THE MEMORY SYSTEM - BASIC CONCEPTS

The Maximum size of the memory that can be used in any computer is determined by the addressing scheme.

Example: A 16-bit computer that generates 16-bit addresses is capable of addressing up to 2¹⁶ = 64K memory locations. Similarly, machines whose instructions generate 32-bit addresses can utilize a memory that contains up to 2³² = 4GB memory locations, whereas machines with 40 – bit addresses can access up to 2⁴⁰ = 1T locations.

The number of locations represents the size of the address space of the computer. The memory is usually designed to store and retrieve data in word-length quantities.

The number of bits actually stored or retrieved in one memory access is the most common definition of the word length of a computer.

Example: A byte addressable computer whose instructions generate 32-bit addresses. A 32-bit address is sent from the processor to the memory unit, the high-order 30 bits determine which word will be accessed. If a byte quantity is specified, the low-order 2 bits of the address specify which byte location is involved.

In a read operation, other bytes may be fetched from the memory, but they are ignored by the processor. If the byte operation is a write, however, the control circuitry of the memory must be ensuring that the contents of other bytes of the same word are not changed.

Fig: Connection of the memory to the processor

Data transfer between the memory and the processor takes place through the use of two processor registers, usually called MAR and MDR. If MAR is k bits long and MDR is n bits long, then the memory unit may contain up to 2^K addressable locations. A memory cycle, n – bits of data are transferred between the memory and the processor. This transfer takes place over the processor bus, which has k address lines and n data lines. The bus also includes the control lines Read/Write and the memory function completed for coordinating data transfers. Other control lines may be added to indicate the number of bytes to be transferred.

Processor reads: It reads data from the memory by loading the address of the required memory location into the MAR register and setting the Read/Write line to 1. The memory responds by placing the data from the addressed location onto the data lines, and confirms this action by asserting the MFC signal.

Processor writes: It writes data into a memory location by loading the address of this location into MAR and loading the data into MDR. It indicates that a write operation is involved by setting the R/W line to 0.

A memory unit is called random access memory (RAM) if any location can be accessed for a read or write operation in some fixed amount of time that is independent of the location’s address. The basic technology for implementing the memory uses semiconductor integrated circuits.