State whether the following statements are true or false. (i) For main memory,…

Concept

A memory system defines three quantities separately, and they are not required to be equal to one another:

• Word — the natural unit of organisation of memory, typically the number of bits used to hold one number or one instruction.

• Addressable unit — the smallest location that carries its own unique address; on most machines this is a byte, on some it is a word.

• Unit of transfer — for main memory, the number of bits read out of or written into memory in one access.

The standard definition of the main-memory unit of transfer is stated in bits, and that count need not equal a word or an addressable unit.

Application

1. Statement (i) describes the main-memory unit of transfer as a number of bytes. The standard definition counts bits, not bytes, so statement (i) does not match the definition and is false.

2. Statement (ii) asserts that the unit of transfer need not equal a word or an addressable unit. That independence is part of the same definition, so statement (ii) is true.

Cross-check and contrast

• Concrete case: a byte-addressable machine has an addressable unit of 8 bits, may use a 32-bit word, and may still move 64 bits to or from main memory in one access — three different values at the same time, which is exactly the independence described in statement (ii).

• Memory capacity is usually quoted in bytes or in words. That is a way of describing how large the memory is, and it does not change the unit in which the transfer size is defined.

• External memory is the contrasting case: data there moves in blocks that are much larger than a word and are counted in bytes. Carrying that block-and-byte picture over to main memory is the confusion that makes a byte-based wording look acceptable.

Result

Statement (i) is false and statement (ii) is true.