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 drivesfor 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.


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