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Which CPU Should You Purchase

by Lloyd Borrett
Technical Cornucopia, February 1990

In recent months many conflicting stories have been circulating about which Intel microprocessor you should have in your next PC. IBM and Microsoft made an announcement at Comdex in November that their "Platform for the 90s" was to be based on the capabilities of the Intel 80386SX, 80386, 80486 and future Intel family microprocessors. Intel themselves have been running an advertising campaign in the USA which stresses you should always choose an 80386SX over an 80286 or earlier microprocessor.

All of this has left the average PC purchaser bewildered. So what are the differences between the various Intel microprocessors, and why are so many major players in the industry now suggesting the Intel 80286 and earlier microprocessors have had their day? Please read on...

In the beginning...

When IBM released the IBM PC back in August 1981 it was the first mass-produced 16-bit personal computer. It was a big break away from the existing 8-bit Zilog Z-80, Motorola 6800 and Intel 8080 based systems then dominating the market. And yet at the same time the IBM PC was also an 8-bit machine.

Instead of choosing the Intel 8086 chip, IBM chose the Intel 8088 chip. The difference was that although both internally were full 16-bit processors with the same instruction set and able to address 1 MByte of memory, the 8088 talked to the outside world via an 8-bit data bus, while the 8086 used a 16-bit data bus. The result was a 50% reduction in I/O performance. But more significantly, it meant a relatively low cost system could be built using the cheaper 8-bit peripheral chips, and that this new system could run a new generation of software.

Enter the Intel 80286

In August 1984, IBM announced the IBM PC/AT. The AT contains an Intel 80286 microprocessor which is able to address up to 16 MBytes of memory. This 16-bit microprocessor operates in either of two incompatible modes real (compatible) mode and protected mode.

Real mode is designed to run programs in exactly the same manner as they run on the 8088/8086 microprocessor. Thus when running DOS, real mode is used. The 80286 runs as nothing but a "fast 8088/8086".

However, Intel's designers of the 80286 wanted it to be much more than that. They wanted to add such features as memory management, memory protection, and the ring protection mechanism, which allows an operating system to protect one application from another. But they weren't able to do this while remaining fully compatible with the 8088/8086 microprocessor, so they added the protected mode.

When the Intel 80286 is running in protected mode it provides these important new features, but it will not run programs written for the 8088/8086. Unfortunately , the designers of the 80286 didn't appreciate the importance of compatibility in the PC marketplace. They designed the 80286 so that it can run in either mode but can't switch back and forth at will. Thus the extended addressing range and protected mode architecture of the Intel 80286 sit unused when running DOS.

32 bit PCs arrive

In October 1985 Intel introduced the 80386, a 32-bit microprocessor. The Intel 80386 addresses 4 gigabytes of memory, compared with 1 megabyte possible on the 8088/8086. With virtual memory, the 80386 addresses 64 tetrabytes, or 70 billion bytes of memory. The magnitude of this is impossible to visualise easily. It is 100 million times the memory available under DOS.

To the operating system designer the 80386 is a magical device. It's capable of running many operating systems simultaneously. It can run 386 software, 286 software and DOS software at the same time. With its Virtual Monitor 8086 mode, the 80386 can run multiple 8086 applications as if each had its own 8086.

The Intel 80386 has a segmented architecture similar to the 80286. But 80286 segments are limited to 64 KBytes while on the 80386 segments can be as large as 4 gigabytes. Writing programs that use large arrays and other data structures is easier using large segments, and because compilers have a hard time generating optimal segmented code, converting large 8086/80286 programs to the 80386 can produce dramatic increases in performance.

During 1986 we saw Compaq and others introduce PCs using the 80386 microprocessor. But without an operating system to exploit the features of the 80386 these PCs were nothing but "super fast 8088/8086" systems.

Something was missing

Software and hardware developers were very aware that there was a lot of power going to waste inside every 80286, and much more in every 80386. What was needed was a new operating system that would let us exploit the power of these microprocessors, while maintaining a degree of compatibility with DOS.

The technical hurdles to developing a multitasking operating system that is at least nominally compatible with earlier versions of DOS are formidable. In fact Microsoft had started working on a multitasking, real mode only version of DOS back in January 1983. Eventually it was released as a special OEM product called MS-DOS version 4.0 (not to be confused with today's PC/MS-DOS 4.00). This project continued and in 1985 IBM and Microsoft signed a Joint Development Agreement that provided for the design and development of what was to become Operating System/2 (OS/2).

In April 1987, along with the introduction of its new PS/2 of systems, IBM announced that together with Microsoft they were developing this new operating system called OS/2, and that they expected to make it available before the end of 1988.

Meanwhile, back in the real world...

During late 1987 and through into 1988 we saw a rapid change in the PC marketplace. Most large purchases of personal computers were of 80286 based systems. Very few corporate and government accounts were now purchasing 8088/8086 based systems.

Intel had licensed other chip manufacturers to produce 80286 microprocessors. These second source manufacturers pushed along increases in the clock speeds of 80286 based systems. 80286 based systems were delivering about 5-10 times the performance of, and cost about half as much as, early 8088/8086 systems. As IBM and Microsoft had promised that OS/2 would run on 80286 based systems, the purchase of such an 80286 based system implied a longer effective working life for the system.

80386 based systems were significantly more expensive and typically only purchased for demanding applications such as LAN file servers, CAD/CAM and desktop publishing.

Enter the Intel 80386SX

In 1988 we saw the first PCs using the Intel 80386SX, a half-way house between the 80286 and 80386 microprocessors. The Intel 80386SX is to the 80386 as the original 8088 is to the 8086. Internally the 80386SX is a full 32-bit 80386 processor with the same instruction set as the real 80386. But as far as the outside world is concerned, it's a 16-bit 80286 like chip. It was created for two reasons.

