Besides Intel, many other manufacturers have produced P6-type processors, but often with a difference. Most of them were designed to interface with P5 class motherboards for the lower-end markets. AMD later offered up the Athlon and Duron processors, which were true sixth-generation designs using their own proprietary connections to the system.

This section examines the various sixth-generation processors from manufacturers other than Intel.

NexGen Nx586

NexGen was founded by Thampy Thomas, who hired some of the people formerly involved with the 486 and Pentium processors at Intel. At NexGen, developers created the Nx586, a processor that was functionally the same as the Pentium but not pin compatible. As such, it was always supplied with a motherboard; in fact, it was usually soldered in. NexGen did not manufacture the chips or the motherboards they came in; for that it hired IBM Microelectronics. Later NexGen was bought by AMD, right before it was ready to introduce the Nx686a greatly improved design by Greg Favor and a true competitor for the Pentium. AMD took the Nx686 design and combined it with a Pentium electrical interface to create a drop-in Pentium-compatible chip called the K6, which actually outperformed the original from Intel.

The Nx586 had all the standard fifth-generation processor features, such as superscalar execution with two internal pipelines and a high-performance integral L1 cache with separate code and data caches. One advantage is that the Nx586 includes separate 16KB instruction and 16KB data caches compared to 8KB each for the Pentium. These caches keep key instruction and data close to the processing engines to increase overall system performance.

The Nx586 also includes branch prediction capabilities, which are one of the hallmarks of a sixth-generation processor. Branch prediction means the processor has internal functions to predict program flow to optimize the instruction execution.

The Nx586 processor also featured a RISC core. A translation unit dynamically translates x86 instructions into RISC86 instructions. These RISC86 instructions were designed specifically with direct support for the x86 architecture while obeying RISC performance principles. They are thus simpler and easier to execute than the complex x86 instructions. This type of capability is another feature normally found only in P6 class processors.

The Nx586 was discontinued after the merger with AMD, which then took the design for the successor Nx686 and released it as the AMD-K6.

AMD-K6 Series

The AMD-K6 processor is a high-performance sixth-generation processor that is physically installable in a P5 (Pentium) motherboard. It essentially was designed for AMD by NexGen and was first known as the Nx686. The NexGen version never appeared because it was purchased by AMD before the chip was due to be released. The AMD-K6 delivers performance levels somewhere between the Pentium and Pentium II processor as a result of its unique hybrid design.

The K6 processor contains an industry-standard, high-performance implementation of the new multimedia instruction set, enabling a high level of multimedia performance for the time period. The K6-2 introduced an upgrade to MMX that AMD calls 3DNow!, which adds even more graphics and sound instructions. AMD designed the K6 processor to fit the low-cost, high-volume Socket 7 infrastructure. Initially, it used AMD's 0.35-micron, five-metal layer process technology; later the 0.25-micron process was used to increase production quantities because of reduced die size, as well as to decrease power consumption.

AMD-K6 processor technical features include

• Sixth-generation internal design, fifth-generation external interface

• Internal RISC core, translates x86 to RISC instructions

• Superscalar parallel execution units (seven)

• Dynamic execution

• Branch prediction

• Speculative execution

• Large 64KB L1 cache (32KB instruction cache plus 32KB write-back dual-ported data cache)

• Built-in floating-point unit

• Industry-standard MMX instruction support

• System Management Mode

• Ceramic pin grid array (CPGA) Socket 7 design

• Manufactured using 0.35-micron and 0.25-micron, five-layer designs

The K6-2 adds the following:

• Higher clock speeds

• Higher bus speeds of up to 100MHz (Super7 motherboards)

• 3DNow!; 21 new graphics and sound processing instructions

The K6-3 adds the following:

• 256KB of on-die full-core speed L2 cache

The addition of the full-speed L2 cache in the K6-3 was significant. It enabled the K6 series to fully compete with the Intel Pentium II processors and the Celeron processors based on the Pentium II. The 3DNow! capability added in the K6-2/3 was also exploited by newer graphics programs.

The AMD-K6 processor architecture is fully x86 binary code compatible, which means it runs all Intel software, including MMX instructions. To make up for the lower L2 cache performance of the Socket 7 design, AMD beefed up the internal L1 cache to 64KB total, twice the size of the Pentium II or III. This, plus the dynamic execution capability, enabled the K6 to outperform the Pentium and come close to the Pentium II and III in performance for a given clock rate. The K6-3 was even better with the addition of full-core speed L2 cache; however, this processor ran very hot and was discontinued after a relatively brief period.

