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- Why does it serve


- The size of the cache memory

- The internal cache memory or L1

- The cache memory of the Pentium II/III and Celeron

- On false cache memories and tricky

- Technologies used in the cache memory

Updated page
11/1/1999


Author:
Juan Herrerías Rey
 

Articles about the cache memory


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What is... the cache memory?


 

In many computers announcements there exist a few numbers that seem extracted of a spies' code, a little how: "256 Kb" or "512 burst-sram". Before similar foolish talk, our computer shift friend does not hesitate to exclaim: "ah, yes, this is the cache memory", in a round tone that does not leave place to any more questions. But in our internal jurisdiction, we keep on wondering: the ca - what?

 

Why does it serve

To start, let's say that the cache memory is not but a type of memory of the computer; therefore, in her there will keep information that the computer needs to work. But was not this the RAM?, you will ask. Very well, partly yes. To tell truth, the principal memory of the computer, (the RAM, the famous persons 8, 16, 32 ó 64 "gentle ones") and the cache memory they are basically equal in many aspects; the difference is in the use that is given to the cache memory.

Due to the big speed reached by the microprocessors from 386, the RAM of the computer is not it sufficiently rapid to store and to transmit the information that the microprocessor (the "mike" from now on) needs, therefore he would have to hope that the memory was free and the work would slow down. To avoid it, there is used a very rapid memory strategically placed between the mike and the RAM: the cache memory.

This is the principal trick of the cache memory: it is very rapid. How much is "very rapid"? Well, approximately 5 ó 6 times more than the RAM. This her gets dearer enough, clearly, and this is one of the motives of that his capacity is much minor than that of the RAM: a maximum concerning 512 kilobytes (512 Kb), that is to say, came up "gentle", opposite to 16 ó 32 RAM gentle ones. Also, her candidate does this high price to falsifications and rip-offs.

But the cache memory not only is rapid; also, it is used by a specific purpose. When a computer works, the mike operates sometimes with a limited number of information, but that it has to bring and lead to the memory in every operation. If we place in the middle of the way of the information a buffer that stores the most secondhand information, which almost sure the mike will need in the next operation that it realizes, one will save a lot of time of the transit and access to the slow memory RAM; this is the second utility of the cache memory. Chips of cache memory of a badge 486

For that they have curiosity to see how there is the cache memory (although frequently it does not turn out to be easy to recognize, for being encapsulated in any type of chip of control or any meeting in the only chip), here they have a photo of a few chips of cache memory:

 

The size of the cache memory

Read the previous thing, you will think: since the bigger, better. True, but not; or rather, almost always yes. Although the cache memory is of major speed that the RAM, if we use a very big cache memory, the mike will take a valuable time in finding the fact that he needs. This would not be very important if the fact was there, but: and if it is not? Then it will have lost the time, and will have to add this time lost to what late in finding it in the RAM.

Therefore, the cache memory acts like a summary, a "chop" of the information of the RAM, and we all know that a summary of 500 pages does not turn out to be useful at all. It is possible to affirm that, for normal uses, from 1 MB (1024 Kb) the cache memory turns out to be ineffective, and it even could go so far as to slow down the functioning of the computer. The suitable size depends on that of the RAM, and it is gathered in the following table:

RAM (MB) Cache memory (Kb)

1 to 4

128 ó 256

4 to 12

256

12 to 32

512

more than 32

512 1024

It is necessary to make to notice that many "chipsets" for Pentium, like the acquaintances Intel "Merman" VX or TX, do not allow to frisk any more than 64 MB of RAM; that is to say, that from this number, IS AS IF Cache memory DID NOT EXIST BY NO MEANS (0 Kb!!).
So if he needs to install more than 64 MB in a badge for Pentium, I looked for a badge that allows to frisk more of this number (like someone - not all - those who have chipsets "Merman" HX). To know more on chipsets,
pulsate here.

 

The internal cache memory or L1

The cache memory to which we have referred till now is the called "external cache memory" or of the second level (L2). There exists other one, which basic beginning is the same, but which is included inside the mike; hence he interns that thing about, or of the first level (L1).

This cache memory works like the day pupil, only that is closer to the mike, is more rapid and more expensive, in addition to complicating the design of the mike, therefore his size measures itself in few tens kilobytes. 486 joined for the first time in the mikes, and at that time it was to 8 Kb (although some 486 of Cyrix had only 1 Kb). Nowadays 64 Kb uses 32 ó, although sure that soon someone will overcome this number.

