Pentium

 The actual Pentium chip shipped on March 22, 1993. Systems that use these chips were only a few months behind.

 The Pentium is fully compatible with previous Intel processors, but it also differs from them in many ways. At least one of these differences is revolutionary: the Pentium features twin data pipelines, which enable it to execute two instructions at the same time. Intel calls the capability to execute two instructions at the same time superscalar technology. This technology provides additional performance compared with the 486.

 The standard 486 chip can execute a single instruction in an average of two clock cycles-cut to an average of one clock cycle with the advent of internal clock multiplication used in the DX2 and DX4 processors. With the superscalar technology, the Pentium can execute many instructions at a rate of two instructions per cycle. Superscalar architecture usually is associated with high-output RISC (Reduced Instruction Set Computer) chips. The Pentium is one of the first CISC (Complex Instruction Set Computer) chips to be considered superscalar. The Pentium is almost like having two 486 chips under the hood.

 The Pentium is 100 percent software-compatible with the 386 and 486, and although all current software will run much faster on the Pentium, many software manufacturers want to recompile their applications to exploit even more of the Pentium's true power. Intel has developed new compilers that will take full advantage of the chip; the company will license the technology to compiler firms so that the software developers can take advantage of the superscalar (parallel processing) capability of the Pentium. Optimised software improves performance by allowing more instructions to execute simultaneously in both pipes.

 To minimize stalls in one or more of the pipes caused by delays in fetching instructions that branch to nonlinear memory locations, the Pentium processor has a Branch Target Buffer (BTB) that employs a technique called branch prediction. The BTB attempts to predict whether a program branch will be taken or not and the fetches the appropriate next instructions. The use of branch prediction enables the Pentium to keep both pipelines operating at full speed.

The Pentium has a 32-bit address-bus width, giving it the same 4G memory-addressing capabilities as the 386DX and 486 processors. But the Pentium expands the data bus to 64 bits, which means that is can move twice as much data into or out of the CPU compared with a 486 of the same clock speed. The 64-bit data bus requires that system memory be accessed 64 bits wide, which means that each bank of memory is 64 bits.

 On most motherboards, memory is installed via SIMMs (Single In-line Memory Module), which are available in 9-bit-wide and 36-bit-wide versions. Most Pentium systems use the 36-bit-wide (32 data bits plus 4 parity bits) SIMMs- four of these SIMMs per bank of memory. Most Pentium motherboards have four of these 36-bit SIMM sockets, providing for a total two banks of memory.

 Even though the Pentium has a 64-bit data bus that transfers information 64 bits at a time into and out of the processor, the Pentium has only 32-bit internat registers. As instructions are being processed internally, they are broken down into 32-bit instructions and data elements, and processed in much the same way as the 486. Some people thought that Intel was misleading them by calling the Pentium a 64-bit processor, but 64-bit transfers do indeed take place. Internally, however, the Pentium has 32-bit registers that are fully compatible with the 486.

 The Pentium has two separate internal 8K caches, compared with a single 8K or 16K cache in the 486. The cache-controller circuitry and the cache memory are embedded in the CPU chip. The cache mirrors the information in normal RAM by keeping a copy of the data and code from different memory locations. The Pentium cache also can hold information to be written to memory when the load of the CPU and other system components is less. (The 486 makes all memory writes immediately.)

 The separate code and data caches are organized in a two-way set associative fashion, with each set split into lines of 32 bytes each. Each cache has a dedicated Translation Lookaside Buffer (TLB), which translates linear addresses to physical addresses. You can configure the data cache as Write-Back or Write-Through on a line-by-line basis. When you use WriteBack capability, the cache can store write operations as well as reads, further improving performance over read-only Write-Through mode. Using Write-Back mode results in less activity between the CPU and system memory-an important improvement, because CPU access to system memory is a bottleneck on fast systems. The code cache is an inherently write-protected cache because is contains only execution instructions and not data, which is updated. Because burst cycles are used, the cache data can be read or written very quickly.

 Systems based on the Pentium can benefit greatly from secondary processor caches (Level 2), which usually consist of up to 512K or more of extremely fast (15ns of less) Static RAM (SRAM) chips. When the CPU fetches data that is not already available in its internal processor (Level 1) cache, wait states slow the CPU. If the data already is in the secondary processor cache, however, the CPU can go ahead with its work without pausing for wait states.

 The Pentium uses a BiCMOS (Bipolar Complementary Metal Oxide Semiconductor) process and superscalar architecture to achieve the level of performance expected from the new chip. BiCMOS adds about 10 percent to the complexity of the chip design, but it adds about 30-35 percent better performance without a size or power penalty. Intel will be transitioning back to conventual CMOS designs as the clock speeds of the Pentium and Pentium Pro processor increase. There is no performance advantage to BiCMOS at lower voltages, and some of the new processors will be running on 2.5v of less. Intel uses the BiCMOS process in most of the processors up to 133 MHZ, but will use CMOS for any faster ones because they will also run at lover voltages.

