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

Basic Input/Output System (BIOS) designed to be the first code run by a PC when powered on. The initial function of the BIOS is to identify, test, and initialize system devices such as the video display card, hard disk, and floppy disk and other hardware. This is to prepare the machine into a known state, so that software stored on compatible media can be loaded, executed, and given control of the PC.This process is known as booting, or booting up, which is short for bootstrapping.

The first thing the BIOS does is check the information stored in a tiny (64 bytes) amount of RAM located on a complementary metal oxide semiconductor (CMOS) chip. The CMOS Setup provides detailed information particular to your system and can be altered as your system changes. The BIOS uses this information to modify or supplement its default programming as needed.

After checking the CMOS Setup and loading the interrupt handlers, the BIOS determines whether the video card is operational.

Next, the BIOS checks to see if this is a cold boot or a reboot.

If it is a cold boot, the BIOS verifies RAM by performing a read/write test of each memory address. It checks the PS/2 ports or USB ports for a keyboard and a mouse. It looks for a peripheral component interconnect (PCI) bus and, if it finds one, checks all the PCI cards. If the BIOS finds any errors during the POST, it will notify you by a series of beeps or a text message displayed on the screen. An error at this point is almost always a hardware problem.

BIOS then displays some details about your system.

and system start begin

Random-access memory (usually known by its acronym, RAM) , is the best known form of computer memory. RAM is considered “random access” because you can access any memory cell directly if you know the row and column that intersect at that cell.

The word RAM is mostly associated with volatile types of memory where the information is lost after the power is switched off.

Some Types of ram :

• SRAM: Static random access memory uses multiple transistors It is used primarily for cache.
• DRAM: Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing.
• FPM DRAM: Fast page mode dynamic random access memory waits through the entire process of locating a bit of data by column and row and then reading the bit before it starts on the next bit.
• EDO DRAM: Extended data-out dynamic random access memory does not wait for all of the processing of the first bit before continuing to the next one. As soon as the address of the first bit is located, EDO DRAM begins looking for the next bit. It is about five percent faster than FPM.
• SDRAM: Synchronous dynamic random access memory takes advantage of the burst mode concept to greatly improve performance. It does this by staying on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes. The idea is that most of the time the data needed by the CPU will be in sequence.
• DDR SDRAM: Double data rate synchronous dynamic RAM is just like SDRAM except that is has higher bandwidth, meaning greater speed.
• RDRAM: Rambus dynamic random access memory is a radical departure from the previous DRAM architecture. What makes RDRAM so different is its use of a special high-speed data bus called the Rambus channel. RDRAM memory chips work in parallel to achieve a data rate of 800 MHz, or 1,600 MBps. Since they operate at such high speeds, they generate much more heat than other types of chips. To help dissipate the excess heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer.there are also SO-RIMMs, designed for notebook computers.
• VRAM: VideoRAM, also known as multiport dynamic random access memory (MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators. VRAM is located on the graphics card and comes in a variety of formats, many of which are proprietary. The amount of VRAM is a determining factor in the resolution and color depth of the display. VRAM is also used to hold graphics-specific information such as 3-D geometry data and texture maps. True multiport VRAM tends to be expensive, so today, many graphics cards use SGRAM (synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is cheaper.