the processing computer would frequently sit twiddling its bits, waiting for the other computer. This observation led to three parallel developments:
  • Buffered input/output: Adding chips inside the computer to handle and store up the results of i/o for the CPU.
  • Multiprogramming: Allowing multiple programs to "execute at the same time" (in reality, they are merely in memory at the same time and the CPU switches from program A to program B when program A is waiting for i/o).
  • Time sharing: Having one computer service multiple users (terminals). Once again, this works because i/o (e.g., time between key presses) is slow. Each user interacts with a program which pretends to be a whole computer; this program is multiprogrammed in the CPU along with everything else.

• Finally, the growth of networks in the form of both local area networks (LANs) and global networks such as the Internet has spawned networking operating systems. For example, UNIX coupled with either the Networked File System (NFS) or the Andrews File System (AFS) create the illusion that files hundreds or thousands of miles away and instantly accessible and modifiable.

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