Firstly, Intel having licensed other chip makers to manufacture the 80286 chip found these other manufacturers had not only been gradually increasing the clock speed of the 80286 but they had also been lowering prices. Intel's share of the 80286 market had been gradually declining. By introducing a baby 386 chip that allowed an 80386 work-alike to be produced at 80286 system prices, Intel could increase sales and profitability.

Secondly, the 80286 is actually a flawed design. It's okay for use as a faster 8088/8086 but it doesn't support multi-user multi-tasking software as well as it could. The 80386 is much, much better at this.

80386 /80386SX software

A few software developers realised that it was possible to exploit some of the features of the Intel 80386 instruction set in programs still running under DOS. Thus we saw the introduction of many 80386 specific programs during 1988 and 1989.

Some of these programs such as Borland's Paradox 386 database delivered significant performance improvements over the non-80386 versions. Other programs such as Quartdeck's QEMM 386 and Qualitas' 386Max Professional used features of the 80386 to extend the limits of DOS.

Microsoft even produced an 80386 specific version of their Windows graphical user interface, and announced plans to make Windows more closely resemble the Presentation Manager graphical user interface of OS/2.

OS/2 has a lot of code built into it to handle the multi-user multi-tasking and memory management instructions that are built into the 80386 but missing from the 80286. That code means increased overhead. Increased overhead means reduced speed.

The OS/2 developers, and many application software developers, were already making noises about how much better a version of OS/2 that allowed them to fully exploit the potential of the 80386 would be. It soon became common knowledge that such a product was in the pipeline.

Already some of the more far sighted system purchasers realised the significance of the Intel 80386SX and proceeded to purchase 80386SX based systems in preference to 80286 based systems. During 1989 we saw the sales of 80386SX and 80386 increase dramatically as more and more people realised the benefits of this more powerful platform.

And now the Intel 80486

Towards the end of 1989 we saw the first Intel 80486 based system shipped. The Intel 80486 microprocessor is essentially a new, very high-performance version of the 80386.

The 80486 typically provides two to three times the performance of an 80386. Frequent instructions execute in one cycle which results in performance levels similar to those found in RISC chips.

The 80486 chip includes integrated memory management with paging, floating point and cache memory units along with a high performance integer unit. Previously these functions resided on other support chips such as the 80387 maths coprocessors. Now one chips does it all, and it's much faster because of the reduced inter-chip communication delays.

Intel also added improved multiprocessor support to the 80486. New instructions speed manipulation of memory based semaphores. On-chip hardware ensures cache consistency and provides hooks for multilevel caches. Thus we can expect to see more systems similar to the Compaq SystemPro that fully exploit the multiprocessor capabilities of the 80486. Software developers are also looking at ways to exploit these multiprocessor capabilities. We are now seeing the power of typical minicomputers and mainframes being put into a desktop personal computer.

Purchasing guidelines

As the gap between entry level and high end personal computers becomes wider so it becomes even more difficult to make the right choice. But there are simple guidelines that can be applied.

  • If you need power and performance then you will be looking to 80486 and 80386 based systems at the top end of the range.
  • If you know you will be migrating to the OS/2 operating system in time then you should only be considering 80386SX, 80386 and 80486 based systems.
  • If you are already networking your systems together, or planning to, then it's fairly certain you will benefit greatly from the use of 80386SX workstations in preference to 80286 based workstations.
  • If you are running a medium size application that today demands an 80286, then the chances are that as you grow you will exceed the limits of the 80286. You are less likely to exceed the limits of an 80386SX, and if a 386 version of the software becomes available you will greatly benefit.
  • If you only want to run simple application stand-alone, but would like to be able to try simple OS/2 applications when they become more common, then get an 80286 based system. But if you can afford the extra money, get an 80386SX anyway. You'll regret not doing so when you realise that like everyone else you too underestimated the power and performance you'd really need.

Why the bias to the 386 family

When the IBM PC was introduced in August 1981 it allowed for 64 KBytes on the motherboard. When the PC/XT was released it allowed for 256 KBytes on the motherboard. Today almost all 8088/8086 based systems are shipped with 640 KBytes and most 80286 based systems ship with 1 MByte of memory.

Very few of today's typical applications will run in a system with less than 640 KBytes of memory. Many software manufacturers are having great difficulty making their programs fit into the 640 KByte limit of DOS and are looking to the features of a new DOS that exploits some of the features of the 386 family.

Some software developers have all but given up on the hope that they can make their applications run under DOS and are only working on OS/2 based applications. Once OS/2 applications that truly exploit the benefits of a multitasking operating system start shipping, you're going to want to run them. And to run these OS/2 applications you really will need the features of an 80386SX, 80386 or 80486. These 386 family microprocessors will become the basis of the next generation of software applications.

More still to come...

As we saw the 80286 replace the 8088/8086, which in turn replaced the early 8-bit microprocessors, so shall we see the 386 family replace the 80286. As the nature of our applications change and demand more resources, and system prices lower, even the 80486 system will become commonplace.

And as to the future. Well we know IBM and Microsoft are working on OS/2 for the Intel i860 RISC chip. Thus we should be able to expect such a product accompanied by a new range of systems based on single and multiple Intel i860s sometime in 1991.

IBM recently showed off a 16 MBit 50 nanosecond memory chip produced on existing production lines. In 1992 we can expect to see personal computers using these chips.

Applications that demand such resources will be developed and will become commonplace. But few, if any, of these applications can be expected to run in the DOS/Windows environment <196> they'll almost certainly use OS/2. At least by purchasing a 386 family computer today, you'll be able to run some of these applications in two years time, thus greatly extending the life of your investment.

Last modified: Saturday, 15 October 2011


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