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Encyclopedia > RAM latency

RAM Latency is the amount of wait time that a computer experiences when trying to access data in its RAM. RAM latency is measured in front side bus clock cycles. Due to the fact that a modern CPU is much faster than RAM, when accessing memory the CPU has to wait for the memory access to complete before it can make further progress. RAM latency is a significant bottleneck for system performance. A computer is a machine capable of undergoing complex calculations. ... Data is the plural of datum. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Front Side Bus (FSB) is the term used to describe the CPU data bus. ... In synchronous digital electronics, such as most computers, a clock signal is a signal used to coordinate the actions of two or more circuits. ... Intel 80486DX2 microprocessor in a ceramic PGA package A central processing unit (CPU), or sometimes simply processor, is the component in a digital computer that interprets instructions and processes data contained in software. ... A bottleneck is literally the neck of a glass or pottery bottle. ...

Contents


RAM access

RAM is notionally organized into a grid like pattern, with "rows", and "columns". The data stored in RAM comes in blocks, defined by the coordinates of the row and column of the specific information. RAM access has four main measurements (quantified in FSB clock cycles) important in defining the RAM latency in a given computer: Front Side Bus (FSB) is the term used to describe the CPU data bus. ...


(the 't' prefixes are for 'time')

  • tCAS - the number of clock cycles needed to access a certain column of Data in RAM. CAS Latency, or simply CAS, is known as Column Address Strobe Latency, sometimes refered to as tCL.
  • tRCD - the number of Clock cycles needed between RAS and CAS. It is the time required between the computer defining the row and column of the given memory block and the actual read or write to that location. Stands for Row address to Column address Delay.
  • tRP - the number of clock cycles needed to terminate access an open row of memory, and open access to the next row. Stands for Row precharge time.
  • tRAS - the miniumum number of clock cycles needed to access certain row of data in RAM between the data request and the precharge command. Known as Active to Precharge Delay.

The steps for the CPU to access data in RAM: CAS is an acronym for column address strobe, or sometimes column address select, both referring to the column of the physical memory location in an array of capacitors (a grid comprised of columns and rows) used in dynamic random access memory modules. ...

  1. defines which row the requested data is in (RAS)
  2. gives the RAM time to access requested row (RCD)
  3. defines the column of requested data (CAS)
  4. waits for the RAM to send requested data to the CPU
  5. precharges the row to recycle it for subsequent use (RP)

Measurements

As with almost all latency issues, the lower, the better. When picking out RAM to buy, these four measurements are only important in high performance computers. RAM speeds are given by the four numbers above, in the format "tCAS-tRCD-tRP-tRAS". So, for example, latency values given as 2.5-3-3-5 would indicate tCAS=2.5, tRCD=3, tRP=3, tRAS=5. (Note that .5 values of latency (such as 2.5) are only possible in Double data rate RAM, where two parts of each clock cycle are used) Latency is the time a message takes to traverse a system. ... In computing, a computer bus operating with double data rate transfers data on both the rising and falling edges of the clock signal, effectively nearly doubling the data transmission rate without having to deal with the additional problems of timing skew that increasing the number of data lines would introduce. ...


Most computer users don't need to worry about RAM latency, because the computer can handle the auto-adjustment to RAM timing based on the Serial Presence Detect (SPD) ROM inside the RAM packaging that defines the four timing values, decided by the RAM manufacturer. Although the RAM latency timing can be adjusted manually, smaller values than those given by the RAM manufacturer is considered over-clocking but it may cause a computer to crash or not boot. Serial Presence Detect (SPD) refers to a standardized way to access manufacturer information from a computer memory module. ... Read-only memory (ROM) is used as a storage medium in computers. ... A crash in computing is a condition where a program (either an application or part of the operating system) stops performing its expected function and also stops responding to other parts of the system. ... An Internet payphone loading Windows XP In computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. ...


See also

DRAM is a type of random access memory that stores each bit of data in a separate capacitor. ...

External links


  Results from FactBites:
 
RAM latency - Wikipedia, the free encyclopedia (494 words)
RAM Latency is the amount of wait time that a computer experiences when trying to access data in its RAM.
RAM latency is measured in front side bus clock cycles.
Although the RAM latency timing can be adjusted manually, smaller values than those given by the RAM manufacturer is considered over-clocking but it may cause a computer to crash or not boot.
CAS Latency - Wikipedia, the free encyclopedia (239 words)
CAS is an acronym for column address strobe, or sometimes column address select, both referring to the column of the physical memory location in an array of capacitors (a grid comprised of columns and rows) used in dynamic random access memory modules.
Latency refers to the amount of time, or lag, that is experienced in traversing a system.
Taken together, the CAS Latency (CL) refers to the length of time, in clock cycles, it takes for a request sent from the memory controller to read a memory location and send it to the module's output pins.
  More results at FactBites »

 
 

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