16-01-2013, 12:45 PM
Project Report ON COMPUTER MEMORY
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INTRODUCTION
From early punched cards to high-tech silicon chips, take a walk down the computer’s memory lane and discover how RAM was born.
When the iconic Manchester Baby computer first ran in 1948, it was revolutionary because it stored its programs in the form of RAM. It sounds obvious now, but if you wanted to run a fresh program on a computer at the time, weeks of rewiring were usually required to make it possible. Baby changed all that. Now you could enter and run new programs in a matter of hours.
Baby’s amazing ability was down to an ingenious storage device called the Williams Tube. The memory worked on the principle that when a beam of electrons was fired down a vacuum tube and hit a phosphorescent coating at the other end, small static charges built up at the points where the beam hit the phosphor. A set of pickup plates in front of the coating then detected the charges. However, because the charges faded quickly, a refresh circuit needed to read which bits were set and use the electron beam to refresh them every few milliseconds. Williams Tubes could store around 1Kb and, although they sound cumbersome, have a modern parallel in today’s DRAM chips. These work by storing tiny electrical charges in microscopic capacitors that are topped up every few microseconds.
TYPES OF MEMORY
Primary Memory
Primary memory is the memory that can be directly accessed by the CPU which constantly interacts with it, retrieves data stored therein, goes through instructions and execute them as per the requirement. All the information, data and application are loaded there in uniform manner. Earlier William tubes, delay lines or rotating magnetic drums were used as primary storage which were later replaced by magnetic core memory. Solid-state silicon chip technology revolutionized the electronic memory and paved the way for Random Access Memory (RAM). RAM is volatile (temporary) but fast form of memory.
Apart from the main large capacity Random Access Memory (RAM), there are two sub-layers of the primary memory.
Processor registers within the processor, which are one of the fastest forms of data storage, contain a word of data (usually 32 or 64 bits). The CPU instructs and helps the Arithmetic and logic unit to perform a number of calculations on this data.
Processor cache, which is meant for enhancing the performance of the computer, links the fast registers to the slower main memory. Cache memory loads the duplicated information that is used most actively. It is much faster than the main memory but relatively can store limited data. It is also much slower but much larger than the processor registers. Cache setup is further split into different levels with smallest and fastest primary cache and relatively larger but slower secondary cache.
RAM
The most familiar form of system memory, Random Access Memory (RAM) derives its name from the fact that any of its memory cells can be accessed directly if you are aware of the row and column that intersect at that cell. The columns are referred to as bitlines while the rows are referred to as word lines. The intersection of a word line and bitliine is the address of the memory cell onto a silicon wafer.
In case of Serial Access Memory (SAM), the opposite of RAM, the data is stored as a series of memory cells and can be accessed sequentially whereas data stored in RAM can be accessed in any order.
ROM
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.
ROM chips also comprise of columns and rows but it is different from RAM in terms of intersection of these. These chips use diodes instead of transistors to connect the lines if the value is 1 whereas if the value is 0 the lines are not connected.
A Programmable read only memory chip.
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 bit line 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 change 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 removed 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 are also not required to change the content the EEPROM chips. These chips are erased and rewritten with the help of electric charge.