Both the AMD-K5 and AMD-K6 processors are Socket 7 bus compatible. However, certain modifications might be necessary for proper voltage setting and BIOS revisions. To ensure reliable operation of the AMD-K6 processor, the motherboard must meet specific voltage requirements.

The AMD processors have specific voltage requirements. Most older split-voltage motherboards default to 2.8V Core/3.3V I/O, which is below specification for the AMD-K6 and could cause erratic operation. To work properly, the motherboard must have Socket 7 with a dual-plane voltage regulator supplying 2.9V or 3.2V (233MHz) to the CPU core voltage (Vcc2) and 3.3V for the I/O (Vcc3). The voltage regulator must be capable of supplying up to 7.5A (9.5A for the 233MHz) to the processor. When used with a 200MHz or slower processor, the voltage regulator must maintain the core voltage within 145mV of nominal (2.9V+/145mV). When used with a 233MHz processor, the voltage regulator must maintain the core voltage within 100mV of nominal (3.2V+/100mV).

If the motherboard has a poorly designed voltage regulator that cannot maintain this performance, unreliable operation can result. If the CPU voltage exceeds the absolute maximum voltage range, the processor can be permanently damaged. Also note that the K6 can run hot. Make sure your heatsink is securely fitted to the processor and that the thermally conductive grease or pad is properly applied.

The motherboard must have an AMD-K6 processor-ready BIOS with support for the K6 built in. Award has that support in its March 1, 1997 or later BIOS; AMI had K6 support in any of its BIOSs with CPU Module 3.31 or later; and Phoenix supports the K6 in version 4.0, release 6.0, or release 5.1 with build dates of 4/7/97 or later.

Because these specifications can be fairly complicated, AMD keeps a list of motherboards that have been verified to work with the AMD-K6 processor on its website.

Older motherboards achieve the 3.5x setting by setting jumpers for 1.5x. The 1.5x setting for older motherboards equates to a 3.5x setting for the AMD-K6 and newer Intel parts. Getting the 4x and higher setting requires a motherboard that controls three BF pins, including BF2. Older motherboards can control only two BF pins.

These settings usually are controlled by jumpers on the motherboard. Consult your motherboard documentation to see where they are and how to set them for the proper multiplier and bus speed settings.

Unlike Cyrix and some of the other Intel competitors, AMD is a manufacturer and a designer. Therefore, it designs and builds its chips in its own fabs. Similar to Intel, AMD has migrated to 0.25-micron process technology and beyond (the AMD Athlon XP is built on a 0.13-micron process). The original K6 has 8.8 million transistors and is built on a 0.35-micron, five-layer process. The die is 12.7mm on each side, or about 162 square mm. The K6-3 uses a 0.25-micron process and incorporates 21.3 million transistors on a die only 10.9mm on each side, or about 118 square mm.

Because of its performance and compatibility with the Socket 7 interface, the K6 series is often looked at as an excellent processor upgrade for motherboards using older Pentium or Pentium MMX processors. Although they do work in Socket 7, the AMD-K6 processors have different voltage and bus speed requirements from the Intel processors. Before attempting any upgrades, you should check the board documentation or contact the manufacturer to see whether your board meets the necessary requirements. In some cases, a BIOS upgrade also is necessary.

AMD Athlon, Duron, and Athlon XP

The Athlon is AMD's successor to the K6 series. The Athlon was designed as a new chip from the ground up and does not interface via the Socket 7 or Super7 sockets like its previous chips. In the initial Athlon versions, AMD used a cartridge design, called Slot A, almost exactly like that of the Intel Pentium II and III. This was due to the fact that the original Athlons used 512KB of external L2 cache, which was mounted on the processor cartridge board. The external cache ran at one-half core, two-fifths core, or one-third core depending on which speed processor you had. In June 2000, AMD introduced a revised version of the Athlon (codenamed Thunderbird) that incorporates 256KB of L2 cache directly on the processor die. This on-die cache runs at full-core speed and eliminates a bottleneck in the original Athlon systems. Along with the change to on-die L2 cache, the Athlon was also introduced in a version for AMD's own Socket A (Socket 462), which replaced the Slot A cartridge version. The most recent Athlon version, called the Athlon XP, has several enhancements such as 3DNow! Professional instructions, which also include the Intel SSE instructions. The latest Athlon XP models have also returned to the use of 512KB L2 cache, but this time at full processor speed

Although the Slot A cartridge looks a lot like the Intel Slot 1, and the Socket A looks like Intel's Socket 370, the pinouts are completely different and the AMD chips do not work in the same motherboards as the Intel chips. This was by design because AMD was looking for ways to improve its chip architecture and distance itself from Intel. Special blocked pins in either socket or slot design prevent accidentally installing the chip in the wrong orientation or wrong slot.

