問78
頻出度★★☆
コンピュータの主記憶中にある命令やデータの格納場所を特定するために, その場所に付けられた値はどれか。
平成24年度 春期 ITパスポート試験 問78
テクノロジ系コンピュータ構成要素 › メモリ
This question's explanation and your answers so far.
解説
問 78
- ア
正解 Main memory is divided into storage units (usually one byte each), and every one of these compartments is given a unique serial number counting up from 0. The CPU specifies the compartment it wants to read or write by this number, so once the number is known, the place in main memory holding an instruction or data can be identified uniquely. This value attached to a storage location is the address, so ア is correct. For example, when the CPU fetches an instruction, it puts the address of the next instruction to execute on the address bus to tell main memory, and then receives the contents of that compartment over the data bus. It works just like the everyday analogy: give the street address and the building (the data) is identified.
Incorrectイ
An index means an index or a subscript. It can be the subscript that says which element of an array you mean, or the separate index built in a database so that a target record can be found quickly. Either way it is an order or a search clue within a table or a collection, not a value attached to the physical storage location in main memory itself. An array subscript becomes a position in main memory only after it is converted into an address (base address + subscript × element size), so it works at a different level from an address.
Incorrectウ
A counter is a mechanism for counting; it holds and adds up a number of times or a quantity. The similarly named program counter is a register that holds the address of the next instruction to execute — it is a container that an address goes into, not the value attached to a storage location itself. Do not mix up the container with the value that goes inside it.
Incorrectエ
A decoder is a circuit that interprets a code and picks out the one output line that corresponds to it. Inside the CPU it is the instruction decoder that interprets instructions; in main memory it works as the selection circuit that picks the matching row from the address it receives. It is a circuit on the side that interprets and uses an address, not a value, so it cannot be the answer to this question, which asks for the value attached to a location. The circuit that performs the reverse conversion is an encoder.
ここがポイント
- An address is the unique serial number assigned to each compartment when main memory is divided into storage units. The CPU uses this value to specify the target location for reading or writing, and sends it to main memory over the address bus.
- Telling the confusable terms apart: an index is a subscript showing a position in a table or array, or an index used for searching; a counter is a counting mechanism (a program counter is a register that holds the address of the next instruction); a decoder is a circuit that interprets codes. Sorting them by whether each one is a value, a container, or a circuit keeps them straight.
用語