One of the most basic functions which a digital computer must perform is that of storing information. The stored information includes both the data to be processed and the instruction specifying the processing steps. The memory unit in a digital computer performs the storage function. It must provide means of access for the retrieval or readout, and alteration or write-in, of selected portions of the stored information. Therefore the memory unit must be able to retain, identify and retrieve digital information upon the appropriate commands.
The memory can be classified as operation memory, inner memory and auxiliary memory. The operation memory is the fastest memory and it normally consist of flip flop register. The earlier versions used transistors but now SSI and MSI memories are available. The flip flops have high cost per stored bit and that is why these are not used for bulk storage. The inner memory or the main memory has moderate cost per stored bit. This memory is normally of passive elements like ferrite cores. Semiconductor memories (MSI and LSI) are now being used as inner memories. The inner memory is supposed to be as fast as possible, because all the information processing is done through the main memory. Normally auxiliary memory or secondary memory is added to most of the computers. The main characteristics of this memory are low cost per bit of information stored. It has a high access time as compared to operation and inner memory. Magnetic tape and magnetic discs from this bulk storage memory.
Types of Memories
The memories can be classified as:
A static storage device is one in which the information does not change positions: flip-flop, registers, magnetic-core registers or even punched cards or tape are examples of static devices.
Dynamic storage devices are devices in which information stored is continually changing position. Circulating register utilizing delay lines are examples of dynamic storage devices, as are magnetic drums, and magnetic disks. Dynamic storage provides the property of compressed time, which has an application in correlation receivers.
The storage can be both temporary and permanent. Also access to the stored data falls into two classification: random access, the facility to go directly to the stored location to read the data; and sequential access, where the data are scanned in a predetermined manner and access to a particular storage location is obtained by waiting until the desired location is reached. A flip-flop register is an example of a random access storage device while a magnetic tape is an example of sequential access memory.
Memory system in which the stored information is lost when the power is turned off, or in which it is lost with elapse of time are known as volatile memories. For example a memory made up of IC flip-flop is volatile as information is lost when power is turned off. A non-volatile memory holds the information even after the power is removed. Example are the magnetic core, magnetic drum, Ferro electric devices and film.
Another way to subdivide memory unit is according to whether it is primary or auxiliary. Primary or internal storage forms an essential part of the memory unit. It performs the following three main functions in a digital computer.
- The temporary storage of numbers and instructions directed to and from the input-out.
- The storage of all data instructions required for the problem begin handled by the computer.
- Temporary storage of the intermediate results of any calculation.
As in the case of computers, digital instruments have instructions or numbers stored in an arrangement of bistables or flip-flop, each capable of storing one bit of information.
In read and write memory or random access memory (RAM), data can be stored (or written) into the memory as well as read out.
In read only memory (ROM) , data are permanently stored by the manufacturer or user and data can only be read later, and the stored data are not changed when the circuit is switched off.
EPROM or electrically programmed read only memory (or known as erasable programmable ROM) allows a user with the required equipment later to change the stored data. In use, such memories only have data read from them.
Now the size of the memory circuit is given in terms of the number of bits of data it can stored. Most instruments use the bits as a group or word of data. Figure 1 (a) shows general RAM with data input lines to the left and the write input below. When the latter goes HIGH, the data on the input lines, would set up an bits of the memory at the section specified by the address lines. The k address lines can specify 2k such sections, each of n-bits, so the total number of elements in the memory is 2k x n bits. For example, a memory of 256x8 bit size would deal with 8-bit data word, it would have eight address lines as 28 = 256 and would be called a 2k memory chip.