The Athlon was manufactured in speeds from 500MHz up to 1.4GHz and uses a 200MHz or 266MHz processor (front-side) bus called the EV6 to connect to the motherboard North Bridge chip as well as other processors. Licensed from Digital Equipment, the EV6 bus is the same as that used for the Alpha 21264 processor, later owned by Compaq. The EV6 bus uses a clock speed of 100MHz or 133MHz but double-clocks the data, transferring data twice per cycle, for a cycling speed of 200MHz or 266MHz. Because the bus is 8 bytes (64 bits) wide, this results in a throughput of 8 bytes times 200MHz/266MHz, which amounts to 1.6GBps or 2.1GBps. This bus is ideal for supporting PC1600 or PC2100 DDR memory, which also runs at those speeds. The AMD bus design eliminates a potential bottleneck between the chipset and processor and enables more efficient transfers compared to other processors. The use of the EV6 bus is one of the primary reasons the Athlon and Duron chips perform so well.

The Athlon has a very large 128KB of L1 cache on the processor die and one-half, two-fifths, or one-third core speed 512KB L2 cache in the cartridge in the older versions; 256KB of full-core speed cache in Socket A Athlon and most Athlon XP models; and 512KB of full-core speed cache in the latest Athlon XP models. All PGA socket A versions have the full-speed cache. The Athlon also has support for MMX and the Enhanced 3DNow! instructions, which are 45 new instructions designed to support graphics and sound processing. 3DNow! is very similar to Intel's SSE in design and intent, but the specific instructions are different and require software support. The Athlon XP adds the Intel SSE instructions, which it calls 3DNow! Professional. Fortunately, most companies producing graphics software have decided to support the 3DNow! instructions along with the Intel SSE instructions, with only a few exceptions.

The initial production of the Athlon used 0.25-micron technology, with newer and faster versions being made on 0.18-micron and 0.13-micron processes. The latest versions are even built using copper metal technology, a first in the PC processor business.

In most benchmarks the AMD Athlon compares as equal, if not superior, to the Intel Pentium III. AMD beat Intel to the 1GHz mark by introducing its 1GHz Athlon two days before Intel introduced the 1GHz Pentium III.

AMD Duron

The AMD Duron processor (originally code named Spitfire) was announced in June 2000 and is a derivative of the AMD Athlon processor in the same fashion as the Celeron is a derivative of the Pentium II and III. Basically, the Duron is an Athlon with less L2 cache; all other capabilities are essentially the same. It is designed to be a lower-cost version with less cache but only slightly less performance. In keeping with the low-cost theme, Duron contains 64KB on-die L2 cache and is designed for Socket A, a socket version of the Athlon Slot A. Except for the Duron markings, the Duron is almost identical externally to the Socket A versions of the original Athlon

Because the Duron processor is derived from the Athlon core, it includes the Athlon 200MHz front-side system bus (interface to the chipset) as well as enhanced 3DNow! instructions in Model 3. Model 7 processors include 3DNow! Professional instructions (which include a full implementation of SSE instructions).

AMD Athlon XP

As mentioned earlier, the most recent version of the Athlon is called the Athlon XP. This is basically an improved version of the previous Athlon, with improvements in the instruction set so it can execute Intel SSE instructions and a new marketing scheme that directly competes with the Pentium 4. The latest Athlon XP models have also adopted a larger (512KB) full-speed on-die cache.

AMD uses the term "QuantiSpeed" (a marketing term, not a technical term) to refer to the architecture of the Athlon XP. AMD defines this as including the following:

• A nine-issue superscalar, fully pipelined microarchitecture. This provides more pathways for instructions to be sent into the execution sections of the CPU and includes three floating-point execution units, three integer units, and three address calculation units.

• A superscalar, fully pipelined floating-point calculation unit. This provides faster operations per clock cycle and cures a long-time deficiency of AMD processors versus Intel processors.

• A hardware data prefetch. This gathers the data needed from system memory and places it in the processor's Level 1 cache to save time.

• Improved translation look-aside buffers (TLBs). These enable the storage of data where the processor can access it more quickly without duplication or stalling for lack of fresh information.

