BCA- 202(N) Subject: Digital Electronics and Computer Organization UNIT III Read-Only Memory (ROM) Read-Only Memory (ROM) is the primary memory unit of any computer system along with the Random Access Memory (RAM), but unlike RAM, in ROM, the binary information is stored permanently. Now, this information to be stored is provided by the designer and is then stored inside the ROM. Once, it is stored, it remains within the unit, even when power is turned off and on again. The information is embedded in the ROM, in the form of bits, by a process known as programming the ROM. Here, programming is used to refer to the hardware procedure which specifies the bits that are going to be inserted in the hardware configuration of the device. And this is what makes ROM a Programmable Logic Device (PLD). Structure of ROM The block diagram for the ROM is as given below- Block Structure It consists of k input lines and n output lines . The k input lines is used to take the input address from where we want to access the content of the ROM. Since each of the k input lines can be either 0 or 1, so there are 2 total addresses which can be referred to by these input lines and each of these addresses contains n bit information, which is given out as the output of the ROM. Such a ROM is specified as 2 x n ROM. Internal Structure It consists of two basic components – Decoder and OR gates. A Decoder is a combinational circuit which is used to decode any encoded form (such as binary, BCD) to a more known form (such as decimal form) . In ROM, the input to a decoder will be in binary form and the output will represent its decimal equivalent. The Decoder is represented as l x 2 , that is, it has l inputs and has 2 outputs, which implies that it will take l-bit binary number and decode it into one of the 2 decimal number . All the OR gates present in the ROM will have outputs of the decoder as their input . Classification of Read-Only Memory (ROM) 1. Mask ROM – In this type of ROM, the specification of the ROM (its contents and their location), is taken by the manufacturer from the customer in tabular form in a specified format and then makes corresponding masks for the paths to produce the desired output . This is costly, as the vendor charges special fee from the customer for making a particular ROM (recommended, only if large quantity of the same ROM is required). Uses – They are used in network operating systems, server operating systems, storing of fonts for laser printers, sound data in electronic musical instruments. 2. PROM – It stands for Programmable Read-Only Memory. It is first prepared as blank memory, and then it is programmed to store the information. The difference between PROM and Mask ROM is that PROM is manufactured as blank memory and programmed after manufacturing, whereas a Mask ROM is programmed during the manufacturing process. To program the PROM, a PROM programmer or PROM burner is used. The process of programming the PROM is called as burning the PROM. Also, the data stored in it cannot be modified, so it is called as one – time programmable device. Uses – They have several different applications, including cell phones, video game consoles, RFID tags, medical devices, and other electronics. 3. EPROM – It stands for Erasable Programmable Read-Only Memory. It overcomes the disadvantage of PROM that once programmed; the fixed pattern is permanent and cannot be altered. If a bit pattern has been established, the PROM becomes unusable, if the bit pattern has to be changed. This problem has been overcome by the EPROM, as when the EPROM is placed under a special ultraviolet light for a length of time, the shortwave radiation makes the EPROM return to its initial state, which then can be programmed accordingly. Again for erasing the content, PROM programmer or PROM burner is used. Uses – Before the advent of EEPROMs, some micro-controllers, like some versions of Intel 8048, the free scale 68HC11 used EPROM to store their program. 4. EEPROM – It stands for Electrically Erasable Programmable Read-Only Memory. It is similar to EPROM, except that in this, the EEPROM is returned to its initial state by application of an electrical signal, in place of ultraviolet light. Thus, it provides the ease of erasing, as this can be done, even if the memory is positioned in the computer. It erases or writes one byte of data at a time. Uses – It is used for storing the computer system BIOS. 5. Flash ROM – It is an enhanced version of EEPROM .The difference between EEPROM and Flash ROM is that in EEPROM, only 1 byte of data can be deleted or written at a particular time, whereas, in flash memory, blocks of data (usually 512 bytes) can be deleted or written at a particular time. So, Flash ROM is much faster than EEPROM. Uses – Many modern PCs have their BIOS stored on a flash memory chip, called as flash BIOS and they are also used in modems as well. RAM (Random Access Memory) RAM (Random Access Memory) is the internal memory of the CPU for storing data, program, and program result. It is a read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased. Access time in RAM is independent of the address, that is, each storage location inside the memory is as easy to reach as other locations and takes the same amount of time. Data in the RAM can be accessed randomly but it is very expensive. RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence, a backup Uninterruptible Power System (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold. RAM is of two types − Static RAM (SRAM) Dynamic RAM (DRAM) Static RAM (SRAM) The word static indicates that the memory retains its contents as long as power is being supplied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not be refreshed on a regular basis. There is extra space in the matrix, hence SRAM uses more chips than DRAM for the same amount of storage space, making the manufacturing costs higher. SRAM is thus used as cache memory and has very fast access. Characteristic of Static RAM Long life No need to refresh Faster Used as cache memory Large size Expensive High power consumption Dynamic RAM (DRAM) DRAM, unlike SRAM, must be continually refreshed in order to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory as it is cheap and small. All DRAMs are made up of memory cells, which are composed of one capacitor and one transistor. Characteristics of Dynamic RAM Short data lifetime Needs to be refreshed continuously Slower as compared to SRAM Used as RAM Smaller in size Less expensive Less power consumption Hard Disk The hard disk drive is the main, and usually largest, data storage hardware device in a computer. The operating system, software titles, and most other files are stored in the hard disk drive. Hard Disk Drive Hard disk drive is made up of a series of circular disks called platters arranged one over the other almost ½ inches apart around a spindle. Disks are made of non-magnetic material like aluminium alloy and coated with 10-20 nm of magnetic material. Standard diameter of these disks is 14 inches and they rotate with speeds varying from 4200 rpm (rotations per minute) for personal computers to 15000 rpm for servers. Data is stored by magnetizing or demagnetizing the magnetic coating. A magnetic reader arm is used to read data from and write data to the disks. A typical modern HDD has capacity in terabytes (TB). Hard Disk Drive Physical Description A hard drive is usually the size of a paperback book, but much heavier. The sides of the hard drive have pre-drilled, threaded holes for easy mounting in the 3.5-inch drive bay in the computer case. Mounting is also possible in a larger 5.25-inch drive bay with an adapter. The hard drive is mounted so the end with the connections faces inside the computer. The back end of the hard drive contains a port for a cable that connects to the motherboard. The type of cable used (SATA or PATA) depends on the type of drive but is almost always included with a hard drive purchase. Also here is a connection for power from the power supply. Most hard drives also have jumper settings on the back end that define how the motherboard is to recognize the drive when more than one is present. These settings vary from drive to drive, so check with your hard drive manufacturer for details. CD Drive CD stands for Compact Disk. CDs are circular disks that use optical rays, usually lasers, to read and write data. They are very cheap as you can get 700 MB of storage space for less than a dollar. CDs are inserted in CD drives built into CPU cabinet.