Posts Tagged ‘memory’

Segementing how a microprocessor manages more than 64K of memory

April 25, 2013

Segmenting is a memory management technique which microprocessors use to address more than 64kilobytes memory.In this example I will demonstrate how
Intel 8086 accomplishes addressing more than 65K memory.

A memory word is addressed by addressing by  adding a segment pointer and a word address as shown
in the picture.

The possibility of addressing one megabyte doesn’t necessary mean that the processor has a physical memory of 1MB.
You can make a program as if the processor had 1MB of memory and the
processor can fetch the segment that is being addressed from the computers hard drive.


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Writen by: Rikard Grossman-Nielsen
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Different kinds of roms and their applications.

March 31, 2012

There are four types of ROM: regular ROM, PROM, EPROM and EAROM.

ROM stands for read only memory. A ROM can’t be changed, which means that once it has been manufcatured the contents of it can’t be changed.
The fact thaht ROMs are permanent gives them a security over programs stored in regular memory which can be changed by powerloss, viruses,
hackers and accidental misstakes. A typical application of a ROM might be an industrial machine that reapets the same sequence over and over
to manufcature auto parts. Another example might be the boot up software for a regular PC, which performs the same task each time the power is turned on.
ROMs however also have som disadvantages. One is that it takes a long time to manufacture a rom, which means that you might loose revenues due to cost delays. Abnother one is that the unit price of a ROM is quite high which means that you have to order at least something like thousand units, this fact makes the intial investment cost high. Once the ROM has been manfactured and in error has been discovered in the information store you can’t change the contents, this means making misstakes are very expensive.

To solve the problems of rom three other types of user programmable ROMs has been developed: PROM, EPROM and EAROM.
PROM stands for Programmable ROM. Information is program into a prom by burning of different electrical connection points. Depending of what’s is burned you code either ao one or a zero into the rom. PROMs are relatively cheap to manfucaturer which means that misstakes are not as expensive. PROMs are often used for testing phases where
you need to try out many different programs before finding the right one. When the right program has been found an eletrically compatbile ROM can take the place of PROM giving the final product a greater relability. However if you need to make frequent changes you have to discared one PROM each time which will be a vaste of money in the long run. Therefore there are ROMs which can be erased and reprogrammed by the user.

EPROM stands from electrically programmable ROM. You can erasure the contents of a an EPROM by submitting it to ultraviolet let for five to ten minutes. The circuits are however relatively expensive per unit and aren’t pin compatbile with PROMS and ROMS, this means that you have to change a design of the circuit board when you wish to make the final product which often uses ROMs. EPROMs loose their charge of many years pass by which means that if you have applications which will run for many years you should choose ROMs or PROMs instead.

Finally there are EAROM which stands for Electrically alterabkle ROM. These ROMS can be both written and read. However writing is very slow so it can be considered as a read often memory. EARoms is an expensive and complex technolog and is therefore most oftenly used for industrial and military applications.
To sum it up the kind of ROM you will use depends on the application type.

Circuit diagram of a 16K static ROM (83164).

Intel 2164 a 64kbit dynamic ROM, with an access time of 150ms.

Deletion of an Intel 2716 16K EPROM. The deletion is performed by directing UV light onto the window of the circuit.

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A lesson on the difference beetween static and dynamic ram

March 13, 2012

The static ram stores information as long as the power is turned on. This type of memory doesn’t need timing since it’s asynchronous.

Dynamic ram on the other needs to be recharged at regular intervals to maintain it’s information. The recharging is timed by an external timing crystal. This type typically needs to be refreshed
every few milliseconds. The refresh is done by reading the information and thereafter writing it back so that memory cell is kept up to date. The refresh is typically done several rows or columns at a time.

Dynamic ram is more cost effective to produce since it has a greater component density than static ram. On the other hand static ram is more reliable since it doesn’t need to refreshed at regular intervals.

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