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Chapter Four CPU Architecture
4.1 Chapter Overview
This chapter discusses history of the 80x86 CPU family and the major improvements occuring along the line. The historical background will help you better understand the design compromises they made as well as understand the legacy issues surrounding the CPU's design. This chapter also discusses the major advances in computer architecture that Intel employed while improving the x861.
4.2 The History of the 80x86 CPU Family
Intel developed and delivered the first commercially viable microprocessor way back in the early 1970's: the 4004 and 4040 devices. These four-bit microprocessors, intended for use in calculators, had very little power. Nevertheless, they demonstrated the future potential of the microprocessor - an entire CPU on a single piece of silicon2. Intel rapidly followed their four-bit offerings with their 8008 and 8080 eight-bit CPUs. A small outfit in Santa Fe, New Mexico, incorporated the 8080 CPU into a box they called the Altair 8800. Although this was not the world's first "personal computer" (there were some limited distribution machines built around the 8008 prior to this), the Altair was the device that sparked the imaginations of hobbyists the world over and the personal computer revolution was born.
Intel soon had competition from Motorola, MOS Technology, and an upstart company formed by disgrunteled Intel employees, Zilog. To compete, Intel produced the 8085 microprocessor. To the software engineer, the 8085 was essentially the same as the 8080. However, the 8085 had lots of hardware improvements that made it easier to design into a circuit. Unfortunately, from a software perspective the other manufacturer's offerings were better. Motorola's 6800 series was easier to program, MOS Technologies' 65xx family was easier to program and very inexpensive, and Zilog's Z80 chip was upwards compatible with the 8080 with lots of additional instructions and other features. By 1978 most personal computers were using the 6502 or Z80 chips, not the Intel offerings.
Sometime between 1976 and 1978 Intel decided that they needed to leap-frog the competition and produce a 16-bit microprocessor that offered substantially more power than their competitor's eight-bit offerings. This initiative led to the design of the 8086 microprocessor. The 8086 microprocessor was not the world's first 16-bit microprocessor (there were some oddball 16-bit microprocessors prior to this point) but it was certainly the highest performance single-chip 16-bit microprocessor when it was first introduced.
During the design timeframe of the 8086 memory was very expensive. Sixteen Kilobytes of RAM was selling above $200 at the time. One problem with a 16-bit CPU is that programs tend to consume more memory than their counterparts on an eight-bit CPU. Intel, ever cogniscent of the fact that designers would reject their CPU if the total system cost was too high, made a special effort to design an instruction set that had a high memory density (that is, packed as many instructions into as little RAM as possible). Intel achieved their design goal and programs written for the 8086 were comparable in size to code running on eight-bit microprocessors. However, those design decisions still haunt us today as you'll soon see.
At the time Intel designed the 8086 CPU the average lifetime of a CPU was only a couple of years. Their experiences with the 4004, 4040, 8008, 8080, and 8085 taught them that designers would quickly ditch the old technology in favor of the new technology as long as the new stuff was radically better. So Intel designed the 8086 assuming that whatever compromises they made in order to achieve a high instruction density would be fixed in newer chips. Based on their experience, this was a reasonable assumption.
Intel's competitors were not standing still. Zilog created their own 16-bit processor that they called the Z8000, Motorola created the 68000, their own 16-bit processor, and National Semicondutor introduced the 16032 device (later to be renamed the 32016). The designers of these chips had different design goals than Intel. Primarily, they were more interested in providing a reasonable instruction set for programmers even if their code density wasn't anywhere near as high as the 8086. The Motorola and National offers even provided 32-bit integer registers, making programming the chips even easier. All in all, these chips were much better (from a software development standpoint) than the Intel chip.
Intel wasn't resting on its laurels with the 8086. Immediately after the release of the 8086 they created an eight-bit version, the 8088. The purpose of this chip was to reduce system cost (since a minimal system could get by with half the memory chips and cheaper peripherals since the 8088 had an eight-bit data bus). In the very early 1980's, Intel also began work on their intended successor to the 8086 - the iAPX432 CPU. Intel fully expected the 8086 and 8088 to die away and that system designers who were creating general purpose computer systems would choose the '432 chip instead.
Then a major event occurred that would forever change history: in 1980 a small group at IBM got the go-ahead to create a "personal computer" along the likes of the Apple II and TRS-80 computers (the most popular PCs at the time). IBM's engineers probably evaluated lots of different CPUs and system designs. Ultimately, they settled on the 8088 chip. Most likely they chose this chip because they could create a minimal system with only 16 Kilobytes of RAM and a set of cheap eight-bit peripheral devices. So Intel's design goals of creating CPUs that worked well in low-cost systems landed them a very big "design win" from IBM.
