Memory Unit Computer Architecture

 

Memory Hierarchy

A memory unit is an essential component in any digital computer since it is needed for storing programs and data.

Typically, a memory unit can be classified into two categories:

1. The memory unit that establishes direct communication with the CPU is called Main Memory. The main memory is often referred to as RAM (Random Access Memory).
2. The memory units that provide backup storage are called Auxiliary Memory. For instance, magnetic disks and magnetic tapes are the most commonly used auxiliary memories.

Apart from the basic classifications of a memory unit, the memory hierarchy consists all of the storage devices available in a computer system ranging from the slow but high-capacity auxiliary memory to relatively faster main memory.

The following image illustrates the components in a typical memory hierarchy.

Auxiliary Memory

Auxiliary memory is known as the lowest-cost, highest-capacity and slowest-access storage in a computer system. Auxiliary memory provides storage for programs and data that are kept for long-term storage or when not in immediate use. The most common examples of auxiliary memories are magnetic tapes and magnetic disks.

A magnetic disk is a digital computer memory that uses a magnetization process to write, rewrite and access data. For example, hard drives, zip disks, and floppy disks.

Magnetic tape is a storage medium that allows for data archiving, collection, and backup for different kinds of data.

Main Memory

The main memory in a computer system is often referred to as Random Access Memory (RAM). This memory unit communicates directly with the CPU and with auxiliary memory devices through an I/O processor.

The programs that are not currently required in the main memory are transferred into auxiliary memory to provide space for currently used programs and data.

I/O Processor

The primary function of an I/O Processor is to manage the data transfers between auxiliary memories and the main memory.

Cache Memory

The data or contents of the main memory that are used frequently by CPU are stored in the cache memory so that the processor can easily access that data in a shorter time. Whenever the CPU requires accessing memory, it first checks the required data into the cache memory. If the data is found in the cache memory, it is read from the fast memory. Otherwise, the CPU moves onto the main memory for the required data.

Main Memory

The main memory acts as the central storage unit in a computer system. It is a relatively large and fast memory which is used to store programs and data during the run time operations.

The primary technology used for the main memory is based on semiconductor integrated circuits. The integrated circuits for the main memory are classified into two major units.

1. RAM (Random Access Memory) integrated circuit chips
2. ROM (Read Only Memory) integrated circuit chips

RAM integrated circuit chips

The RAM integrated circuit chips are further classified into two possible operating modes, static and dynamic.

The primary compositions of a static RAM are flip-flops that store the binary information. The nature of the stored information is volatile, i.e. it remains valid as long as power is applied to the system. The static RAM is easy to use and takes less time performing read and write operations as compared to dynamic RAM.

The dynamic RAM exhibits the binary information in the form of electric charges that are applied to capacitors. The capacitors are integrated inside the chip by MOS transistors. The dynamic RAM consumes less power and provides large storage capacity in a single memory chip.

RAM chips are available in a variety of sizes and are used as per the system requirement. The following block diagram demonstrates the chip interconnection in a 128 * 8 RAM chip.

• A 128 * 8 RAM chip has a memory capacity of 128 words of eight bits (one byte) per word. This requires a 7-bit address and an 8-bit bidirectional data bus.
• The 8-bit bidirectional data bus allows the transfer of data either from memory to CPU during a read operation or from CPU to memory during a write operation.
• The read and write inputs specify the memory operation, and the two chip select (CS) control inputs are for enabling the chip only when the microprocessor selects it.
• The bidirectional data bus is constructed using three-state buffers.
• The output generated by three-state buffers can be placed in one of the three possible states which include a signal equivalent to logic 1, a signal equal to logic 0, or a high-impedance state.

• ROM integrated circuit

  The primary component of the main memory is RAM integrated circuit chips, but a portion of memory may be constructed with ROM chips.

  A ROM memory is used for keeping programs and data that are permanently resident in the computer.

  Apart from the permanent storage of data, the ROM portion of main memory is needed for storing an initial program called a bootstrap loader. The primary function of the bootstrap loader program is to start the computer software operating when power is turned on.

  ROM chips are also available in a variety of sizes and are also used as per the system requirement. The following block diagram demonstrates the chip interconnection in a 512 * 8 ROM chip.

  

A ROM chip has a similar organization as a RAM chip. However, a ROM can only perform read operation; the data bus can only operate in an output mode.

The 9-bit address lines in the ROM chip specify any one of the 512 bytes stored in it.
The value for chip select 1 and chip select 2 must be 1 and 0 for the unit to operate. Otherwise, the data bus is said to be in a high-impedance state.