A communication link must be established between the cooperating processes before messages can be sent.
The level of charge on the memory cell capacitor determines whether that particular bit is a logical "1" or "0" - the presence of charge in the capacitor indicates a logic "1" and the absence of charge indicates a logical "0".
The basic dynamic RAM memory cell has the format that is shown below.
It is very simple and as a result it can be densely packed on a silicon chip and this makes it very cheap. The basic memory cell shown would be one of many thousands or millions of such cells in a complete memory chip.
Memories may have capacities of Mbit and more. To improve the write or read capabilities and speed, the overall dynamic RAM memory may be split into sub-arrays. The presence of multiple sub-arrays shortens the word and bit lines and this reduces the time to access the individual cells.
The word lines control the gates of the transfer lines, while the bit bines are connected to the FET channel and are ultimately connected to the sense amplifiers.
There are two ways in which the bit lines can be organised: It is possible to consider a pair of adjacent bit lines as a single bit line folded in half with the connection on the fold broken and connected to a shared sense amplifier. This format provides additional noise immunity, but at the expense of being less compact.
In this configuration the sense lines are placed between two sub-arrays, thereby connecting each sense amplifier to one bit line in each array.
This offers a more compact solution than the folded bit lines, but at the expense of noise immunity. As voltages on the charge capacitors are small, noise immunity is a key issue.
There are several lines that are used in the read and write operations: This line selects the column to be addressed. It enables a column to be selected from the open row for read or write operations. Low enables the write action, while high enables a read action.
Dynamic RAM refresh One of the problems with this arrangement is that the capacitors do not hold their charge indefinitely as there is some leakage across the capacitor. It would not be acceptable for the memory to lose its data, and to overcome this problem the data is refreshed periodically.
The data is sensed and written and this then ensures that any leakage is overcome, and the data is re-instated. One of the key elements of DRAM memory is the fact that the data is refreshed periodically to overcome the fact that charge on the storage capacitor leaks away and the data would disappear after a short while.
Typically manufacturers specify that each row should be refreshed every 64 ms. There are a number of ways in which the refresh activity can be accomplished. Some processor systems refresh every row together once every 64 ms. Other systems refresh one row at a time, but this has the disadvantage that for large memories the refresh rate becomes very fast.
Some other systems especially real time systems where speed is of the essence adopt an approach whereby a portion of the semiconductor memory at a time based on an external timer that governs the operation of the rest of the system.
In this way it does not interfere with the operation of the system. Whatever method is use, there is a necessity for a counter to be able to track the next row in the DRAM memory is to be refreshed. Some DRAM chips include a counter, otherwise it is necessary to include an additional counter for this purpose.
It may appear that the refresh circuitry required for DRAM memory would over complicate the overall memory circuit making it more expensive. However it is found that DRAM the additional circuitry is not a major concern if it can be integrated into the memory chip itself.
This depends upon the ratio of the capacitance of the storage capacitor within the DRAM memory to the capacitance of the Word or Bit line on which the charge is dumped when the cell is accessed. As the bit density per chip is increased, the ratio is degraded since the cell area is decreased as more cells are added on the bit line.
It is for this reason that it is important to store as high a voltage on the cell capacitor, and also to increase the capacitance of the DRAM storage capacitor for a given areas as much as possible. This is a very important consideration because sensing the small charge on the memory cell capacitor is one of the most challenging areas of the DRAM memory chip design.
As a result of this some elaborate circuit designs have been incorporated onto DRAM memory chips.EEPROM occupies more die area than flash memory for the same capacity, because each cell usually needs a read, a write, and an erase transistor, while flash memory erase circuits are shared by large blocks of cells (often ×8).
A memory unit stores binary information in groups of bits called words. Data input lines provide the information to be stored into the memory, Data output lines carry the information out from the memory.
The control lines Read and write specifies the direction of transfer of data. Basically, in the. A read-modify-write bus cycle is nothing but a read cycle followed by a write cycle to the same address. This bus cycle is important when designing multiprocessor systems.
Since the read and write operations are performed in a single bus cycle, the processor cannot loose the bus when these operations are being carried out.
A read memory barrier orders only the memory read operations and a write memory barrier orders only the memory write operations.
These instructions also ensure that the compiler disables any optimizations that could reorder memory operations across the barriers. As of , V-NAND flash architecture allows read and write operations twice as fast as conventional NAND and can last up to 10 times as long, while consuming 50 percent less power.
The method used to read NAND flash memory can cause nearby cells in the same memory block to change over time (become programmed). This is known as read . I/O Systems References: Abraham Silberschatz, Greg Gagne, and Peter Baer Galvin, "Operating System Concepts, Eighth Edition ", Chapter 13 and communications occur by reading and writing directly to/from those memory areas.
For example, the completion of a disk read operation involves two interrupts.