 I am Dr. Srutish Rubabharali, Assistant Professor in the Department of Computer Science. In this video, I am going to talk about Computer Memory. This topic is part of the course Computer Fundamentals. In this video, I am going to talk about the different types of computer memory and the memory hierarchy. I am also going to talk about the different CPU resistors. The binary digit or bit is the smallest unit of memory. A bit is the basic unit of representation of data in a computer. A bit can have only two possible states, one or zero. We may find it useful to think this in terms of yes and no, or on and off, or true and false, where one will be yes, true or on, and zero would be no, false or off. Most computers do not work with bit individually, but instead a group of eight bits together from a byte. One byte can store two data power eight, that is 256 different combinations of bits, and thus can be used to represent 256 different symbols. In a byte, the different combinations of bits fall in the range eight zeros to eight ones. A group of bytes can be further combined to form a word. A word can be a group of two, four or eight bytes. The relationship between bit and byte is given in this slide. One bit can have either the value of zero or one. One byte is equivalent to eight bits. One kilobyte or one kb is equivalent to two to the power ten or one zero to four bytes. Similarly, one megabyte or mb is equivalent to two to the power twenty or one zero to four kb. One gigabyte or GB is equivalent to two to the power three zero or one zero to four mb or one zero to four into one zero to four kb. Similarly, we have the bytes equivalent for one terabyte and one petabyte. In this figure, we can see that there is an organization for 16 mb block of memory for a processor. Now this processor uses a 32 bit word length. Now this can be seen as the bits are arranged horizontally. So the first square is for bit one, the second bit two, and in this way 32 bits are aligned. And this 32 bit forms one word. Similarly, if you see vertically, then we have from zero to 16 mb of block of memory. Now let us look at the different aspects of computer memory. The computer memory is characterized on the basis of two key factors. First one is storage capacity and the second one is access time. Storage capacity is the amount of information that a memory can store. While access time is the time that is required to locate and retrieve stored data from the memory unit. This is done in response to a program instruction. Now the lesser the access time, the faster will be the speed of memory. Ideally, the memory with fast access time and large capacity is preferred. However, the cost of fast memory is very high. The computer uses a hierarchy of memory that is organized in a manner to enable the fastest speed and largest capacity of memory. The hierarchy of the different memory types are shown in this slide. Three parameters that is cost, access speed and storage capacity are taken into consideration for the different types of computer memory. If we consider the registers, then the cost of the registers is very high and the access speed is also very high, whereas the storage capacity is low. If we consider the second type of memory that is cache memory, then the cost and access speed is kind of lower than the registers. If we consider the primary memory, then the cost and access speed will be lower than the registers and cache memory, but the storage capacity will be more. Now these three types of memories were semiconductor memories. If we are looking to send secondary memories, then we have the magnetic disc and optical disc and also magnetic tapes. The secondary memories costs are very low and also the access speed is low. But if we look into the storage capacity of secondary memories, then it is very high when compared to the semiconductor memories. Now let us look at the different types of computer memory. We have CPU registers which are at the top of the computer memory hierarchy and are the fastest way for the system to manipulate data. In a very simple MAC processor, it consists of a single memory location which is usually an accommodator. Registers are built from fast multi-portal memory cell. The result of arithmetic and logic unit operations is stored in registers and could be reused in a subsequent operation or saved into memory. Registers are usually measured by the number of bits that they can hold. So for example, we will have an 8-bit resistor, a 16-bit resistor or a 32-bit resistor. Now let us look at the different CPU registers. The number of registers may vary from computer to computer, but there are some registers which are common to all computers. Some of these are the accumulator, general purpose registers and spatial purpose registers. The accumulator is one of the most frequently used registers. It holds the data to be operated upon the intermediate results and also the results of processing. It is used during execution of most instructions. The results of arithmetic operations are returned to the accumulator resistor for transfer to main memory through the memory buffer resistor. Next we have the general purpose registers. These registers are used to store data and intermediate results during program execution. The contents can be accessed using assembly language programming. Next we have special purpose registers. Users do not access these registers. These are used by the computer system at the time of program execution. Some of the examples of special purpose registers are memory address register and instruction register. Now let us look at the different special purpose registers. The first one is the memory address register or MAR. It holds the address of the active memory location. Another special purpose register is memory buffer register or MBR. It holds the contents of the accessed memory world. A third special purpose register is the instruction register which holds the current instruction being executed. Then we have the program counter or PC which holds the address of the next instruction that is to be executed. Lastly we also have input output resistor or ION resistor. It is used to communicate with the input output devices. Thank you for listening to the video.