Data transferred from memory to the CPU

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CPUs - Part 1

There are many factors which determine your system's performance.The most significant, in decreasing order of importance, are:

File fragmentation
Disk access time
RAM
CPU speed
Cache

On some systems, the order will be different, but in general, thisis true. File fragmentation is handled with Diskeeper. The basics of disk access time and RAM are covered in earlierarticles. This article deals with the last two items in the above list, CPU speed and Cache.

Cache - What it Is and What it Does

Cache is a relatively small amount of memory (RAM) which is positionedclose to the CPU (or in it), and is used to cut down on delays caused by the CPU having to wait for data to arrive.Cache is pipelined; this is a technique of breaking down instructions into sequential steps and using differentresources to do each step. This means the instructions get done faster.

Level 1 (L1) cache has a small capacity but extremely high speed.Level 2 (L2) is much larger, but slower. When the CPU needs data, it checks L1, then L2, then goes on to the systemmemory as a last resort. This kind of cache allows data to be transferred from memory while the CPU is processing(that's the "pipelined" part), thus speeding up overall performance. The keys to the speed-up are thebus speed (the speed at which data is transferred between the cache and the CPU) and the bus width (how many bitscan be transferred simultaneously). The external bus speed (for data transfer between the CPU and motherboard)is usually 66 MHz (MHz = megahertz, or millions of cycles per second). The external bus width is usually 64 bits.

Let's illustrate this with an example of a CPU with no cache and thesame CPU with 256 kilobytes of cache, with an internal speed of 150 MHz and an internal bus width of 300 bits.It really takes more than one clock cycle to transfer a bit of data, but for simplicity, we will assume that onecycle results in one transfer. The resulting number of bits per second will be wrong, but we are interested inthe ratio between the two CPUs, and this ratio will be correct.

Data transferred from memory to the CPU without cache comes 64 bitsat a time at 66 MHz, or 4,224 million bits per second. On the CPU with cache, the data loads the cache at the samerate, but the CPU can perform other tasks while the cache is loading. The data transfers from the cache to theCPU, moving 300 bits at 150 MHz, or 45,000 million bits per second. That's more than ten times as fast.

The CPU

The CPU (Central Processing Unit) is the glory hog. It gets most ofthe attention and is the standard that most people believe determines the overall performance of a system. Whilethis usually is a mistaken belief, the CPU is still very important in determining performance. However, to makean informed decision about a CPU, you need to know what is available.

The current Intel processors are the 486, Pentium, Pentium Pro, andPentium II. They are not interchangeable, as they all require different means of mounting them on the motherboard(mounts), so upgrading from one to another means you have to replace the motherboard.

The 486 is similar to the original Pentium, but slower. The clockspeed can be as low as 25 MHz. There's nothing wrong with it, it's a fine chip, but obsolete. We recommend notusing a 486 for Windows NT; we have tried it and, while it does work, it is very slow.

Pentium

The original Pentium comes in speeds ranging from 66 to 200 MHz, anduses 3.3 volts power. The Pentium with MMX technology ranges from 166 to 233 MHz, and uses 2.8 volts power. TheMMX technology can handle more colors, better graphics, and full-screen full-motion video. According to Intel benchmarks,MMX is faster by 60% on multimedia, and 10-20% on other applications, but the difference in voltage means thateven though both types of Pentium use the Socket 7 standard, some motherboards cannot support an MMX CPU. If youwant to upgrade a Pentium to MMX, first make sure that your motherboard supports 2.8 volts.

On Pentium systems, the cache is in the motherboard, either builtin or on a board similar to those used for system RAM. The amount of cache is either 0, 256, or 512 kilobytes.The bus speed between the cache and the CPU is 66 MHz. Thus, the effectiveness of the cache depends on the buswidth.

