Why Memory Cards, Pen Drives comes in the form of 1GB,2GB,4GB not 3GB ?

As we all know computer use Binary Language to save and processing the information given by user. Binary Language is a language of two digits which is 0 and 1.

Like


2^0 = 1GB
2^1=  2GB
2^2=  4GB
2^3=  8GB
2^4=  16GB  ................... and so on to 2^10=1024

Thats why Memory cards and Pendrives comes in the form of 1,2,4,8,16,32,........,


To connect the CPU to the memory it needs fewer address lines if you specify the memory location as a binary address rather than as a row column select.
So the processor specifies which memory cell it wants to use by giving a binary address.

mem
Address lines are converted to row/column selects

Notice that at the moment we are looking at a single memory chip and this arrangement can only store a single bit. If you want to store a byte you need eight such chips, one for each bit, and eight data input and eight data output lines.
The eight data lines are grouped together into a data input bus and a data output “bus” – a bus is just a group of wires. Early computers really did have separate buses for input and output but today’s machines have a single unified data bus that can be used for either input or output.
If you want more storage than a bank of eight chips can provide then you have to add another bank of eight chips and some additional address decoding logic to select the correct bank. The address lines that come from the processor are generally referred to as an address bus and now we have the fundamental architecture of a simple but modern computer.

bus
The data and address bus

This is all very straightforward but you can see how it gets increasingly complicated in practice. However the use of a binary address explains why for example, 1KByte is 1024 bytes and not 1000 like any sane K should be.
To see why this is natural first ask yourself how many memory locations can be addressed using a single address line? Answer – 2. With two address lines you can address 4. With three you can address 8 and if you carry on in this way you discover the following natural memory sizes:

Address
lines    Locations
 1         2
 2         4
 3         8
 4        16
 5        32
 6        64
 7       128
 8       256
 9       512
10      1024


Notice that with ten address lines you can address the magic number of 1024 memory locations, and this is the closest you can get to 1000 using this system – hence 1K = 1024.
If you follow the same reasoning, adding another ten address lines allows you to address 1K x 1K memory locations and this we call 1MByte, which is 1024x1024 or 1048576 bytes. An odd number unless you realise that it is the nearest power of 2 to one million.(source=howstuffworks)
 

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