- A program that acts as an intermediary between the user of a computer and the computer hardware
- Operating system goals
- Convenience: make the computer system convenient to use
- Efficiency: Manage the computer system resources in an efficient manner
- "The one program running at all times on the computer" is the kernel. Everything else is either a system program or an application program.
2. OS Features needed for multiprogramming
- Job Scheduling: must choose the processes that will be brought to memory
- Memory Management: must allocate the memory to several jobs
- CPU Scheduling: must choose among several jobs ready to run
3. Parallel System
- Multiprocessor systems with more than one CPU in close communication
- Tightly coupled system
- processors share memory and a clock;
- communication usually takes place through the shared memory
- Advantage of parallel system
- increased throughput
- economy of scale
- increased reliability
4. Distributed Systems
- Distributed the computation among several physical processors
- Loosely coupled system
- each processor has its own local memory
- processors communicate with one another through various communication lines
- Advantage of distributed systems
- resource and load sharing
- reliability
- communications
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