Subsystem Managers

COMP124 Lectures 2021-03-06

Memory Manager

This manager is in charge of main memory and making sure requests are legal.

Tasks

• Preserves and protects the space in main memory that is occupied by the OS itself.
• Checks validity of each request for memory space.
• For legal requests, allocates areas of memory not already in use.
• In a multi-user system, keeps track of which users are using which section of memory.
• De-allocates sections of memory that are no longer needed.

Processor Manager

The processor manager decides how to allocate the central processing unit (CPU).

Tasks

• Creates processes when necessary to carry out tasks.
• Performs initialisation of new processes.
• Keeps track of the status of processes.
• Assigns processes to the CPU when available.
• Changes process states as events occur.
• Handles termination of processes on completion or abort.
• Handles inter-process communication.
• Manages process queues and prioritisation.

Device Manager

The device manager monitors every device and control unit.

Tasks

• Allocates the system’s devices (printers, disk drives), in accordance with the system’s scheduling policy.
• Deals with multiple requests for same device.
• Performs communication with the device during operation.
• De-allocates devices when no longer required.

File Manager

The file manager keeps track of every file in the system.

Tasks

• Organises all files, including data files, compilers, application programs, into directory structures.
• Manages the locations of files on disk.
• Enforces restrictions on who has access to which files (using a pre-determined access policy).
• Deals with all standard file operations.
  • Open, close, delete, read, write.

Network Manager

The Network Manager provides the facilities for users to share resources while controlling user access to them:

• Resources connected to local network.
• Disks, printers…

The network stack provides access to the network.

We will not be looking at networking on this module.

Interaction Between OS Managers

Each OS manager has specific, individual tasks to perform but, it is not enough for each to operate on its own.

Example

Suppose a user types in a command at the keyboard to execute a program. The following steps must occur in sequence:

1. Device manager
   • Receives keystrokes and builds up command line. Informs processor manager that input has been received.
2. Processor Manager
   • Activates process awaiting a command. Process checks validity of command, and then tries to execute the corresponding program.
3. File Manager
   • Checks to see if the program file is already in memory. If not, it finds it on external storage and issues request for it.
4. Device Manager
   • Communicates with the disk drive and retrieves the program file from disk.
5. Memory Manager
   • Allocates space for the program and records its location. Continues to deal with changes to memory requirements as program runs.
6. Processor Manager
   • Creates a process for the new program and begins its execution.