国外论文网站 System Organization And Architecture English Language Essay

EEPROM (Electrically Erasable Programmable ROM)

OK, EPROMs are cool but they sure have their disadvantages. An EPROM may be usable for programming for many times but to reprogram it or to edit an existing program in an EPROM, we have to first remove or obliterate the entire chip (i.e. to reset it completely) and to burn the whole program again. If any particular program takes 5 to 6 times to compile successfully (i.e. as we desire it to work), we might have to erase and burn the complete program this so many times. Now consider the timing that it takes to obliterate the whole chip that is almost 20 to 30 minutes (at max.), one must be crazy enough to wait again and again until finally it’s good to go.

Another problem may occur when you just want a particular part of the program to edit, there is no way you can do it without erasing the whole chip and to burn it completely again with the modified part (now this must suck). Another drawback is the use of different equipment (plotter) to burn EPROM and another equipment (UV light container) to erase it.

The above told drawbacks (not all but mostly) were pretty neatly addressed in the EEPROM (e-e-p-ROM or double E-P-ROM) chip which is an Electrically Erasable Programmable ROM chip with everything almost like its predecessor EPROM. It also has one drawback i.e. it was slow on erasing but for other problems like editing just a single part of a program and need another instrument for the erasing was not required for these kinds of chips. BTW there are two kinds of EEPROM (as I must write it for no reason, because the other type is obsolete now and no one cares);

Serial EEPROM

Parallel EEPROM

Well, what I will be discussing in my paper here is Serial EEPROM, Parallel EEPROM are now vanishing from the surface of earth due to the existence of so many legs (like a centipede of course, the larger seems the uglier). But as serial circuits have some of their own notorious cons like consumption of more voltage and dissipation of heat which in addition makes it slower to work, is seriously a pain, still it wins because it is not ugly looking chip like parallel EEPROM and its small size makes it cuter to install in a large circuitry (design wins over performance).

So how it works, well quite simple, just provide with more electricity in terms of voltages to the oxide layer that may generate electric flux and since we already have field effect transistors in place (Floating gate and Control gate are electric field effect transistors), they will simply nullify the negative ion on the oxide lay and the two gates shall merged which will eventually create a binary 1. Since each transistor is a field effect transistor in the EEPROM, it is a bit wide operational transistor. Programmers for EEPROM carry an inbox utility to perform a bit by bit erasure for EEPROM chip. This is also very obvious that like EPROM they are not very dense due to carrying two transistor, an oxide layer and a capacitor for containing only 1 bit and of course they are still slow in erasing mechanism because they are only bit wide operational but the good thing is we can program it, edit it or do anything we like with it in real time. They also don’t need any quartz window attached over their tops which is the only major difference between EPROM and EEPROM.

EEPROMs are the predecessor of a new kind of ROM which almost addresses all the possible short-comings of the PROM, EPROM and EEPROM.

FLASH Memory

The most updated form of the ROM today is in shape of flash memory. Flash memory as it predecessors EPROM and double-EPROM are manufactured using the same fundamental architecture of Floating Gate and Tunneling. It possesses all the same difference from EPROM like EEPROM does, the only feature that it possesses other EEPROM is that it can erase or obliterate the data within it in chunks and sectors. So unlike a bit wide operation, it is a chunk wide operational chip. The size of the chip may vary as how it is formatted to keep the data.

Since it can erase data in chunks and not in bits, lesser circuitry is required to manufacture a flash memory which makes it denser than EEPROM and less expensive than EEPROM. This feature also gives it a head above than HDD which contains a spinning media and a larger size than a flash memory but still the cost of flash memory per bit is much higher than a HDD which uses magnetic disk and electric flux to contain data but not any logic gates. The cost of flash memory is getting cheaper each day which makes it available in terms of a technology called flash based Solid State Drive that has started replacing the current HDD market by a good factor. Apart from that, Flash memory is slower than all kind of ROMs and RAMs due to lesser transistors installed and more populated for each bit.

EEPROM are essentially a NOR based technology which contains 2 transistor to hold one bit which makes it less denser in serial circuit but flash memory can be manufactured using either NAND or NOR based technology. When a NOR based technology is used to manufactured a flash memory, in a serial circuit, only a single transistor is required to hold 1 bit which eventually gives it an extra level of addressing and makes it denser in terms of capacity but slows it down too when it comes to writing. Erasing on the contrary is much faster since all it has to do is to reset a whole chuck of bits or a complete memory using the built-in Wiring within the chip circuit.

The most common kind of a flash memory available today is a USB Flash drive, a Smart Drive (Solid State Floppy Disk Card) in mobile phones and cameras, SSDs and of course the BIOS chip in our PCs.

NOR Based Flash Memory architecture, as we can see that only on transistor exists for each Floating gate per memory cell and one Control gate per row which is sufficient for the whole column it is attached with. The whole row will decide how big a complete chuck of data it can contain. To erase the data, simply the bit line is “flashed” which ultimately gives the complete chuck a default 1.File:Nand flash structure.svg