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Terms for computer junkies
PUMP up your computer-related vocabulary! Load your brains with these frequenty used i-terms.
Address space: The set of all legal addresses in memory for a given application. The address space represents the amount of memory available to a program. Interestingly, the address space can be larger than physical memory through a technique called virtual memory.
MMU: Short for memory management unit, the hardware component that manages virtual memory systems. Typically, the MMU is part of the CPU, though in some designs it is a separate chip. The MMU includes a small amount of memory that holds a table matching virtual addresses to physical addresses. This table is called the Translation Look-aside Buffer (TLB). All requests for data are sent to the MMU, which determines whether the data is in RAM or needs to be fetched from the mass storage device. If the data is not in memory, the MMU issues a page fault interrupt.
Access time: The time a program or device takes to locate a single piece of information and make it available to the computer for processing. DRAM (dynamic random access memory) chips for personal computers have access times of 50 to 150 nanoseconds (billionths of a second). Static RAM (SRAM) has access times as low as 10 nanoseconds. Ideally, the access time of memory should be fast enough to keep up with the CPU. If not, the CPU will waste a certain number of clock cycles, which makes it slower.
Note, however, that reported access times can be misleading because most memory chips, especially DRAM chips, require a pause between back-to-back accesses. This is one reason why SRAM is so much faster than DRAM, even when the reported access times are equivalent; SRAM doesn’t require any refreshes, so there is no pause between back-to-back accesses. A more important measurement of a chip’s speed, therefore, is its cycle time, which measures how quickly two back-to-back accesses can be made.
Access time is also frequently used to describe the speed of disk drives. Disk access times are measured in milliseconds (thousandths of a second), often abbreviated as ms. Fast hard disk drives for personal computers boast access times of about 9 to 15 milliseconds. Note that this is about 200 times slower than average DRAM. The access time for disk drives includes the time it actually takes for the read/write head to locate a sector on the disk (called the seek time). This is an average time since it depends on how far away the head is from the desired data.
Burst mode: A data transmission mode in which data is sent faster than normal. There are a number of techniques for implementing burst modes. In a data bus, for example, a burst mode is usually implemented by allowing a device to seize control of the bus and not permitting other devices to interrupt. In RAM, burst modes are implemented by automatically fetching the next memory contents before they are requested. This is essentially the same technique used by disk caches. The one characteristic that all burst modes have in common is that they are temporary and unsustainable. They allow faster data transfer rates than normal, but only for a limited period of time and only under special conditions.
Disk cache: A portion of RAM used to speed up access to data on a disk. The RAM can be part of the disk drive itself (sometimes called a hard disk cache or buffer) or it can be general-purpose RAM in the computer that is reserved for use by the disk drive (sometimes called a soft disk cache). Hard disk caches are more effective, but they are also much more expensive, and therefore smaller. Nearly all modern disk drives include a small amount of internal cache.
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