Access time:-
Access time is the time from the start of one storage device access to the time when the next access can be started. Access time consists of latency (the overhead of getting to the right place on the device and preparing to access it) and transfer time.
The term is applied to both random access memory (RAM) access and to hard disk and CD-ROM access. For RAM access, IBM prefers the term cycle time. However, the use of access time for RAM access is common. Access time to RAM is usually measured in nanoseconds. Access time to a hard disk or CD-ROM is usually measured in milliseconds.
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.
Its depends on given below
These are the 3 parameters on which the disk access time is dependent.Disk access time is the interval b/n the time a computer makes a request for transfer of data from a disk system to the primary storage and the time this operation is completed.
Seek Time - As soon as the read/write command is received by the disk unit,the read/write heads are 1st positioned on to the specified track/cylinder number.The time required to position the read/write head over the desired track is called seek time.
Latency - Once the heads are positioned on the desired track,the head on the specified surface is activated.Since the disk is continuously rotating,the head should wait for the specified sector to come under it.This rotational waiting time required to spin the desired sector under the head is called latency(Rotational Delay Time).
Transfer Rate - Transfer rate refers to the rate at which data are read from or written to the disk.
Suppose,rotational speed = 3600 rpm ,no. of sectors/track = 125 ,no.of bytes/sector = 512.
Amount of data transferred in one full revolution of the disk = 125 * 512 = 64,000 bytes = 64K bytes.
Transfer rate of the disk system = 64,000 * 3600/60 bytes/second = 38,40,000 bytes/second = 3/8 MB/second.
Access time is the time from the start of one storage device access to the time when the next access can be started. Access time consists of latency (the overhead of getting to the right place on the device and preparing to access it) and transfer time.
The term is applied to both random access memory (RAM) access and to hard disk and CD-ROM access. For RAM access, IBM prefers the term cycle time. However, the use of access time for RAM access is common. Access time to RAM is usually measured in nanoseconds. Access time to a hard disk or CD-ROM is usually measured in milliseconds.
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.
Its depends on given below
These are the 3 parameters on which the disk access time is dependent.Disk access time is the interval b/n the time a computer makes a request for transfer of data from a disk system to the primary storage and the time this operation is completed.
Seek Time - As soon as the read/write command is received by the disk unit,the read/write heads are 1st positioned on to the specified track/cylinder number.The time required to position the read/write head over the desired track is called seek time.
Latency - Once the heads are positioned on the desired track,the head on the specified surface is activated.Since the disk is continuously rotating,the head should wait for the specified sector to come under it.This rotational waiting time required to spin the desired sector under the head is called latency(Rotational Delay Time).
Transfer Rate - Transfer rate refers to the rate at which data are read from or written to the disk.
Suppose,rotational speed = 3600 rpm ,no. of sectors/track = 125 ,no.of bytes/sector = 512.
Amount of data transferred in one full revolution of the disk = 125 * 512 = 64,000 bytes = 64K bytes.
Transfer rate of the disk system = 64,000 * 3600/60 bytes/second = 38,40,000 bytes/second = 3/8 MB/second.
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