Computer System

The parts of a computer (CPU, memory, input and output devices, buses), the units of memory from a bit to a petabyte, and the software that runs it.

Key ideas

Input devices take data in, the CPU (ALU, control unit and registers) processes it, and output devices give out the results.
Memory gets larger, cheaper and slower away from the CPU: registers, cache, RAM, then secondary storage such as an SSD, a hard disk or an optical disc.
RAM, cache and registers are volatile and lose their contents without power; ROM, disks, SSDs and pen drives are non-volatile.
The address bus carries memory addresses, the data bus carries data both ways, and the control bus carries signals such as read and write.
System software (the operating system, device drivers, utilities) runs the computer; application software does a task for the user.
A compiler translates the whole program before it runs, an interpreter one statement at a time, and an assembler turns assembly language into machine code.
The operating system manages processes, memory, files and devices, and gives the user a command-line or graphical interface.

Remember

Small units	4 bits = 1 nibble; 8 bits = 1 byte; 1 KB = 1024 bytes
Each step up	1024 KB = 1 MB, 1024 MB = 1 GB, 1024 GB = 1 TB, 1024 TB = 1 PB
Converting	to a smaller unit × 1024, to a larger unit ÷ 1024: 5 GB = 5,120 MB
Bits in a file	3 KB = 3 × 1024 × 8 = 24,576 bits
Address bus of n lines	2ⁿ locations: 16 lines address 65,536
Data bus of d bits	d ÷ 8 bytes in one transfer: 64 bits move 8 bytes
How many files fit	take the whole part: 4 GB of 6 MB photos = 4,096 ÷ 6 → 682 photos

Watch out

Using 1000 instead of 1024: 2 GB is 2,048 MB, not 2,000.
Rounding up the number of files that fit: only whole files count.
Calling ROM or a pen drive volatile: only RAM, cache and registers lose their data when the power goes off.
Mixing up the compiler (whole program first) and the interpreter (one statement at a time).

Worked example

A 16 GB pen drive holds 336 photos of 5 MB each and 11 videos of 661 MB each. (a) How many MB are used? (b) How many MB are free? (c) How many more photos of 5 MB can it take?

Working: 336 × 5 + 11 × 661 = 8,951; 16,384 − 8,951 = 7,433; 7,433 ÷ 5

Answer: (a) 8,951 MB; (b) 7,433 MB; (c) 1,486