Components of an OS

Process Operations

OS provides the following kinds of operations:

• Create a process
• Delete a process
• Suspend a process
• Resume a process
• Clone a process
• Inter-process communication
• Inter-process synchronization
• Create/delete child processes (subprocess)

Memory Management

primary memory is directly accessible to the CPU.

• Programs must be in memory to execute
• Memory access is fast
• Memory is non-persistent (volatile)

OS must:

• Allocate memory for process
• deallocate memory when process terminates
• maintain mappings from physical -> virtual memory though page tables
• decide how much memory to allocate to each process

I/O

• a "big" chunk of kernel deals with I/O. Soft of like the "glue" that connects devices to the rest of the system.
• OS must provide a uniform interface to devices

Device Drivers

Routines that interact with specific device types. Encapsulate the details of the device.

• e.g. how to initialize the device, how to request I/O, how to handle interrupts, etc.
• ex: SCSI device drivers, Ethernet card drivers, video card drivers.

Note: Windows has ~35,000 device drivers.

File Systems

A nice abstraction on top of physica storage device drivers. Provides the usual list of operations (open, close, read, write, seek, etc.), but also higher level operations on the fs itself:

• create/delete files
• create/delete directories
• accounting and quotas
• backup and restore
• (sometimes) indexing and searching
• (sometimes) file versioning

Structure

Monolith:

+----------------------+
|    User Programs     |
+----------------------+
|      Everything      |
|      Else in OS      |
+----------------------+
|     Device Driver/   |
|     hardware stuff   |
+----------------------+

advantages: - cost of module interaction is low (procedure call)

disadvantages: - hard to maintain - hard to extend

Layering

Dikjstra's "THE" multiprogramming system:

Each layer presents a "virtual machine" to the layer above it. Each layer only uses the services of the layer below it.

• Layer 5: Job Managers, execute user programs
• Layer 4: Device Managers, handle devices and provide buffering
• Layer 3: Console Manager, implements virtual consoles
• Layer 2: Page Manager, implements virtual memory
• Layer 1: Kernel, implements virtual processor for each process
• Layer 0: Hardware

Each layer can be tested and verified independently.

Hardware Abstraction Layer

Goal: seperate hardwarte-specific routines from the "core" OS. Provides protability and improves readability.

Microkernels

Introduced in the late 80s, early 90s. Goal: minimize the kernel, move as much as possible into user space.

Results in: - better reliability (isolation between components) - better portability (less code to port) - better extensibility (easier to add new features)

OS is split into two parts:

• microkernel: provides basic OS services (process management, memory management, I/O, etc.)
• system processes (servers): provide higher level services (file system, networking, scheduling, etc.)

Probably slower than monolithic kernel because of all the expensive context switches.