Computer Organization and Operating Systems
What this domain tests
Computer-organization questions ask you to trace an instruction through the CPU and to distinguish memory from storage. Operating-system questions test functions, resource management, and the difference between an interface and a kernel.
Core rules
- The CPU contains the ALU (arithmetic/logic), control unit (fetch, decode, direct), and registers. The instruction cycle is fetch → decode → execute → store.
- Key registers: program counter (address of the next instruction), instruction register (current instruction), memory address register, memory buffer/data register, and accumulator.
- Buses carry address, data, and control signals. Address-bus width fixes how many locations can be addressed: 2^n.
- Memory hierarchy: registers → cache → RAM → secondary storage. Speed falls and capacity rises as you move outward.
- RAM is volatile; ROM is non-volatile and holds firmware such as the BIOS.
- OS functions: process, memory, file, device, and security management. The OS also provides a user interface and services such as spooling and virtual memory.
- OS types include batch, multiprogramming, time-sharing, real-time, and distributed.
Worked example
A processor has a 16-bit address bus and an 8-bit data bus. How much memory can it address? Each address selects one byte, and 2^16 = 65,536 addresses, so 65,536 bytes = 64 KB. The 8-bit data bus means one byte is transferred per data cycle. If the address bus were 20 bits, 2^20 = 1,048,576 bytes = 1 MB.
Traps
- Saying cache is larger than RAM; it is smaller but faster.
- Confusing addressable locations with total bytes when the word size is not one byte.
- Calling a GUI an operating system; it is an interface.
- Mixing up multiprogramming (several jobs in memory) and multitasking (rapid switching).