Intel was still hard at work on the (ill-fated) iAPX432 project, but a funny thing happened - IBM PCs started selling far better than anyone had ever dreamed. As the popularity of the IBM PCs increased (and as people began "cloning" the PC), lots of software developers began writing software for the 8088 (and 8086) CPU, mostly in assembly language. In the meantime, Intel was pushing their iAPX432 with the Ada programming language (which was supposed to be the next big thing after Pascal, a popular language at the time). Unfortunately for Intel, no one was interested in the '432. Their PC software, written mostly in assembly language wouldn't run on the '432 and the '432 was notoriously slow. It took a while, but the iAPX432 project eventually died off completely and remains a black spot on Intel's record to this day.
Intel wasn't sitting pretty on the 8086 and 8088 CPUs, however. In the late 1970's and early 1980's they developed the 80186 and 80188 CPUs. These CPUs, unlike their previous CPU offerings, were fully upwards compatible with the 8086 and 8088 CPUs. In the past, whenever Intel produced a new CPU it did not necessarily run the programs written for the previous processors. For example, the 8086 did not run 8080 software and the 8080 did not run 4040 software. Intel, recognizing that there was a tremendous investment in 8086 software, decided to create an upgrade to the 8086 that was superior (both in terms of hardware capability and with respect to the software it would execute). Although the 80186 did not find its way into many PCs, it was a very popular chip in embedded applications (i.e., non-computer devices that use a CPU to control their functions). Indeed, variants of the 80186 are in common use even today.
The unexpected popularity of the IBM PC created a problem for Intel. This popularity obliterated the assumption that designers would be willing to switch to a better chip when such a chip arrived, even if it meant rewriting their software. Unfortunately, IBM and tens of thousands of software developers weren't willing to do this to make life easy for Intel. They wanted to stick with the 8086 software they'd written but they also wanted something a little better than the 8086. If they were going to be forced into jumping ship to a new CPU, the Motorola, Zilog, and National offerings were starting to look pretty good. So Intel did something that saved their bacon and has infuriated computer architects ever since: they started creating upwards compatible CPUs that continued to execute programs written for previous members of their growing CPU family while adding new features.
As noted earlier, memory was very expensive when Intel first designed the 8086 CPU. At that time, computer systems with a megabyte of memory usually cost megabucks. Intel was expecting a typical computer system employing the 8086 to have somewhere between 4 Kilobytes and 64 Kilobytes of memory. So when they designed in a one megabyte limitation, they figured no one would ever install that much memory in a system. Of course, by 1983 people were still using 8086 and 8088 CPUs in their systems and memory prices had dropped to the point where it was very common to install 640 Kilobytes of memory on a PC (the IBM PC design effectively limited the amount of RAM to 640 Kilobytes even though the 8086 was capable of addressing one megabyte). By this time software developers were starting to write more sophisticated programs and users were starting to use these programs in more sophisticated ways. The bottom line was that everyone was bumping up against the one megabyte limit of the 8086. Despite the investment in existing software, Intel was about to lose their cash cow if they didn't do something about the memory addressing limitations of their 8086 family (the 68000 and 32016 CPUs could address up to 16 Megbytes at the time and many system designers [e.g., Apple] were defecting to these other chips). So Intel introduced the 80286 which was a big improvement over the previous CPUs. The 80286 added lots of new instructions to make programming a whole lot easier and they added a new "protected" mode of operation that allowed access to as much as 16 megabytes of memory. They also improved the internal operation of the CPU and bumped up the clock frequency so that the 80286 ran about 10 times faster than the 8088 in IBM PC systems.
IBM introduced the 80286 in their IBM PC/AT (AT = "advanced technology"). This change proved enourmously popular. PC/AT clones based on the 80286 started appearing everywhere and Intel's financial future was assured.
Realizing that the 80x86 (x = "", "1", or "2") family was a big money maker, Intel immediately began the process of designing new chips that continued to execute the old code while improving performance and adding new features. Intel was still playing catch-up with their competitors in the CPU arena with respect to features, but they were definitely the king of the hill with respect to CPUs installed in PCs. One significant difference between Intel's chips and many of their competitors was that their competitors (noteably Motorola and National) had a 32-bit internal architecture while the 80x86 family was stuck at 16-bits. Again, concerned that people would eventually switch to the 32-bit devices their competitors offered, Intel upgraded the 80x86 family to 32 bits by adding the 80386 to the product line.
