PRIMARY MEMORY

INTRODUCTION

            Computers are used to perform various tasks in science, engineering, business, education, entertainment and many other human endeavors. They work at high speed, are able to handle large amounts of data with great accuracy, and have the ability to carry out specified sequence of operations (Program) without human intervention. The CPU handles the processing of data and after processing, presents the results with the help of output devices. However, to process the data and to store the output, computers require memory.

            Memory refers to the electronic holding place for instructions and data where the computer’s microprocessor can reach quickly. The primary memory allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. For primary memory, computers use RAM (Random Access Memory).

Memory representation

            All the quantities, physical or otherwise, can be measured in some units. For example, length is measured in meters and mass in grams. Likewise, for measuring computer memory, we need units. The basic unit of memory is bit.

The various units, used to measure computer memory, are as follows:

Bit: bit, abbreviation for binary digit, is the basic unit of memory. It is the smallest unit of information on a machine and a single bit can only one of two values: 0 or 1. Bit is represented by a lower case b.

Byte: a unit of 8 bits is known as a byte. Hence, a byte is able to contain any binary number between 00000000 and 11111111. It is represented by an upper case B.

Kilobyte: in decimal system, kilo stands for 1000, but in binary system, kilo refers to 1024. Therefore, a kilobyte is equal to 1024 bytes, but it is often used loosely as a synonym for 1000 bytes. It is usually represented by KB.

Megabyte: it comprises 1024 kilobytes, or 1,048,576 bytes. However, since this number is hard to remember, a megabyte can be thought of as a million bytes. Megabyte is the standard unit of measurement of RAM and is abbreviated as MB.

Gigabyte: a gigabyte consists of 1024 megabytes (1,073,741,824 bytes). It is the standard unit of measurement for hard disks and is often abbreviated as GB.

Terabyte: the term terabyte refers to 1024 gigabytes of memory. Often abbreviated as TB, terabyte memory is usually associated with super computers only.

MEMORY HIERARCHY:

            The processor is the ‘brain’ of the computer where all the essential computing takes place. Unlike a human brain, which combines memory with processing power, a human brain, which combines memory with processing power, a computer processor has very little memory. The memory in a computer system is of three fundamental types:

1.      Internal processor memory: this memory is placed in the CPU or is attached to a special fast bus. Internal memory usually includes cache memory and special registers, both of which can be directly accessed by the processor.

2.      Primary memory: random access memory (RAM) and Read Only Memory (ROM) fall in the category of the primary memory, also known as main memory. Every computer comes with a small amount of ROM, which contains the boot firmware.

3.      Secondary memory: also known as auxiliary memory, secondary memory provides backup storage for instruction and data. Most commonly used secondary memory devices are hard disk, magnetic disk, and magnetic tapes. These are the least expensive among all the memories.

Storage Technologies

Much of the success of computer technology stems from the tremendous progress in storage technology. Early computers had a few kilobytes of random-access memory. The earliest IBM PCs didn’t even have a hard disk. That changed with the introduction of the IBM PC-XT in 1982, with its 10-megabyte disk. By the year 2010, typical machines had 150,000 times as much disk storage, and the amount of storage was increasing by a factor of 2 every couple of years.

Random-Access Memory

Random-access memory (RAM) comes in two varieties—static and dynamic. Static RAM (SRAM) is faster and significantly more expensive than Dynamic RAM (DRAM).

Static RAM: This RAM retains the data as long as power is provided to the memory chips. It does not need to be ‘refresher’ periodically SRAM is very fast but much more expensive than DRAM. SRAM is often used as cache memory due to its high speed.

Dynamic RAM: DRAM, unlike SRAM, must be continually ‘refreshed’ in order for it 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 because it is cheap and small.

Static RAM (SRAM): SRAM stands for Static Random Access Memory. The word ‘static’ indicated that the memory retains its contents as long as power remains applied. However, data is lost when the power gets down, which makes SRAM a volatile memory as opposed to read only memory. SRAM does not need to be re-written periodically. In fact, the only time the data on the memory is refreshed or changed is when a write command is

executed. SRAM uses a clock, which is built onto the memory module. This allows the module to be synchronized to the motherboard’s clock. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis. Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. Static RAM us used in specific application in computers where its strong points outweigh its weaknesses, thus they are well suited for the level-1 and level-2 cache memory and not for the system memory, as cache memory needs to very fast and small.

