Class 11 Computer System

The parts of a computer system and how they work together: the CPU, memory from bit to petabyte, input and output devices, kinds of software and the translators for programs.

Key ideas

The CPU has the ALU for arithmetic and logic, the control unit to direct the order of steps, and registers; buses carry data, addresses and control signals between the parts.

Input devices send data in (keyboard, scanner, microphone, barcode reader); output devices give results (monitor, printer, speaker, plotter).

RAM is volatile and holds the programs in use; ROM is non-volatile and holds the start-up program; cache sits between the CPU and RAM and keeps often-used data.

Secondary memory (hard disk, SSD, pen drive) is large and non-volatile; the CPU cannot read it directly, so its contents are first loaded into RAM.

System software runs the computer (operating system, device drivers, utilities); programming tools include compilers, interpreters and assemblers; application software is generic (for many users) or customised (for one).

A compiler translates the whole program first and lists all its errors; an interpreter translates and runs one statement at a time and stops at the first error; an assembler turns mnemonics into machine code.

Deleting a file usually only marks its space as free, so the data can be recovered until it is overwritten; shredding software or destroying the device prevents that.

Remember

Bits and bytes

1 nibble = 4 bits; 1 byte = 8 bits = 2 nibbles

Units, each 1024 of the one below

byte → KB → MB → GB → TB → PB (1 KB = 1024 bytes)

Bigger unit to smaller

multiply by 1024 for each step: 3 GB = 3 × 1024 = 3,072 MB

Smaller unit to bigger

divide by 1024 for each step: 8,192 KB = 8 MB

Files that fit

whole part of (space in MB ÷ file size): 8 GB = 8,192 MB, ÷ 5 MB → 1,638 photos

Fastest to slowest

CPU registers → cache → RAM → secondary storage

Timeline

abacus → Pascaline → Analytical Engine → punched-card tabulator → Turing machine idea → stored-program computers → transistor → IC → microprocessor (LSI) → VLSI

Watch out

Using 1000 instead of 1024 between units: 8 GB is 8,192 MB, not 8,000.

Calling ROM or a hard disk volatile: only RAM and cache lose their contents without power.

Rounding up the number of files that fit: only whole files count, so take the whole part.

Mixing up compiler and interpreter: the interpreter is the one that stops at the first error.

Worked example

A phone has 128 GB of storage. 800 photos of 5 MB each and 22 videos of 570 MB each are copied to it. (a) How many MB do the photos take? (b) How many MB do the videos take? (c) How many MB are still free?

Working: 128 GB = 131,072 MB; minus 16,540 MB

Answer: (a) 4,000 MB; (b) 12,540 MB; (c) 114,532 MB