In Computer Memory what is CAS Latency?

Written by Ron Merts

This isrepparttar question we are asked more than any other question. So, I figured I'd put together a bulletin containing my $0.02 worth!

First of all, what is CAS?

"CAS" is short for "Column Address Strobe". A DRAM memory can be thought of as a matrix, kind of like a spreadsheet with memory cells instead of numbers and formulas. Likerepparttar 107774 spreadsheet, each cell has a row address and a column address (like "AA57" or "R23C34" inrepparttar 107775 spreadsheet). As you might have guessed, there is also a RAS signal, which is shorthand for "Row Address Strobe".

And, what do you mean by "latency"?

Latency refers torepparttar 107776 time that you are waiting to get what you need. Merriam-Webster dictionary defines it as "the interval between stimulus and response".

Now, how does CAS work?

To understand this let's walk through a simplified version of howrepparttar 107777 memory controller actually readsrepparttar 107778 memory. First,repparttar 107779 chip set accessesrepparttar 107780 ROW ofrepparttar 107781 memory matrix by putting an address onrepparttar 107782 memory's address pins and activatingrepparttar 107783 RAS signal. Then, we have to wait a few clock cycles (known as RAS-to-CAS Delay). Then,repparttar 107784 column address is put onrepparttar 107785 address pins, andrepparttar 107786 CAS signal is activated, to accessrepparttar 107787 correct COLUMN ofrepparttar 107788 memory matrix. Then, we wait a few clock cycles -- THIS IS KNOWN AS CAS LATENCY! -- and thenrepparttar 107789 data appears onrepparttar 107790 pins ofrepparttar 107791 RAM. So, for CAS-2 you wait 2 clock cycles and for CAS-3 you wait 3 clock cycles? Bingo!

So, CAS-2 is 33% faster than CAS-3?

Whoa, not so fast! There are a LOT of other factors inrepparttar 107792 memory performance. Here are a few ofrepparttar 107793 main ones: ·Sometimes you have to move to a different row in memory. This means activating RAS, waiting RAS-to-CAS delay, then doingrepparttar 107794 CAS latency thing. ·Other times, you do a "burst" read, when you pull in a lot of data in one big block. In that case, CAS is only activated ONCE, atrepparttar 107795 beginning ofrepparttar 107796 burst. ·Also, don't forgetrepparttar 107797 most important thing: processors have big caches! The cache is whererepparttar 107798 processor stores recently accessed instructions and data. The cache "hit rate", i.e.,repparttar 107799 percentage of timesrepparttar 107800 processor findsrepparttar 107801 information it needs in its own cache, is typically greater than 95%!

Running a Program on a Remote Server Using SSH

Written by C.S. Deam

How do you run a program on a remote server using ssh?

For this example we'll have two servers, one named Johnny and another named Cash. Both are running openssh. Our goal is to have a program on Johnny login to Cash and run a program on Cash. To makerepparttar task a little more complex we'll be using different users on each machine.

The first thing we'll need to do is generate public and private keys on Johnny. So, logged into Johnny as user 'boy' we create public and private keys by creating them inrepparttar 107773 .ssh directory as follows:

Johnny$> pwd /home/boy/.ssh Johnny$> ssh-keygen -t rsa -f sue Generating public/private rsa key pair. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in sue. Your public key has been saved in The key fingerprint is: 8d:e9:c0:g1:c7:1f:e3:b3:2f:38:12:aa:b5:3b:2e:b3 boy@Johnny Johnny$>

Inrepparttar 107774 example above we picked an arbitrary name, sue, to identifyrepparttar 107775 files that holdrepparttar 107776 generated keys. When prompted to enter a passphrase (twice) we simply hit enter twice.

Asrepparttar 107777 output of ssh-keygen indicates,repparttar 107778 public key has been saved in a file named

The output of ssh-keygen implies, but doesn't indicate directly, thatrepparttar 107779 private key is in a file named sue (Yes, a user named boy created a file named sue.)

Johnny$>ls -l -rwx------ 1 sue suegrp 887 Oct 17 14:27 sue -rwx------ 1 sue suegrp 223 Oct 17 14:27

The private key file, sue, will remain on Johnny forrepparttar 107780 reaminder of this exercise, butrepparttar 107781 public key must be moved torepparttar 107782 remote server, Cash. Note thatrepparttar 107783 .ssh directory itself, as well asrepparttar 107784 sue and files should have permissions of 700.

Now you'll need to ftprepparttar 107785 file from Johnny to Cash. The user on Cash that we'll login as is user 'named'.

Johnny$> ftp Cash Connected to Cash 220 Cash - Propery of Xyz. - Authorized users only Name (Cash:boy): named 331 Password required for named. Password: 230-Last unsuccessful login: Fri Oct 17 13:12:55 2003 on ftp from Johnny 230-Last login: Fri Oct 17 16:02:11 2003 on /dev/pts/1 from Johnny 230 User named logged in. ftp> cd .ssh ftp> mput mput y 200 PORT command successful. 150 Opening data connection for 226 Transfer complete. 224 bytes sent in 0.000781 seconds (280.1 Kbytes/s) local: remote: ftp>bye 221 Goodbye. Johnny$>

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