Bit and byte

Junior

Essence

A bit is the smallest unit of information: 0 or 1. A byte is the smallest addressable unit of memory; on every modern platform, Apple's included, it is 8 bits, i.e. 256 (2⁸) possible values. Everything a program stores or sends — numbers, strings, images, pointers — is a sequence of bytes, and the type tells the compiler how to interpret those bytes.

In Swift

• Int8/UInt8 are exactly one byte, Int/UInt are word-sized (8 bytes on a 64-bit system), Int32, Int64 are fixed-width.
• MemoryLayout<T>.size shows how many bytes a value occupies.
• Bitwise operations: &, |, ^, ~, shifts <</>>. OptionSet is built on them ([.alignLeft, .wrap] is just bits in one integer), as are flags, masks and hashes.
• Literals: 0b1010 (binary), 0xFF (hex), 1_000_000.

let a: UInt8 = 0b1010_0101           // 165
print(UInt8.max)                     // 255
print(MemoryLayout<Int>.size)        // 8 on a 64-bit platform
print(MemoryLayout<Bool>.size)  // 1 — even a single flag takes a whole byte

let flags: UInt8 = 0b0000_0101
print(flags & 0b100 != 0)            // true — the third bit is set
print(flags << 1)                    // 10 — left shift = multiply by 2
print(Int8(bitPattern: 0xFF))  // -1 — same 8 bits, different interpretation

What the interviewer looks for

• Do you understand that the sign of a number is an interpretation convention (two's complement), not a property of the bytes.
• Do you know that Int is platform-dependent, and that network and file formats need fixed-width types and an explicit byte order (bigEndian/littleEndian).
• The link to memory: 1 KB = 1024 bytes, a 1000×1000 RGBA image takes 4 MB in memory regardless of the PNG file size.