Pentium Pro

The Pentium Pro runs at 166 to 200 MHz. No Pentium Pro includes MMX.It uses the Socket 8 mount, so it is not interchangeable with the Pentium. The L2 cache is built into the chipand is either 256, 512 or 1000 kilobytes. The bus speed between cache and the CPU is the same as the CPU clockspeed, and the bus width is 300 bits. In addition, the Pentium Pro has two internal buses (DIB, or Dual IndependentBus) which can be used simultaneously, so one bus can be dedicated to "cache flow" (please excuse thepun).

The Pentium Pro is generally the best CPU for a Server, because ofthe fast and large cache. Servers are primarily used to transfer data to other machines in the network. A PentiumPro 200 with a one-megabyte cache should give you the fastest data transfer rate of any Intel CPU.

Pentium II

The Pentium II is in effect a Pentium Pro with MMX technology. Itcomes with speeds of 233 to 300 MHz, but there will undoubtably be faster models in the near future. It uses thesame DIB architecture as the Pentium Pro and includes a built-in 512-kilobyte L2 cache, with a bus speed of halfthe clock speed of the CPU. We believe the bus width to be 300 bits, but could not confirm this.

The Pentium II mounts in a Slot 1 mount. While Socket 7 is very commonand is used by other CPU manufacturers (Cyrix, AMD, etc.), Slot 1 is pretty much nailed down by Intel patents.It is very unlikely that any other manufacturer will be able to use it, so when other companies come out with theirown versions of the Pentium II, they will have to use some other mount. The other CPU vendors are already discussinga new open interface. We'll have either a completely different motherboard, or a motherboard with a slot to accepta "CPU card", with interchangeable mounts for different CPUs.

This is important when planning for future expansion. If the PentiumII is cheap enough and offers what the user community wants, Intel should remain the dominant CPU manufacturer.But, if other companies produce a comparable CPU at a substantially lower cost and use an open, competitive mount,this could change. Then, an upgrade might require changing a lot more than just the CPU, which could get very expensive.

This brings us to the "non-Intel" manufacturers. Part 2 of this article willcover these other CPUs.

If you have any comments about this article orany requests for new technical articles e-mail

