Operating System Architecture and Types in Detail
Software and Operating Systems · 4 min read
System software at a glance
| Type | Role | Examples |
|---|
| Operating system | Controls the whole computer | Windows, Linux, Android, macOS |
| Device drivers | Let hardware talk to the OS | Printer, graphics and sound drivers |
| Utility software | Maintain and protect the system | Antivirus, disk cleanup, backup |
| Language translators | Convert code to machine language | Compiler, interpreter, assembler |
| Firmware | Software stored permanently in hardware | BIOS, router firmware |
Layers of an operating system
| Layer | Component | Role |
|---|
| 1 (top) | User | Clicks, types, opens apps |
| 2 | Shell | Takes the user's commands and passes them to the kernel. CLI (Command Prompt) or GUI (desktop) |
| 3 | Application software | Does the user's task; asks the OS for services and cannot touch the hardware directly |
| 4 | Kernel | The core. Talks to the hardware and manages the CPU, memory, devices, files, processes and security |
| 5 (bottom) | Hardware | CPU, RAM, storage, devices |
Flow: User → Shell → Application → Kernel → Hardware.
Types of kernel
| Kernel | Idea | Advantage | Examples |
|---|
| Monolithic | All OS services run together in kernel space | Fast | Linux, Unix |
| Microkernel | Only essential functions in kernel space; the rest run in user space | Stable and secure: one failing service does not crash the system | Mach, MINIX, QNX |
| Hybrid | Mix of the two | Balance of speed and stability | Windows NT, macOS (XNU) |
| Exokernel | Almost no abstraction; programs get direct hardware access | Peak performance for special uses | Research systems |
| Nanokernel | Even smaller than a microkernel; handles little more than hardware interrupts | Embedded and real-time use | EROS, embedded RTOS |
Well-known operating systems
| OS | Developer | Used on | Interface |
|---|
| Windows | Microsoft | PCs, laptops, servers | GUI |
| macOS | Apple | Mac computers | GUI |
| Linux | Linus Torvalds (open source) | Servers, PCs, embedded | CLI and GUI |
| ChromeOS | Google | Chromebooks | GUI |
| Android | Google (Linux-based) | Phones, tablets | Touch GUI |
| iOS | Apple | iPhone, iPad | Touch GUI |
| UNIX | AT&T Bell Labs | Servers, workstations | CLI and GUI |
Batch, multiprogramming, multitasking and multiprocessing
- Batch OS: from the second generation. An operator groups similar jobs into a batch, which then runs one after another with no user contact. Good for payroll and bank processing; a fault in one job can delay the batch and the CPU may sit idle during I/O.
- Multiprogramming: several programs sit in memory together. When one needs I/O, the OS switches the CPU to another ready program, so the CPU is rarely idle. It is not parallel: the CPU still runs one at a time.
- Multitasking (time-sharing): each task gets a small time slice (time quantum) in turn, round-robin, so quickly that everything seems to run at once. Fast response, interactive.
- Multiprocessing: two or more CPUs or cores run different processes truly in parallel, sharing the same RAM.
| Feature | Multiprogramming | Multitasking | Multiprocessing |
|---|
| Goal | Keep the CPU busy | Fast response to users | Higher throughput |
| When CPU switches | When a process waits for I/O | At fixed time intervals | Not needed: each CPU has its own process |
| CPUs | One | One | Two or more |
| Examples | Early mainframes | Windows, Linux, macOS | Modern multi-core PCs |
Real-time and network operating systems
- Real-time OS (RTOS): guarantees a response within a strict deadline. Hard real-time: missing a deadline is a complete failure (airbag, missile guidance, aircraft control). Soft real-time: a missed deadline only reduces quality (video streaming, online games). Examples: VxWorks, FreeRTOS, QNX, ThreadX.
- Network OS (NOS): lets computers share files, printers, users and an internet connection. Peer-to-peer: all computers are equal, no dedicated server. Client-server: a dedicated server provides services to clients. Examples: Windows Server, Linux server, UNIX.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from standard computer textbooks, Microsoft documentation and official sources. Features differ between Excel and Word versions (2016, 2019, 2021, 365) so check which version your exam follows. Found a mistake? Tell us.