Computing Systems
Computing Systems
The exam samples the machine layer: how a processor executes instructions, how data is represented in binary, how the memory hierarchy trades speed for capacity, and how an operating system manages processes, memory, files, and devices. Questions often ask you to convert values, predict overflow, or identify which component performs a task.
Core rules
- Von Neumann cycle: fetch the instruction at the program counter, decode it, execute it in the ALU or control unit, then write back; registers are the fastest storage.
- Memory hierarchy: registers, then L1/L2/L3 cache, then RAM, then SSD/HDD; caches exploit temporal and spatial locality.
- Integers: an n-bit unsigned value ranges from 0 to 2^n - 1; two's complement ranges from -2^(n-1) to 2^(n-1) - 1. Overflow wraps or raises an error depending on the language.
- OS duties: scheduling, virtual memory and paging, file systems, device drivers, and permission checks.
Worked example
Represent -5 in 8-bit two's complement. Write +5 as 0000 0101. Invert the bits to get 1111 1010. Add 1 to get 1111 1011. The leading 1 confirms a negative value, and 1111 1011 equals -128 + 64 + 32 + 16 + 8 + 2 + 1 = -5. The range check matters: 8-bit two's complement holds -128 through 127, so -5 is representable but +200 is not.
Traps
Mixing bits and bytes (8 bits = 1 byte); forgetting that two's complement has one more negative value than positive; calling cache secondary storage; and assuming a compiler and an interpreter differ in what language features they allow rather than in translation timing.