 All Pentium processors are SL Enhanced, meaning that the incorporate the SMM to provide full control of power-management features, which helps reduce power consumption. The second-generation Pentium processors (75 MHZ and faster) incorporate a more advanced form of SMM that includes processor clock control, which allows you to throttle the processor up or down to control power use. With these more advanced Pentium processors, you can even stop the clock, Putting the processor in a state of suspension that requires very little power. The second-generation Pentium Processors run of 3.3v power (instead of 5v), reducing power requirements and heat generation even further.

 For even lower power consumption, Intel has introduced special Mobile Pentium processors in the 75/90/100 MHZ family. These do not use a conventional chip package and are instead mounted using a new format called Tape Carrier Packaging (TCP). The Tape carrier packaging does not encase the chip in ceramic or plastic as with a conventional chip package, but instead covers the actual processor die directly with a thin, protective plastic coating. The entire processor is less than 1mm thick and weighs less than 1 gram. They are sold to system manufactures in a roll that looks very much like filmstrip. The TCP processor is directly affixed (soldered) to the motherboard by a special machine, resulting in a smaller package, lower height, better thermal transfer, and lower power consumption. Special solder plugs on the circuit board located directly under the processor draw heat away and provide better cooling in the tight confines of a typical notebook or laptop system, and no cooling fans are required.

 The Pentium, tike the 486, contains an internal math coprocessor or FPU. The FPU in the Pentium has been rewritten and performs significantly better that the FPU in the 486, yet is fully compatible with the 486 and 386 math processor. The Pentium FPU is estimated to be two to as much as 10 times faster than the FPU in the 486. In addition, the two standard instruction pipelines in the Pentium provide two units to handle standard integer math. (The math coprocessor handles only more complex calculations.) Other processors, such as the 486, have only a single standard execution pipe and one integer-math unit.

First-Generation Pentium Processor

 Much of the criticism levelled at Intel for the first-generation Pentuim was justified. Some people realized that the first-generation design was just that; they knew that new Pentium versions, made in a more advanced manufacturing process, were coming. Many of those people advised against purchasing any Pentium system until the second-generation version became available.

 A cardinal rule of Computing is never to buy the first generation of any processor. Although you can wait forever because something better always will be on the horizon, a little waiting is worthwhile in some cases.

Second-Generation Pentium Processor

 The P54C second-generation Pentium processors come in a 296-pin SPGA form factor that is physically incompatible with the first-generation versions. The only way to up-grade from the first-generation to the second is to replace the motherboard. The second-generation Pentium processors also have 3.3 million transistors-more than the earlier chips. The extra transistors exist because additional clock-control SL enhancements were added, as were an on-chip Advanced Programmable Interrupt Controller (APIC) and dual-processor interface.

 The APIC and dual-processor interface are responsible for orchestrating dual-processor configurations in which two second-generation Pentium chips can process on the same motherboard simultaneously. Many of the new Pentium motherboards come with dual Socket 5 or Socket 7 specifications sockets, which fully support the multiprocessing capability of the new chips. Already, software support for what is called Symmetric Multi-Processing (SMP) is being integrated into operating systems such as Windows and OS/2.

 The second-generation Pentium processors use clock-multiplier circuitry to run the processor at speeds faster than the bus. The 90 MHZ Pentium processor can run at 1.5 times the bus frequency, which normally is 60 MHZ. The 100 MHZ Pentium processor can run at a 1.5x clock in a system using a 66 MHZ bus speed or at a 2x clock on a motherboard that is running at 50 MHZ.

 Currently, running the motherboard faster than 66 MHZ is impractical because of memory and local-bus performance constraints. The fastest Pentium systems would combine the 66 MHZ motherboard operation with a 2x internat processor clock and 133 MHZ processor operation. If you think that 2 times 66 equals 132 and not 133, you may be right, but in nearly all cases, 66 MHZ operation really means 66.6666 MHZ actual speed.

 Virtually all Pentium motherboards have three speed settings: 50, 60 and 66 MHZ. Pentium chips are available with a variety of different internal clock multipliers that cause the processor to operate at various multiples of these motherboard speeds.

 How the state of Bfx pins affect the clock multiplication in the Pentium processor

Third-Generation Pentium processor

 Most of the older motherboards could only supply 3.465v or 3.3v. The 3.465v setting is called VRE (Voltage Reduced Extended) by Intel and is required by some versions of the Pentium, particularly some of the 100 MHZ versions. The standard 3.3v setting is called STD (Standard), which most of the second-generation Pentium's use. STD-voltage means anything in a range from 3.135v to 3.465v with 3.3v nominal. There is also a special 3.3v setting called VR (Voltage Reduced), which reduces the range from 3.300v to 3.465 v with 3.38 nominal. Some of the processors require this narrower specification, which most motherboards provide.

 If you want the maximum future upgradeablity to the P55C third-generation Pentium's, Make sure that your Pentium motherboard includes 321-pin processor sockets that fully meet the Intel Socket 7 specification. These would also include the VRM socket. If you have dual sockets, you can add a second Pentium processor to take advantage of SMP support in the newer operating systems.

