Memory access i s When memory is read or written  , a specific procedure is used to control each access to memory, which consists of having the memory controller generate the correct signals to specify which memory location needs to be accessed, and then having the data show up on the data bus to be read by the processor or whatever other device requested it.

Memory controller is the part of the system that controls the memory byt generating the necessary signals to control the reading and writing of information from and to the memory, and interfaces the memory with the other major parts of the system. It can be a separate chip or integrated into another chip, such as on the  microprocessor.

Memory controllers contain the logic necessary to read and write dynamic random access memory (DRAM), and to “refresh” the DRAM by sending current through the entire device. Without constant refreshes, DRAM will lose the data written to it as the capacitors leak their charge within a fraction of a second.

DRAM VS RAM :

DRAM is dynamic in that, unlike static RAM , it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds.  RAM does not need refreshing because it operates on the principle of moving current that is switched in one of two directions rather than a storage cell that holds a charge in place.  RAM is generally used for cache memory, which can be accessed more quickly than DRAM.

Only one transistor and a capacitor are required per bit, compared to four transistors in RAM. This allows DRAM to reach very high density. Unlike flash memory, it is volatile memory , since it loses its data when the power is off.

in brief :

Advantages: it’s compatible with just about everything. Disadvantates: it’s the slowest of the memory types currently available and because it isn’t in demand

computer  bus is a subsystem that transfers data between computer components inside a computer or between computers. Unlike a point-to-point connection, a bus can logically connect several peripherals over the same set of wires. Each bus defines its set of connectors to physically plug devices, cards or cables together.

A bus is characterised by the amount of information that can be transmitted at once. This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously. A 32-wire ribbon cable can transmit 32 bits in parallel. The term “width” is used to refer to the number of bits that a bus can transmit at once.

Additionally, the bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second. Each time that data is sent or received is called a cycle.

it may be external or internal

internal  as  PCI

external as USB