1.2.1 operating systems purpose and memory management

operating systems

→ software which makes the hardware of the computer available for use

→software which manages the computer

purposes of OS

* manage hardware

→ input/output

→ processor

→ secondary storage filing system
* manages programs (installation and running)
* manages system security
* manages interfaces

OS in managing system resources

* some resources are limited
* manages resources for the maximum benefit of the system

→ the processor

→ secondary storage

OS in interfaces

* between the computer and user
* hides complexity of the system
* access rights and passwords

Kernel

* core of the operating system
* sends and receives date to and from other devices
* lies below the interface
* user only becomes aware when there is a kernel error

memory management

* jobs must be loaded into main memory for the processor to use it
* several jobs can be loaded at the same time if there is enough memory to allow it
* each programs data must be protected from the other programs

memory management module of OS

1. must keep track of memory used
2. must keep track of memory available
3. make memory available when the program execution finishes
4. check if there is enough memory available to load new program
5. ensure two programs don’t try to occupy the same memory location

Paging

* programs split into predetermined blocks called pages.
* OS keeps track of all the pages in memory
* this means programs don’t need to be stored contiguously
* may waste space if the page is only half filled

Segmentation

* programs split into logical sized blocks called segments
* the system cannot predict the size of the segments
* harder for the OS to control as the segments are all different sizes

virtual memory

1. when program is run, the pages are loaded into main memory and moved back when they are finished with.
2. process of moving pages back and forth is slow so virtual memory is used
3. virtual memory provides a small and fast to access location where the pages can quickly be accessed
4. the OS is able to predict which pages are needed and can load them first to save time