The 80386 was truly a remarkable chip. It maintained almost complete compatibility with the previous 16-bit CPUs while fixing most of the real complaints people had with those older chips. In addition to supporting 32-bit computing, the 80386 also bumped up the maximum addressablility to four gigabytes as well as solving some problems with the "segmented" organization of the previous chips (a big complaint by software developers at the time). The 80386 also represented the most radical change to ever occur in the 80x86 family. Intel more than doubled the total number of instructions, added new memory management facilities, added hardware debugging support for software, and introduced many other features. Continuing the trend they set with the 80286, the 80386 executed instructions faster than previous generation chips, even when running at the same clock speed plus the new chip ran at a higher clock speed than the previous generation chips. Therefore, it ran existing 8088 and 80286 programs faster than on these older chips. Unfortunately, while people adopted the new chip for its higher performance, they didn't write new software to take advantage of the chip's new features. But more on that in a moment.
Although the 80386 represented the most radical change in the 80x86 architecture from the programmer's view, Intel wasn't done wringing all the performance out of the x86 family. By the time the 80386 appeared, computer architects were making a big noise about the so-called RISC (Reduced Instruction Set Computer) CPUs. While there were several advantages to these new RISC chips, a important advantage of these chips is that they purported to execute one instruction every clock cycle. The 80386 instructions required a wildly varying number of cycles to execute ranging from a few cycles per instruction to well over a hundred. Although comparing RISC processors directly with the 80386 was dangerous (because many 80386 instructions actually did the work of two or more RISC instructions), there was a general perception that, at the same clock speed, the 80386 was slower since it executed fewer instructions in a given amount of time.
The 80486 CPU introduced two major advances in the x86 design. First, the 80486 integrated the floating point unit (or FPU) directly onto the CPU die. Prior to this point Intel supplied a separate, external, chip to provide floating point calculations (these were the 8087, 80287, and 80387 devices). By incorporating the FPU with the CPU, Intel was able to speed up floating point operations and provide this capability at a lower cost (at least on systems that required floating point arithmetic). The second major architectural advance was the use of pipelined instruction execution. This feature (which we will discuss in detail a little later in this chapter) allowed Intel to overlap the execution of two or more instructions. The end result of pipelining is that they effectively reduced the number of cycles each instruction required for execution. With pipelining, many of the simpler instructions had an aggregate throughput of one instruction per clock cycle (under ideal conditions) so the 80486 was able to compete with RISC chips in terms of clocks per instruction cycle.
While Intel was busy adding pipelining to their x86 family, the companies building RISC CPUs weren't standing still. To create ever faster and faster CPU offerings, RISC designers began creating superscalar CPUs that could actually execute more than one instruction per clock cycle. Once again, Intel's CPUs were perceived as following the leaders in terms of CPU performance. Another problem with Intel's CPU is that the integrated FPU, though faster than the earlier models, was significantly slower than the FPUs on the RISC chips. As a result, those designing high-end engineering workstations (that typically require good floating point hardware support) began using the RISC chips because they were faster than Intel's offerings.
From the programmer's perspective, there was very little difference between an 80386 with an 80387 FPU and an 80486 CPU. There were only a handful of new instructions (most of which had very little utility in standard applications) and not much in the way of other architectural features that software could use. The 80486, from the software engineer's point of view, was just a really fast 80386/80387 combination.
So Intel went back to their CAD3 tools and began work on their next CPU. This new CPU featured a superscalar design with vastly improved floating point performance. Finally, Intel was closing in on the performance of the RISC chips. Like the 80486 before it, this new CPU added only a small number of new instructions and most of those were intended for use by operating systems, not application software.
Intel did not designate this new chip the 80586. Instead, they called it the Pentium Processor4. The reason they discontinued referring to processors by number and started naming them was because of confusion in the marketplace. Intel was not the only company producing x86 compatible CPUs. AMD, Cyrix, and a host of others were also building and selling these chips in direct competition with Intel. Until the 80486 came along, the internal design of the CPUs were relatively simple and even small companies could faithfully reproduce the functionality of Intel's CPUs. The 80486 was a different story altogether. This chip was quite complex and taxed the design capabilities of the smaller companies. Some companies, like AMD, actually licensed Intel's design and they were able to produce chips that were compatible with Intel's (since they were, effectively, Intel's chips). Other companies attempted to create their own version of the 80486 and fell short of the goal. Perhaps they didn't integrate an FPU or the new instructions on the 80486. Many didn't support pipelining. Some chips lacked other features found on the 80486. In fact, most of the (non-Intel) chips were really 80386 devices with some very slight improvements. Nevertheless, they called these chips 80486 CPUs.