DRAM: DRAM stands for Dynamic Random Access Memory. It is named so because it is very unstable. The data moves in and out of the memory repeatedly for as long as power is available. Electricity always has to be flowing through the chip to keep refreshing the memory. DRAM, depending on its type, has to be refreshed approximately every 64ms or 15.6 times per second. The reason for this lies in DRAM’s composition. All DRAMs are made up of memory cells. These cells are composed of one capacitor and one transistor. Capacitors by nature hold electrons. A capacitor that is full of electrons is considered ON or having the value of 1, and an empty capacitor is considered to be OFF, or having the value of 0. When the capacitor is charged, it powers the gate of the transistor. The gate will then allow electrons to move across the transistor. If the capacitor is not charged, then very few electrons will be able to traverse the transistor. However, the gate is not hundred percent efficient, so electrons slowly leaks out of the capacitor until it is discharged. As a result, the cell may lose its value, and therefore to prevent this from happening, the capacitor needs to be refreshed continuously, at least every 64ms. This puts DRAM in the class of volatile memory devices. The capacitor also has to be recharged whenever it is read from, because reading from it discharges the capacitor. If the capacitor is not refreshed, it will loose its electrical charge and the cell will have a value of 0. DRAM is slower, less expensive and covers less space on computer’s motherboard.

Video RAM (VRAM): VRAM is dual-ported version of DRAM formerly used in graphics adapters. It was designed to provide two access paths to the same memory address. VRAM can be compared to a house that has two doors, one on the front side and the other on the backside of the house. Information comes in from entrance at the same time, other information flows out to the exit. When the video controller reads the memory for information. It accesses an  address with one of the paths and when the CPU writes data to memory,  it access the address from other path. The unique feature of VRAM allows data to be read from and written to at the same time.

SDRAM: Synchronous dynamic random access memory enhances the system’s performance by utilizing the burst mode concept. With maximum transfer rate to L2 cache of 528 mbps, SDRAM stays on the row that contains the requested bit and moves quickly through the columns to read each bit as it moves on.

FPM DRAM: The pristine form of DRAM, Fast Page Mode Dynamic Random Access memory waits through the process of locating a bit of data by column and row and then reading the bit and then starts on the next bit. It has a maximum transfer rate to L2 cache of about 176 mbps.

EDO DRAM: Contrary to FPM DRAM, Extended data-out dynamic random access memory does not wait and as the address of the first bit is located it starts looking for the next bit. With maximum transfer rate to L2 cache of about 264 mbps, it is 5 percent faster than the FPM.

DDR SDRAM: Double data rate synchronous dynamic random access memory is somewhat similar to SDRAM but has higher bandwidth. It has a maximum transfer rate to L2cache of about 1064 mbps.

ROM

            Just as human brain has a tendency to recall events, which occurred in the past repeatedly, a computer system also needs special instructions every time during a login operation. This process is required because during login, main memory of the computer is empty due to its volatile property, so there must be some sort of instruction to be stored in the special chips, which enables the computer system to perform start operations and transfer the control to operating system.

Read Only Memory (ROM) is an integrated circuit programmed with data that holds instructions for starting up the computer. Data stored in ROM is non volatile and is not lost when powered off. These data cannot be changed or a special operation is needed to be performed to change it.

TYPES OF ROM:

MASKED ROM: The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kinds of ROMs are known as masked ROMs. The contents of such ROMs have to be specified before chip production, so the actual data could be used to arrange the transistors inside the chip.

PROM: Programmable read only memory (PROM) is a type of ROM. These chips are non volatile and cannot be purged to store something else once it has been used. Blank PROM chips can be coded with the help of a tool known as a programmer.

A charge towards the columns passes through the fuse in a cell to a grounded row and indicates a value of 1. Initially PROM chips are all 1s for all cells have a fuse. In order to change the value of a cell to 0, a programmer is used to send the current to the cell. The connection between the column and the row snaps as the higher voltages while passing through burns out the fuse.

EPROM: Erasable Programmable Read Only Memory can be erased with the help of ultraviolet light and rewritten many times. These chips are configured by the EPROM programmer, providing the voltage at the specified levels. The floating gate is linked to the row through the control gate. The cell has a value of 1 till the link remains established. A process known as Fowler-Nordheim tunneling is performed to change the value to zero. The tunneling changes the placement of electrons in the floating gate. An electrical charge of 10 to 13 volts is passed through the bitline which drains to a ground after entering the floating gate.

The electrical charge excites the electrons of the transistor at the floating gate and they are pushed through and trapped on the side of the flimsy oxide layer to give a negative charge. A call sensor monitors the level of the charge that passes through the threshold of floating gate. Is shows a value of 1 if the flow is more than fifty percent. On the other hand if the flow is below fifty percent, the value changes to 0. Blank EPROM chips have a value of 1 for each cell as the have all of the gates opened completely.

EEPROM: Electrically Erasable Programmable Read Only Memory chips are not required to remove to be erased or rewritten. These chips do not require to be erased altogether and specific portion of it can be easily altered. Additional dedicated equipment is also not required to change the content the EEPROM chips. These chips are erased and rewritten with the help of electric charge.

FLASH ROM: Flash Rom, also called flash BIOS or flash memory, is a type of constantly powered non-volatile memory that can be erased and re-programmed in blocks. It is a variation of EEPROM, which, unlike flash memory, is erased and rewritten at the byte level. Flash memory is often used to hold control code such as the BIOS in a personal computer. When BIOS needs to be changed or rewritten, the flash memory can be written in block sizes, making it easy to update.