 Also make sure that and Pentium motherboard you buy can be jumpered or reconfigured for both 60 and 66 MHZ operation, which will enable you to take advantage of future Pentium Overdrive processors that will support the higher motherboard clock speeds.

 These simple recommendations will enable you to perform several dramatic upgrades without changing the entire motherboard.

Pentium Pro Processor

 The architecture of the Pentium Pro includes three internal instruction pipes, which can execute multiple instructions in one cycle. The main processor die includes a 16K split L1 cache with an 8K 2-Way Set Associative Cache for primary instructions, and an 8K 4-Way set Associative Cache for data. The Pentium Pro can execute instructions out of order and has dynamic branch prediction and speculative execution capabilities.

 In many ways, the Pentium Pro seems more of an evolutionary design compared to the Pentium rather than something totally new. The core of the chip is very RISC-like, while the external instruction interface is classic Intel CISC (Complex Instruction Set Computer). By breaking down the CISC instructions into several different RISC instructions and running them down parallel execution pipelines, the overall performance is increased.

 Compared to the Pentium, Intel claims that the Pentium Pro is twice as fast, but it is comparing a 133 MHZ Pentium Pro against a 100 MHZ Pentium. It claims that this is a fair comparison because the improved design of the Pentium Pro can run on the older 0.6-micron BiCMOS process used by the Pentium 100 rather than the 0.35-micron CMOS process used by the Pentium 150 and higher. Intel has indicated that when the Pentium Pro is migrated to the 0.35-micron and eventually 0.25-micron CMOS process, absolutely incredible speeds will be possible.

 Notwithstanding that the Pentium Pro can be manufactured at higher clock speeds on a given process than the Pentium, comparing a 133 MHZ operating version of both the Pentium and a Pentium Pro shows that the Pentium Pro has only about 33 percent advantage in power. This is enhanced further, however, by the fact that Intel Has this chip running at speeds of 150, 166 and 200 MHZ. This has been improved with the newer Pentium II version with higher speeds and an even larger L2 cache.

 The integrated L2 cache is one of the really outstanding features of the Pentium Pro. By building the L2 cache into the CPU and getting it off the motherboard, the cache can now run at full processor speed rather than the slower 60 or 66 MHZ motherboard bus speeds. In fact, the L2 cache features its own internal 64-bit backside bus, which does not share time with the external 64-bit frontside bus used by the CPU. The internal registers and data paths are still 32-bit as with the Pentium. By building the L2 cache into the system, motherboards will become cheaper because they will no longer require separate cache memory. Some boards may still try to include cache memory in their design, but the general consensus is that Level 3 cache (as it would be called) would offer less improvement with the Pentium Pro than with the Pentium.

 One of the features of the built-in L2 cache is that multi processing is greatly improved. Rather than just dual processor configurations (SMP) as with the Pentium, the Pentium Pro supports a new type of mulit processor configuration called the Multi-Processor Specification (MP 1.1). The Pentium Pro with MPS will allow configurations of up to four processors running together. Unlike other multi-processor configurations, the Pentium Pro will avoid cache coherency problems because each chip maintains a separate L1 and L2 cache internally.

 Pentium Pro-based motherboards are pretty much exclusively PCI and ISA based, and Intel is producing its own chipset for these new motherboards. The first chipset was called Orion, while the newest version is called Natoma. Along with the new capsize, Intel created a motherboard form factor change. The new form factor is called ATX, and is different from the Baby-AT form factor used by most PC-compatibles. The ATX form factor is about the same 22.5 x 32.5cm size as the Baby-AT, but the board is turned 90 degrees from the way the Baby-AT boards mount. In other words the long side is now against the back of the case, and the expansion slots are parallel with the short side of the board. The main reason for this form factor is to move the CPU to an area where the expansion cards will not be located, which allows much better cooling. Current systems with the CPU under the slots can have problems in this area, sometimes preventing one from using all the available bus slots.

 Another benefit of the ATX form factor is that the long edge of the board will be against the back of the case, allowing room for many built-in connectors. ATX boards will be highly integrated, featuring built-in dual serial ports, a parallel port, floppy controller, dual enhanced IDE ports, integrated sound, SVGA video, and optional SCAI and networking interfaces. Of course, this new motherboard form factor will require retooled cases and power supplies, as well. Intel is sharing the specifications of the new ATX form factor, and many other motherboard manufacturers already have designs ready.

 Other Pentium Pro system manufacturers intend on sticking with the Baby-AT form factor, at least for the time being. The big problem with the standard Baby-AT form factor is keeping the CPU properly cooled. The massive Pentium Pro processor consumes 20 watts or more of power and generate an appreciable amount of heat.

 Intel has just developed the P7 or Pentium II processor with over 10 million transistors. If current predictions hold up the P8 processors will be released around 1999 with 20 million transistors at 300 MHZ, and possibly a P9 chip during 2000 with 40 million transistors and 400 MHZ or more! This helps to keep in perspective the fact that whatever you purchase today will be obsolete in two to three years.

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