The importance of this cache memory is fundamental; for example, the Pentium MMX are more rapid than the normal Pentium in applications not optimized for MMX, thanks to having the double of internal cache memory. To tell truth, the efficacy of the "optimization MMX" of applications like Microsoft Office, it is still to see...

 

The cache memory of the Pentium II/III and Celeron

It is possible that he has heard speaking that Pentium II and Pentium III (and his predecessor the dead Pentium Pro) have 512 Kb of internal cache memory; this is inaccurate, when not a "interested confusion" on the part of Intel and the sellers. Pentium II and III have 32 Kb of internal cache memory, and 512 cache memory Kb inside the cartridge SEC but day pupil to the encapsulated one of the microchip.

This absurdity explains to itself if a Pentium II "spoiled" like this one is seen:

Pentium II in all his interiority

Inside the big black casing we find a badge of circuit in which the mike is welded in itself (in the center of the image), together with several chips that form the cache memory, day pupil to what it is properly the mike. Nevertheless, this cache memory works to a frequency that is half of that of the mike (that is to say, to 133, 150 MHz or more), while the classic external cache memory works to that of the motherboard (from 50 to 66 MHz in the Pentium and 100 MHz in the AMD K6-2).

Those who almost can presume to have a big internal cache memory are the most modern mikes: Pentium III Coppermine, Celeron Mendocino (not the ancient Celeron, who were lacking cache memory L2 by no means) and AMD Athlon Thunderbird and Duron. These mikes have the cache memory L2 integrated to the encapsulated proper one of the mike and make her work at the same speed as this one, so that it does not become so rapid as the cache memory L1 but yes enough how to be more rapid than the models with external cache memory, despite having the half or less of cache memory.

Let's say, anyway, that Pentium II and the first Pentium III and Celeron have an internal cache memory and a semi-day pupil, which is not little a merit by no means; but the things are like a sound, an evil that him despite the magicians of the publicity.


Appendix 1 - On false cache memories and tricky

As we said already, the cache memory is a valued, and valued good in enough pesetas. For it, the natural greed of certain personages has led them to making basic badges with chips of cache memory of vulgar solid plastic, method that it can enrich in approximately 2.000 pesetas for badge and limit the yield of the computer of 5 to 10 %.

This phenomenon had his heyday with the basic badges for 486, although it is not possible to assure that should be completely eradicated. Unfortunately, there are few methods to know if a cache memory chip is good or false, and almost none bases on the direct observation (as it is not for radiography). The principal means to detect the fraud are:

  • to observe chips extremely coarse and badly finished off, with plastic rims and serigrafiados of low quality (supposing that he can identify or the cache memory chips, which can be difficult);
  • to use some diagnosis tool for software that detects the presence or cache memory absence.

On these diagnosis programs, it is necessary to mention that they are not infallible, therefore if it does not detect anyone the cache memory is convenient to try with other (but condition one detects it, it is almost sure that it is authentic). Also, they can fail with very rapid and modern certain types of cache memory, therefore it usually does not serve the same program for the badge of 486 and that of a Pentium. Some of these programs (for badges 486, which are usually the most falsified) can be in Internet.

If you finish convinced that his badge has a false cache memory (although if it is a question of one for Pentium it can take his time to him), the best thing that can do is to not buy again in the shop where he acquired it or, if it has time and desire, to go to complain. It is not probable that they him pay attention, but: that hear him! (And if his provider cheated them, it is not an excuse, but lacking in professionalism).

Appendix 2 - Technologies used in the cache memory

Although in general it is not possible to choose what cache memory to acquire with the computer, since it sells jointly with the motherboard (or with the mike, if it is a Pentium II, a Pentium III or a Mendocino), it is convenient to have a few concepts clear if there was happening the case to have several options at our disposal.

First of all, the type of memory used to make the cache memory is one of the most important factors. It is usually a memory of a very rapid type (like for example SRAM or SDRAM) and with special characteristics, as burst pipeline: to transmit information "to blasts" (burst).

The speed of the cache memory influences his yield, as it is obvious. The cache memories move concerning 10 nanoseconds (ns) of refreshment speed; that is to say, that every 10 ns can admit a new series of information. Therefore, to minor time of refreshment, major speed. If he wants to know how to detect "roughly discovered" the speed of a chip of memory, pulsate on this hypertie.

The last parameter that influences the cache memories is the way of writing the information in them. This is selected usually in the BIOS, good to hand or leaving that does the computer automatically; two principal forms are:

  • Write-Througth: unpronounceable term that indicates the classic way of work of the cache memory;
  • Write-Back: a more modern and effective way of managing the cache memory.

For more information on the mysterious BIOS and how to operate on her, go to the article dedicated to her: What is... the BIOS?.

 


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