School of Computing. Dublin City University.
At start, there were no Operating Systems. Computers were like the abstract model of a machine, running one program literally and nothing else. 1 user at a time 1 program at a time Programmer operates machine himself. Manually load program, run until crashed (dump memory and registers) or finished (print output), revise program, run again, or run next program. Interactive (great if you're the lone programmer) but CPU idle for long periods (e.g. while program being revised, or when program halts/crashes when programmer not watching). Long wait to use machine for other programmers. DRIVING FORCES FOR CHANGE: LOT MORE PROGRAMMERS WANTING TO USE MACHINE COMPUTERS EXPENSIVE (ANY CPU IDLE TIME BAD)
Programmers have to submit jobs, receive results later. Operator schedules jobs. Jobs still stored on sequential (tape) access (no random access medium yet). Sequential tapes of jobs are prepared by operator for the CPU to run through once. Resident monitor, always in memory (first OS). Doesn't decide order of jobs, but does co-ordinate their sequencing, automatic starting and terminating. Jobs come with control cards to say what they need to run. Downside for programmer: long queues, not interactive any more, if error in program may have to wait days to retry it. Have to think things through in advance! DRIVING FORCE FOR CHANGE: RANDOM (DISK) ACCESS BECOMES AVAILABLE.
OS can now implement the human operator's algorithms in deciding sequence of jobs to run. Scheduling of jobs now totally automated (true OS). DRIVING FORCE FOR CHANGE: I/O DEVICE SPEED IS MUCH SLOWER THAN CPU SPEED, SO CPU STILL OFTEN IDLE WHILE I/O GOING ON.
Device controller is a piece of hardware that can work in parallel with the CPU. Not a parallel computer - it is just a specialised device for data transfer, not a general-purpose computer. Spooling - Print device controller is still copying data from disk to printer while CPU has already begun working on next job. Next job can begin while previous is still printing out. CPU can store output on disk if printer full and move on. DRIVING FORCES FOR CHANGE: WAIT TIMES STILL TOO LONG. Long jobs delay everything else. Run short jobs first? But if we have a small job that must be run once a day, then we cannot EVER run a program that will take 1.1 days. Also, program might do I/O half-way through its execution (rather than only at start/end). When program stops to wait for this I/O, CPU is idle. Maybe only short time, but it all adds up.
Multiple jobs in memory (and running) at the same time. Each memory space protected from each other. OS picks one, executes it for a while, stops (e.g. when program waits for input, or else randomly), picks other to run. CPU busy, but still batch model, not interactive. Scheduling: Job scheduling (or long-term scheduling) - which jobs get into memory at all CPU scheduling (or short-term scheduling) - which to run at each step (many may be ready) DRIVING FORCES FOR CHANGE: INCREASING NUMBER OF -USERS- WHO WANT TO INTERACT WITH PROGRAMS WHILE THEY ARE RUNNING. COMPUTERS CHEAPER - CHEAP DUMB TERMINALS AVAILABLE. HUMANS' TIME IS EXPENSIVE - DON'T WANT THEM TO WAIT.
Multi-programming where the program may be waiting on a USER. OS will in the meantime service another program, which may be interacting with another user. Result: Multiple users share the CPU. If the time-slicing is quick enough, they all feel as if they have their own dedicated machine! Programmers delighted - CPU kept busy, yet interactive again, just like in (1). User interaction at run-time allows a whole world of programs that were never possible before. In practice shared systems can get overloaded and slow down.
DRIVING FORCES FOR CHANGE: REAL COMPUTERS GET VERY CHEAP
To some extent a return to simplicity of (5) and earlier - no traffic problems. Internet still unimportant. LANs beginning to be used to coordinate file sharing.
DRIVING FORCES FOR CHANGE: INTERNET BECOMES USABLE AND USEFUL. STANDALONE MACHINE NOW SEEN AS TOO ISOLATED.
Web is killer app for Internet 1993. Much use of information on shared remote web servers. Shared file systems. Shared email systems. Return to some of the problems of (6) - traffic problems.
DRIVING FORCES FOR CHANGE: BROADBAND (AT HOME). THEN LATER: MOBILE BROADBAND.
The mobile world before 2005:
No or limited Internet.
Image from here.