Wait State
A wait state is a processing cycle of a microprocessor during which it briefly waits for an operation to occur and complete before resuming activity. A program or process in a wait state is inactive for the duration of the wait state. For example, the central processing unit (CPU) working with a word processing program might communicate with main memory (sometimes called random-access memory or RAM) for a particular instruction, and then go into a wait state until it receives a message back from memory. A wait state may refer to a "variable" length of time that a program has to wait before it can be processed, or to a "fixed" duration of time, such as one or more machine cycles. Wait state is also called a "time-out" period during which the CPU (often interchangeably called a microprocessor) or bus (the collection of wires through which data is transmitted from one part of a computer to another) remains idle. When memory is too slow to respond to the CPU's request for it, wait states are introduced until the memory can catch up. Normally the CPU works at a faster clock speed (usually expressed in megahertz (MHz) or millions of cycles per second) than RAM. So it may need to sit idle during some clock cycles (the oscillation speed of the computer's internal clock that determines execution times of instructions) in order for RAM to catch up. When this happens the CPU is automatically set to a wait state for one or more clock cycles so that it can synchronize with RAM speed. A delay of one or more clock cycles added to the execution time of a microprocessor's instruction allows it to communicate with slower external devices (such as printers and scanners). In addition, wait states are sometimes required because different components function at different clock cycles.
Likewise, buses sometimes require wait states if expansion boards (a printed circuit board that is inserted into a computer for added capabilities, such as a sound card) run slower than the bus. While one wait state is not perceptible, the cumulative effect of many wait states is to slow system performance.
An example of how a wait state affects computers is seen with Dynamic RAM (DRAM), which is a form of semiconductor RAM. DRAMs store information in integrated circuits that contain capacitors. Because capacitors lose their charge over time, DRAM boards must include the ability to repeatedly "refresh" (recharge) the RAM chips. While a DRAM is being refreshed, it cannot be read by the microprocessor. If the microprocessor must read the RAM while it is being refreshed, one or more wait states occur. Because their circuitry is relatively simple (a DRAM chip can hold approximately four times as much data as a comparable static RAM chip) DRAMs are more commonly used than static RAMs, even though they are slower. But there is a trade-off with using more DRAM memory, because wait state times are increased (versus if static RAM was used instead).
On the opposite side of a wait state is a zero wait state. A zero wait state system refers to high-speed memory that does not wait for machine cycles to respond before transferring data. In a zero wait state the microprocessor runs at the maximum speed without any "time-outs" to compensate for slower memory. Zero wait states are desirable because the more a microprocessor spends in wait states, the slower its processing performance. Wait states can be avoided, and moved into zero wait states, by using a variety of techniques, including burst mode, page-mode (RAM) memory, interleaved memory, and CPU cache.
This is the complete article, containing 598 words
(approx. 2 pages at 300 words per page).