These design improvements wring more work out of each clock cycle, enabling a "slower" Athlon XP to beat a "faster" Pentium 4 processor in doing actual work (and play).

The first models of the Athlon XP used the Palomino core, which is also shared by the Athlon 4 mobile (laptop) processor. Later models have used the Thoroughbred core, which was later revised to improve thermal characteristics. The different Thoroughbred cores are sometimes referred to as Thoroughbred-A and Thoroughbred-B. The latest Athlon XP processors use a core with 512KB on-die full-speed L2 cache known as Barton. Additional features include

• 3DNow! Professional multimedia instructions (adding compatibility with the 70 additional SSE instructions in the Pentium III but not the 144 additional SSE2 instructions in the Pentium 4)

• 266MHz or 333MHz FSB

• 128KB Level 1 and 256KB or 512KB on-die Level 2 memory caches running at full CPU speed

• Copper interconnects (instead of aluminum) for more electrical efficiency and less heat

Also new to the Athlon XP is the use of a thinner, lighter organic chip packaging compound similar to that used by recent Intel processors
This packaging allows for a more efficient layout of electrical components. The latest versions of the Athlon XP are made using a new 0.13-micron die process that results in a chip with a smaller die that uses less power, generates less heat, and is capable of running faster as compared to the previous models. The newest 0.13-micron versions of the Athlon XP run at actual clock speeds exceeding 2GHz.

The Athlon XP has been replaced by Socket A versions of the Sempron.

Athlon MP

The Athlon MP is AMD's first processor designed for multiprocessor support. Thus, it can be used in servers and workstations that demand multiprocessor support. The Athlon MP comes in the following three versions, which are similar to various Athlon and Athlon XP models:

• Model 6 (1GHz, 1.2GHz). This model is similar to the Athlon Model 4.
• Model 6 OPGA (1500+ through 2100+). This model is similar to the Athlon XP Model 6.
• Model 8 (2000+, 2200+, 2400+, 2600+). This model is similar to the Athlon XP Model 8.
• Model 10 (2500+, 2800+, 3000+). This model is similar to the Athlon XP Model 8, but with 512KB of L2 cache.

All Athlon MP processors use the same Socket A interface used by later models of the Athlon and all Duron and Athlon XP processors.

The Athlon MP has been replaced by the AMD Opteron. For more details about the Athlon MP, see the AMD website.

Sempron (Socket A)

AMD introduced the Sempron line of processors in 2004 to provide an economy line of processors designed to compete with the Intel Celeron D. As with the Celeron, the Sempron is a chameleon because the Sempron brand is used for both Socket A processors (based on and replacing the Athlon XP series) and Socket 754 processors (based on the Athlon 64). This section discusses Socket A versions of the Sempron.

The Socket A version of the AMD Sempron is a replacement for, and is closely based on, the Athlon XP processor's Thoroughbred (Model 8) and Barton (Model 10) versions. The major features of the Sempron are the same as the Athlon XP. Although the Sempron uses processor numbers that appear similar to those used by the Athlon XP, a Sempron with features similar to an Athlon XP does not use the same processor number. As with other AMD processorsand with Intel processors that use one of Intel's new numbering schemesyou need to look up the specifics for a particular processor to determine its exact features.

Cyrix/IBM 6x86 (M1) and 6x86MX (MII)

The Cyrix 6x86 processor family consists of the now-discontinued 6x86 and the newer 6x86MX processors. They are similar to the AMD-K5 and K6 in that they offer sixth-generation internal designs in a fifth-generation P5 Pentium-compatible Socket 7 exterior.

The Cyrix 6x86 and 6x86MX (renamed MII) processors incorporate two optimized superpipelined integer units and an on-chip floating-point unit. These processors include the dynamic execution capability that is the hallmark of a sixth-generation CPU design. This includes branch prediction and speculative execution.

The 6x86MX/MII processor is compatible with MMX technology to run MMX games and multimedia software. With its enhanced memory-management unit, a 64KB internal cache, and other advanced architectural features, the 6x86MX processor achieves higher performance and offers better value than competitive processors.

Features and benefits of the 6x86 processors include

• Superscalar architecture. Two pipelines to execute multiple instructions in parallel

• Branch prediction. Predicts with high accuracy the next instructions needed

• Speculative execution. Enables the pipelines to continuously execute instructions following a branch without stalling the pipelines

• Out-of-order completion. Lets the faster instruction exit the pipeline out of order, saving processing time without disrupting program flow

The 6x86 incorporates two caches: a 16KB dual-ported unified cache and a 256-byte instruction line cache. The unified cache is supplemented with a small, quarter-K-size, high-speed, fully associative instruction line cache. The improved 6x86MX design quadruples the internal cache size to 64KB, which significantly improves performance.