Executive Software Europe





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	CPUs - Part 1 There are many factors which determine your system's performance.The most significant, in decreasing order of importance, are: File fragmentation Disk access time RAM CPU speed Cache On some systems, the order will be different, but in general, thisis true. File fragmentation is handled with Diskeeper. The basics of disk access time and RAM are covered in earlierarticles. This article deals with the last two items in the above list, CPU speed and Cache. Cache - What it Is and What it Does Cache is a relatively small amount of memory (RAM) which is positionedclose to the CPU (or in it), and is used to cut down on delays caused by the CPU having to wait for data to arrive.Cache is pipelined; this is a technique of breaking down instructions into sequential steps and using differentresources to do each step. This means the instructions get done faster. Level 1 (L1) cache has a small capacity but extremely high speed.Level 2 (L2) is much larger, but slower. When the CPU needs data, it checks L1, then L2, then goes on to the systemmemory as a last resort. This kind of cache allows data to be transferred from memory while the CPU is processing(that's the "pipelined" part), thus speeding up overall performance. The keys to the speed-up are thebus speed (the speed at which data is transferred between the cache and the CPU) and the bus width (how many bitscan be transferred simultaneously). The external bus speed (for data transfer between the CPU and motherboard)is usually 66 MHz (MHz = megahertz, or millions of cycles per second). The external bus width is usually 64 bits. Let's illustrate this with an example of a CPU with no cache and thesame CPU with 256 kilobytes of cache, with an internal speed of 150 MHz and an internal bus width of 300 bits.It really takes more than one clock cycle to transfer a bit of data, but for simplicity, we will assume that onecycle results in one transfer. The resulting number of bits per second will be wrong, but we are interested inthe ratio between the two CPUs, and this ratio will be correct. Data transferred from memory to the CPU without cache comes 64 bitsat a time at 66 MHz, or 4,224 million bits per second. On the CPU with cache, the data loads the cache at the samerate, but the CPU can perform other tasks while the cache is loading. The data transfers from the cache to theCPU, moving 300 bits at 150 MHz, or 45,000 million bits per second. That's more than ten times as fast. The CPU The CPU (Central Processing Unit) is the glory hog. It gets most ofthe attention and is the standard that most people believe determines the overall performance of a system. Whilethis usually is a mistaken belief, the CPU is still very important in determining performance. However, to makean informed decision about a CPU, you need to know what is available. The current Intel processors are the 486, Pentium, Pentium Pro, andPentium II. They are not interchangeable, as they all require different means of mounting them on the motherboard(mounts), so upgrading from one to another means you have to replace the motherboard. The 486 is similar to the original Pentium, but slower. The clockspeed can be as low as 25 MHz. There's nothing wrong with it, it's a fine chip, but obsolete. We recommend notusing a 486 for Windows NT; we have tried it and, while it does work, it is very slow. Pentium The original Pentium comes in speeds ranging from 66 to 200 MHz, anduses 3.3 volts power. The Pentium with MMX technology ranges from 166 to 233 MHz, and uses 2.8 volts power. TheMMX technology can handle more colors, better graphics, and full-screen full-motion video. According to Intel benchmarks,MMX is faster by 60% on multimedia, and 10-20% on other applications, but the difference in voltage means thateven though both types of Pentium use the Socket 7 standard, some motherboards cannot support an MMX CPU. If youwant to upgrade a Pentium to MMX, first make sure that your motherboard supports 2.8 volts. On Pentium systems, the cache is in the motherboard, either builtin or on a board similar to those used for system RAM. The amount of cache is either 0, 256, or 512 kilobytes.The bus speed between the cache and the CPU is 66 MHz. Thus, the effectiveness of the cache depends on the buswidth. Pentium Pro The Pentium Pro runs at 166 to 200 MHz. No Pentium Pro includes MMX.It uses the Socket 8 mount, so it is not interchangeable with the Pentium. The L2 cache is built into the chipand is either 256, 512 or 1000 kilobytes. The bus speed between cache and the CPU is the same as the CPU clockspeed, and the bus width is 300 bits. In addition, the Pentium Pro has two internal buses (DIB, or Dual IndependentBus) which can be used simultaneously, so one bus can be dedicated to "cache flow" (please excuse thepun). The Pentium Pro is generally the best CPU for a Server, because ofthe fast and large cache. Servers are primarily used to transfer data to other machines in the network. A PentiumPro 200 with a one-megabyte cache should give you the fastest data transfer rate of any Intel CPU. Pentium II The Pentium II is in effect a Pentium Pro with MMX technology. Itcomes with speeds of 233 to 300 MHz, but there will undoubtably be faster models in the near future. It uses thesame DIB architecture as the Pentium Pro and includes a built-in 512-kilobyte L2 cache, with a bus speed of halfthe clock speed of the CPU. We believe the bus width to be 300 bits, but could not confirm this. The Pentium II mounts in a Slot 1 mount. While Socket 7 is very commonand is used by other CPU manufacturers (Cyrix, AMD, etc.), Slot 1 is pretty much nailed down by Intel patents.It is very unlikely that any other manufacturer will be able to use it, so when other companies come out with theirown versions of the Pentium II, they will have to use some other mount. The other CPU vendors are already discussinga new open interface. We'll have either a completely different motherboard, or a motherboard with a slot to accepta "CPU card", with interchangeable mounts for different CPUs. This is important when planning for future expansion. If the PentiumII is cheap enough and offers what the user community wants, Intel should remain the dominant CPU manufacturer.But, if other companies produce a comparable CPU at a substantially lower cost and use an open, competitive mount,this could change. Then, an upgrade might require changing a lot more than just the CPU, which could get very expensive. This brings us to the "non-Intel" manufacturers. Part 2 of this article willcover these other CPUs. If you have any comments about this article orany requests for new technical articles e-mail
	Executive Software Europe