A computer’s main memory is organized in manageable units called cells, with a typical cell size being eight bits (eight bits = one byte). Small computers used in such household devices as microwave ovens may have main memories consisting of only a few hundred cells, whereas large computers may have billions of cells in their main memories. Although there is no left or right within a computer, we normally envision he bits within a memory cell as being arranged in a row. The left end of this row is called the high-order end, and the right end is called the low-order end. The last bit at the high-order end is called either the high-order bit or the most significant bit in reference to the fact that if the contents of the cell were interpreted as representing a numeric value, this bit would be the most significant digit in the number. Similarly, the bit at the right end is referred to as the low-order bit or the least significant bit.
To identify individual cells in a computer’s main memory, each cell is assigned a unique “name”, called its address. The system is analogous to the technique of identifying houses in a city by addresses. In the case of memory cells however, the addresses used are entirely numeric. To be more precise, we envision all the cells being placed in a single row and numbered in this order starting with the value zero. Such an addressing system not only gives us a way of uniquely identifying each cell but also associates order to the cells giving us phrases such as “the next cell” or “the previous cell”.
An important consequence of assigning an order to both the cells in main meory and the bits within each cell is that the entire collection of bits within a computer’s main memory is essentially ordered in one long row. Pieces of this long row can therefore be used to store bit patterns that may be longer than the length of a single cell. In particular, we can still store a string of 16 bits merely by using to consecutive memory cells.
To complete the main memory of a computer, the circuitry that actually holds the bits is combined with the circuitry to allow other circuit to store and retrieve data from the memory cells. In this way, other circuits can get data from the memory cells. In this way, other circuits can get data from the memory by electronically asking for the contents of a certain address (read operation), or they can record information in the memory by requesting that a certain bit pattern be placed in the cell at a particular address (write operation).
Because a computer’s main memory is organized as individual, addressable cells, the cells can be accessed independently as required. To reflect the ability to cells, the cells can be accessed independently as required. To reflect the ability to access cells in any order, a computer’s main memory is often called random access memory (RAM). This random access feature of main memory is in stark contrast to the mass storage systems that will be discussed on the next article, in which long strings of bits are manipulated as amalgamated blocks.
Memory cells arranged by address
Although flip-flops that introduced earlier can be used as a means of storing bits, the RAM in most modern computers is constructed using other technologies that provide greater miniaturization and faster response time. Many of these technologies store bits as tiny electric charges that dissipate quickly. Thus these devices require additional circuitry, known as refresh circuit, that repeatedly replenishes the charges many times a second. In recognition of this volatility, computer memory must be constructed from such technology called dynamic memory, leading to term DRAM (Dynamic RAM). Or, at times the term SDRAM (Synchronous DRAM) is used in reference to DRAM that applies additional technique to decrease the time needed to retrieve the contents from its memory cells.
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