This created massive confusion in the marketplace. Prior to this, if you'd purchased a computer with an 80386 chip you knew the capabilities of the CPU. All 80386 chips were equivalent. However, when the 80486 came along and you purchased a computer system with an 80486, you didn't know if you were getting an actual 80486 or a remarked 80386 CPU. To counter this, Intel began their enormously successful "Intel Inside" campaign to let people know that there was a difference between Intel CPUs and CPUs from other vendors. This marketing campaign was so successful that people began specifying Intel CPUs even though some other vendor's chips (i.e., AMD) were completely compatible.
Not wanting to repeat this problem with the 80586 generation, Intel ditched the numeric designation of their chips. They created the term "Pentium Processor" to describe their new CPU so they could trademark the name and prevent other manufacturers from using the same designation for their chip. Initially, of course, savvy computer users griped about Intel's strong-arm tactics but the average user benefited quite a bit from Intel's marketing strategy. Other manufacturers release their own 80586 chips (some even used the "586" designation), but they couldn't use the Pentium Processor name on their parts so when someone purchased a system with a Pentium in it, they knew it was going to have all the capabilities of Intel's chip since it had to be Intel's chip. This was a good thing because most of the other '586 class chips that people produced at that time were not as powerful as the Pentium.
The Pentium cemented Intel's position as champ of the personal computer. It had near RISC performance and ran tons of existing software. Only the Apple Macintosh and high-end UNIX workstations and servers went the RISC route. Together, these other machines comprised less than 10% of the total desktop computer market.
Intel still was not satisfied. They wanted to control the server market as well. So they developed the Pentium Pro CPU. The Pentium Pro had a couple of features that made it ideal for servers. Intel improved the 32-bit performance of the CPU (at the expense of its 16-bit performance), they added better support for multiprocessing to allow multiple CPUs in a system (high-end servers usually have two or more processors), and they added a handful of new instructions to improve the performance of certain instruction sequences on the pipelined architecture. Unfortunately, most application software written at the time of the Pentium Pro's release was 16-bit software which actually ran slower on the Pentium Pro than it did on a Pentium at equivalent clock frequencies. So although the Pentium Pro did wind up in a few server machines, it was never as popular as the other chips in the Intel line.
The Pentium Pro had another big strike against it: shortly after the introduction of the Pentium Pro, Intel's engineers introduced an upgrade to the standard Pentium chip, the MMX (multimedia extension) instruction set. These new instructions (nearly 60 in all) gave the Pentium additional power to handle computer video and audio applications. These extensions became popular overnight, putting the last nail in the Pentium Pro's coffin. The Pentium Pro was slower than the standard Pentium chip and slower than high-end RISC chips, so it didn't see much use.
Intel corrected the 16-bit performance in the Pentium Pro, added the MMX extensions and called the result the Pentium II5. The Pentium II demonstrated an interesting point. Computers had reached a point where they were powerful enough for most people's everyday activities. Prior to the introduction of the Pentium II, Intel (and most industry pundits) had assumed that people would always want more power out of their computer systems. Even if they didn't need the machines to run faster, surely the software developers would write larger (and slower) systems requiring more and more CPU power. The Pentium II proved this idea wrong. The average user needed email, word processing, Internet access, multimedia support, simple graphics editing capabilities, and a spreadsheet now and then. Most of these applications, at least as home users employed them, were fast enough on existing CPUs. The applications that were slow (e.g., Internet access) were generally beyond the control of the CPU (i.e., the modem was the bottleneck not the CPU). As a result, when Intel introduced their pricey Pentium II CPUs, they discovered that system manufacturers started buying other people's x86 chips because they were far less expensive and quite suitable for their customer's applications. This nearly stunned Intel since it contradicted their experience up to that point.
Realizing that the competition was capturing the low-end market and stealing sales away, Intel devised a low-cost (lower performance) version of the Pentium II that they named Celeron6. The initial Celerons consisted of a Pentium II CPU without the on-board level two cache. Without the cache, the chip ran only a little bit better than half the speed of the Pentium II part. Nevertheless, the performance was comparable to other low-cost parts so Intel's fortunes improved once more.
While designing the low-end Celeron, Intel had not lost sight of the fact that they wanted to capture a chunk of the high-end workstation and server market as well. So they created a third version of the Pentium II, the Xeon Processor with improved cache and the capability of multiprocessor more than two CPUs. The Pentium II supports a two CPU multiprocessor system but it isn't easy to expand it beyond this number; the Xeon processor corrected this limitation. With the introduction of the Xeon processor (plus special versions of Unix and Windows NT), Intel finally started to make some serious inroads into the server and high-end workstation markets.
