Introduction
A computer stores and processes everything — numbers, letters, punctuation, emojis, images, sound — using only two symbols: 0 and 1 (bits). Data representation is the set of agreed rules (codes) that decide which pattern of bits stands for which piece of information. This lesson covers four classic coding schemes that appear again and again in BCS, Bank and IT-officer exams:
- BCD — how decimal digits are packed into bits.
- ASCII — how English letters and symbols are numbered.
- Unicode — how every script on Earth (Bangla included) gets a number.
- Two's complement — how computers store negative numbers and do subtraction with addition.
These four topics together answer one big exam theme: "How does a machine that only knows 0 and 1 represent the real world?" Expect direct questions ("How many bits in a byte?", "ASCII code of A?", "Two's complement of -5 in 8 bits?") and conceptual ones ("Difference between BCD and binary", "Why Unicode over ASCII?").
Learning objectives
By the end of this lesson you will be able to:
- Explain why computers need coding schemes and what a bit, nibble and byte are.
- Convert decimal numbers to and from BCD and distinguish BCD from pure binary.
- Recall the structure of ASCII, its ranges, and key code values (A, a, 0, space).
- Explain what Unicode is, its main encodings (UTF-8, UTF-16, UTF-32), and why it replaced ASCII globally.
- Compute one's complement and two's complement, represent signed numbers, and perform binary subtraction by addition.
- Detect overflow and read the value range of an n-bit two's-complement number.