Memory holds the information that the CPU needs to process. The capacity of memory chips and modules are described in megabits (Mb) and megabytes (MB). A module consists of a group of chips. If you add together the capacities of all the chips on the module, you get the total capacity of the module.
|
COMPONENT |
CAPACITY EXPRESSION |
CAPACITY UNITS |
EXAMPLE |
|
Chips |
Chip Density |
Mbit (megabits) |
64Mbit |
|
Memory Modules |
Module Capacity |
MB (megabytes) |
64MB |
CHIP DENSITY
Each memory chip is a matrix of tiny cells. Each cell
holds one bit of information. Memory chips are often described by how much
information they could hold. We call this chip density. You may have encountered
examples of chip densities, such as "64Mbit SDRAM" or "8M by
8". A 64Mbit chip has 64 million cells and is capable of holding 64 million
bits of data. The expression "8M by 8" describes one kind of 64Mbit
chip in more detail.
In the memory industry, DRAM chip densities are often
described by their cell organization. The first number in the expression indicates
the depth of the chip (in locations) and the second number indicates the width
of the chip (in bits). Multiplying the depth by the width, gives the density
of the chip.
MODULE CAPACITY
It's easy to calculate the capacity of a memory module
the capacities of the chips on it is known. If there are eight 64Mbit chips,
it's a 512Mbit module. However, because the capacity of a module is described
in megabytes, not megabits, it has to be converted from bits to bytes. To
do this, divide the number of bits by 8. In the case of the 512Mbit module:
512 Mb
------------------- = 64 MB
8 bits per byte
NEXT STOP: Memory: Survey of Available Memory Devices