You can probably imagine what followed the Pentium II. Yep, the Pentium III. The Pentium III introduced the SIMD (pronounced SIM-DEE) extensions to the instruction set. These new instructions provided high performance floating point operations for certain types of computations that allow the Pentium III to compete with high-end RISC CPUs. The Pentium III also introduced another handful of integer instructions to aid certain applications.
With the introduction of the Pentium III, nearly all serious claims about RISC chips offering better performance were fading away. In fact, for most applications, the Intel chips were actually faster than the RISC chips available at the time. Next, of course, Intel introduced the Pentium IV chip (it was running at 2 GHz as this was being written, a much higher clock frequency than its RISC contemporaries). An interesting issues concerning the Pentium IV is that it does not execute code faster than the Pentium III when running at the same clock frequency (it runs slower, in fact). The Pentium IV makes up for this problem by executing at a much higher clock frequency than is possible with the Pentium III. One would think that Intel would soon own it all. Surely by the time of the Pentium V, the RISC competition wouldn't be a factor anymore.
There is one problem with this theory: even Intel is admiting that they've pushed the x86 architecture about as far as they can. For nearly 20 years, computer architects have blasted Intel's architecture as being gross and bloated having to support code written for the 8086 processor way back in 1978. Indeed, Intel's design decisions (like high instruction density) that seemed so important in 1978 are holding back the CPU today. So-called "clean" designs, that don't have to support legacy applications, allow CPU designers to create high-performance CPUs with far less effort than Intel's. Worse, those decisions Intel made in the 1976-1978 time frame are beginning to catch up with them and will eventually stall further development of the CPU. Computer architects have been warning everyone about this problem for twenty years; it is a testament to Intel's design effort (and willingness to put money into R&D) that they've taken the CPU as far as they have.
The biggest problem on the horizon is that most RISC manufacturers are now extending their architectures to 64-bits. This has two important impacts on computer systems. First, arithmetic calculations will be somewhat faster as will many internal operations and second, the CPUs will be able to directly address more than four gigabytes of main memory. This last factor is probably the most important for server and workstation systems. Already, high-end servers have more than four gigabytes installed. In the future, the ability to address more than four gigabytes of physical RAM will become essential for servers and high-end workstations. As the price of a gigabyte or more of memory drops below $100, you'll see low-end personal computers with more than four gigabytes installed. To effectively handle this kind of memory, Intel will need a 64-bit processor to compete with the RISC chips.
Perhaps Intel has seen the light and decided it's time to give up on the x86 architecture. Towards the middle to end of the 1990's Intel announced that they were going to create a partnership with Hewlet-Packard to create a new 64-bit processor based around HP's PA-RISC architecture. This new 64-bit chip would execute x86 code in a special "emulation" mode and run native 64-bit code using a new instruction set. It's too early to tell if Intel will be successful with this strategy, but there are some major risks (pardon the pun) with this approach. The first such CPUs (just becoming available as this is being written) run 32-bit code far slower than the Pentium III and IV chips. Not only does the emulation of the x86 instruction set slow things down, but the clock speeds of the early CPUs are half the speed of the Pentium IVs. This is roughly the same situation Intel had with the Pentium Pro running 16-bit code slower than the Pentium. Second, the 64-bit CPUs (the IA64 family) rely heavily on compiler technology and are using a commercially untested architecture. This is similar to the situation with the iAPX432 project that failed quite miserably. Hopefully Intel knows what they're doing and ten years from now we'll all be using IA64 processors and wondering why anyone ever stuck with the IA32. On the other hand, hopefully Intel has a back-up plan in case the IA64 intiative fails.
Intel is betting that people will move to the IA64 when they need 64-bit computing capabilities. AMD, on the other hand, is betting that people would rather have a 64-bit x86 processor. Although the details are sketchy, AMD has announced that they will extend the x86 architecture to 64 bits in much the same way that Intel extend the 8086 and 80286 to 32-bits with the introduction of the the 80386 microprocessor. Only time will tell if Intel or AMD (or both) are successful with their visions.
1Note that Intel wasn't the inventor of most of these new technological advances. They simply duplicated research long since commercially employed by mainframe designers.
2Prior to this point, commerical computer systems used multiple semiconductor devices to implement the CPU.
3Computer aided design.
4Pentium Processor is a registered trademark of Intel Corporation. For legal reasons Intel could not trademark the name Pentium by itself, hence the full name of the CPU is the "Pentium Processor".
5Interestingly enough, by the time the Pentium II appeared, the 16-bit efficiency was no longer a facter since most software was written as 32-bit code.
6The term "Celeron Processor" is also an Intel trademark.
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