1.2.2. Memory basics

Memory is that unit in the computer which holds program instructions and data. To execute a program, the CPU fetches the program instructions from memory, it also loads the data corresponding to the operands of these instructions from the memory. After the execution of the instructions, it stores the data produced after executing that instruction also into the memory.

Memory is a crucial component of a computer system. It temporarily holds the program instructions that the CPU needs to execute tasks and the data that the program operates on. The CPU continuously interacts with memory, fetching instructions and the data required for executing those instructions. After executing, any results or changes are also saved back into memory for future use.

Think of memory as a chalkboard in a classroom. The teacher writes lessons (instructions) on the board, and students refer to them to work on their assignments (data). Once students complete their tasks, they might write their results back on the board, which represents saving their work.

Memory is broadly classified into two categories: Inboard memory and Outboard memory. Inboard memory are those memory units which are directly plugged into the motherboard of the computer, such as processor registers, cache memory, and main memory. Outboard memory, on the other hand, includes magnetic disks, hard disks, and optical disks, which are not on the motherboard. There is also offline storage like magnetic tapes, which are used for bulk storage.

Memory can be categorized into inboard and outboard types. Inboard memory refers to the storage that is integrated into the computer's motherboard, facilitating fast access because it’s physically closer to the CPU. Examples include registers, cache, and RAM. Outboard memory, however, refers to storage devices like hard drives and CDs that are external to the motherboard and are used for larger, long-term storage. Offline storage, like magnetic tapes, serves as a backup solution.

Imagine your study area. The books you keep on your desk (inboard memory) are easily accessible and quick to refer to, while the books stored in a cabinet (outboard memory) take more time to retrieve. The entire library where you don’t visit often represents offline storage.

The capacity of a memory module is characterized in terms of the number of distinctly addressable memory locations and the size of each of those locations. Typically, the size of a memory location is 1 byte for byte addressable memory. In the case of word addressable memory, a word can consist of multiple bytes.

Memory capacity is defined by how many locations it has and how large each location is. For most systems, each addressable location holds one byte. Some systems might group bytes into larger units called words, which can hold several bytes together. This impacts how the memory is addressed and accessed.

Think of memory like a bookshelf organized into sections (addressable locations). Each section can hold a specific number of books (bytes). If you only have a small shelf, you can only fit a limited number of books, just as a smaller memory module can only hold a certain amount of data.

The number of data bits read out or written into the memory at a given time is called the unit of transfer. The unit of transfer is determined by the size of the data bus in the computer. For a 32-bit computer, 4 bytes is the unit of transfer.

The unit of transfer is how much data can be moved into or out of memory in a single operation, and this is influenced by the data bus width of the CPU. If a computer can handle 32 bits at once, that translates to 4 bytes because each byte consists of 8 bits. This ability to move larger chunks of data at a time improves overall performance.

Consider a delivery truck that can carry 4 boxes at a time (unit of transfer). If you have a truck that can carry more boxes (a wider data bus), you can deliver goods (data) to your destination more efficiently, needing fewer trips.

Consider a 32-bit memory. The word size is 32 bits in my computer. Unit of transfer equals to word size equals to 32 bits, and this computer is byte addressable. The address of a word is always an integer multiple of 4.

In a computer with 32 bits of memory, each address corresponds to a byte, but words (groups of bytes) are addressed as multiples of 4 because each word contains 4 bytes. This means every even byte is part of a word, and it simplifies how we address data storage in memory.

Imagine a street where every fourth house has a mailbox (word), and the mailboxes (addresses) are labeled 0, 1, 2, 3 (bytes) sequentially. If you know the address of the mailbox (memory location), you can easily find which house corresponds to it.

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