The 6x86MX also includes the 57 MMX instructions that speed up the processing of certain computing-intensive loops found in multimedia and communication applications.

All 6x86 processors feature support for SMM. This provides an interrupt that can be used for system power management or software transparent emulation of I/O peripherals. Additionally, the 6x86 supports a hardware interface that enables the CPU to be placed into a low-power suspend mode.

The 6x86 is compatible with x86 software and all popular x86 operating systems, including Windows 95/98/Me, Windows NT/2000, OS/2, DOS, Solaris, and Unix. Additionally, the 6x86 processor has been certified Windows 95 compatible by Microsoft.

As with the AMD-K6, there are some unique motherboard and BIOS requirements for the 6x86 processors. The 6x86 processor has been discontinued since Cyrix was absorbed into VIA, but the 6x86MX (MII) design is still sold and supported by VIA. Check motherboard compatibility with the 6x86MX or MII processors before integrating one into an existing Socket 7/Super7 system. A BIOS update might be necessary in some cases. When installing or configuring a system with the 6x86 processors, you have to set the correct motherboard bus speed and multiplier settings. The Cyrix processors are numbered based on a P-Rating scale, which is not the same as the true megahertz clock speed of the processor.

Note that because of the use of the P-Rating system, the actual speed of the chip is not the same number at which it is advertised. For example, the 6x86MX-PR300 is not a 300MHz chip; it actually runs at only 263MHz or 266MHz, depending on exactly how the motherboard bus speed and CPU clock multipliers are set. Cyrix says it runs as fast as a 300MHz Pentium, hence the P-Rating. Personally, I wish it would label the chip at the correct speed and then say that it runs faster than a Pentium at the same speed.

To install the 6x86 processors in a motherboard, you also must set the correct voltage. Normally, the markings on top of the chip indicate which voltage setting is appropriate. Various versions of the 6x86 run at 3.52V (use VRE setting), 3.3V (VR setting), or 2.8V (MMX) settings. The MMX versions use the standard split-plane 2.8V core 3.3V I/O settings.

The Cyrix MII is now sold by VIA Technologies.

VIA C3

The VIA C3 was originally known as the VIA Cyrix III and was designed to fit into the same Socket 370 used by the Pentium III and Celeron III. The initial versions of the C3, code named Joshua and Samuel, had 128KB L1 cache but didn't contain any L2 cache. As a consequence, they had much lower performance than similar 500MHz-class processors. The original Cyrix III/C3, code named Joshua, was developed by former Cyrix engineers after VIA bought Cyrix in late 1998, but the Samuel and subsequent versions are based on the Centaur Winchip (VIA purchased Centaur in 1999). The Samuel was built with a .18-micron process, whereas the Samuel 2 is a development of the Samuel with 64KB of L2 cache on board and is built on a .15-micron process. The Ezra core was the first .13-micron process C3 processor, but it, like previous C3 processors, was not compatible with Tualatin (late Pentium III-compatible) motherboards. The Ezra-T core was the first C3 to reach 1GHz and the first to support Tualatin motherboards. The latest C3 uses the Nehemiah core and features clock speeds over 1GHz and built-in encryption. C3 models feature 100MHz FSB (750MHz and 900MHz models) or 133MHz FSB (733MHz, 800MHz, 866MHz, 933MHz, and higher).

The C3 is fully software compatible with other x86 processors, including Pentium III and Celeron, but its microarchitecture is designed to enhance the performance of most frequently used instructions while reducing the performance of seldom-used instructions. This design feature significantly reduces the die size needed for C3 processors, but it also reduces performance in multimedia and graphics operations. By reducing the die size, the C3 in its Nehemiah version offers typical power consumption of only 11.25 watts, making it the coolest running processor available for Socket 370 applications.

Because of its low power consumption, cool operation, and relatively low performance compared to the Intel Celeron, the C3 processor should be considered primarily for computing appliances, set-top boxes, and portable computers in which small size and low power/cooling requirements (rather than performance) are paramount.

The C3 is also available in an enhanced ball grid array (EBGA) package called the E-series. E-series C3 processors are used for permanent installation on motherboards such as the Mini-ITX ultra-compact form factor designs also produced by VIA.