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# Memory, Bits, & Bytes - PowerPoint PPT Presentation

Memory, Bits, & Bytes. Memory. Part of the computer where programs and data are stored. Read and written (changed). Bit Binary digit Basic unit of memory 1 or 0 Why binary? Because we can most reliably (electronically) distinguish between 1 and 0. Byte = 8 bits

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### Memory, Bits, & Bytes

• Part of the computer where programs and data are stored.

• Bit

• Binary digit

• Basic unit of memory

• 1 or 0

• Why binary? Because we can most reliably (electronically) distinguish between 1 and 0.

• Byte = 8 bits

• Smallest unit of memory that can be read or written.

• Each individually addressable “cell” is an 8-bit byte containing 28 = 256 possible values (0..255).

• The number of memory cells is independent of the cell size.

• Most modern processors have at least a 32-bit address space.

232 = 4G bytes arranged 0..232-1

• Each byte will contain the ASCII value of a character.

• So “hello” is the sequence of bytes:

104 101 108 108 111

• But how do we determine the end of the string?

104 101 108 108 111 0

• This method is also employed by C.

• Other methods are possible.

• Each individually addressable “cell” is an 8-bit byte containing 28 = 256 possible values (0..255).

• To allow for larger values, we group bytes together.

• byte = 8 bits

• word = 16 bits

• double word = 32 bits (long word)

• Consider a word consisting of 2 bytes in memory with a value of 080116 at address 10.

• It is a word (2 bytes) so it occupies memory location 10 and memory location 11.

• It can be stored in memory as either:

M[10] M[11]

08 01 - big endian (Motorola)

01 08 - little endian (IA32, VAX)

- either (switchable): IA64, ultraSparc

• Extends from 4 to 8 (and 16) byte integers too.

• (Note: For integers larger than 2 bytes, other orderings are possible but they are not used.)

• big endian: Motorola

• little endian: IA32, VAX

• bi-endian: IA64, ultraSparc

• either/both supported

• typically switchable at boot time

• What happens if one sends a message (that contains multi-byte integers) from one system to another across the internet, and they have different endian-ness?

• We need a way to convert from one format to another (future topic; also see htonl Unix/Linux function).