Intel 80486
From Wikipedia, the free encyclopedia
| 486 Central processing unit |
|
 The exposed die of an Intel 80486DX2 microprocessor. |
|
| Produced: | From 1989 to 2007 |
| Common manufacturers: | |
| Max CPU clock: | 16 MHz to 133 MHz |
| FSB speeds: | 16 MHz to 50 MHz |
| Min feature size: | 0.8 µm |
| Instruction set: | x86 (i386) |
| Cores: | 1 |
| Sockets: | |
The Intel486[1] brand encompasses Intel's family of i486 (incl. i487) CPUs - the second generation of 32-bit x86 microprocessors, and the first truly pipelined x86 design. Their predecessor was the Intel386, the very first 32-bit x86 processor. The first line of 486 processors was introduced in 1989 containing 1.2 million transistors (800 nanometer scale). The i486 was so named without the usual 80-prefix, because of a court ruling that prohibited trademarking numbers (like 80486). Intel dropped number-based naming altogether with the successor to the i486 – the Pentium processor.
Contents |
[edit] Improvements
The instruction set of the i486 is very similar to its predecessor, the Intel 80386, with the addition of only a few extra instructions, such as CMPXCHG which executes the Compare-and-swap atomic operation and the XADD which executes the Fetch-and-add atomic operation. Though many atomic instructions have existed since the 8086/8088, they did not correspond to the atomic instructions implemented in certain RISC processors, which made it harder to port some applications from these processors.
From a performance point of view, the architecture of the i486 is a vast improvement over the 80386. It has an on-chip unified instruction and data cache, an on-chip floating-point unit (FPU), and an enhanced bus interface unit. In addition, simple instructions (such as ALU reg,reg) has a single clock troughput. These improvements yield a rough doubling in performance over the 386 at the same clock rate. A 386 (or 286) chip therefore has to reach 50 MHz to be comparable with low end parts in the 486 series.
[edit] Differences between the 386 and 486
- An 8192-bytes on chip SRAM cache stores the most commonly used instructions and data. The 386 supported a slower off-chip cache though.
- Tightly coupled pipelining allows the 486 to complete a simple instruction like ALU reg,reg or ALU reg,im every clock cycle. The 386 needed two clock cycles for this.
- Integrated FPU (disabled in SX models) with a dedicated local bus gives faster floating point calculations compared to the i386+i387 combination.
- Improved MMU performance.
The 486 has a 32-bit data bus and a 32-bit address bus. This required either four matched 30-pin SIMMs or one 72-pin SIMM on a typical PC motherboard. The 32-bit address bus means that 4 GiB of memory can be directly addressed.
The Intel project manager for the 80486 was Pat Gelsinger.
In May 2006 Intel announced that production of the 80486 would cease at the end of September 2007. [1] Although the chip had long been obsolete for personal computer applications, Intel had continued production for use in embedded systems. The 80486 was able to compute at up to 41 million instructions per second. [2]
[edit] Models
There are several suffixes and variants including:
- i486DX — The original chip (without any clock doubling).
- i486SX — an i486DX with the FPU part disabled or missing. Early variants were parts with disabled (defective) FPUs, later versions had the FPU removed from the die to reduce area and hence cost.
- i486DX2 — the internal processor clock runs at twice the clock rate of the external bus clock.
- i486SX2 — i486DX2 with the FPU disabled.
- i486SL — low power version of the i486DX, reduced VCore, power conservation circuitry - mainly for use in portable computers.
- i486SL-NM — i486SL based on i486SX
- i487 — i486DX with a slightly different pinout sold as an FPU to i486SX systems; it was widely documented that i487 when installed completely disabled the existing i486SX on the motherboard.
- i486 OverDrive — i486SX, i486SX2, i486DX2 or i486DX4. Marked as upgrade processors, some models had different pinouts or voltage handling abilities from 'standard' chips of the same speed stepping.
- i486DX4 — designed to run at triple clock rate (not quadruple as often believed; the DX3, which was meant to run at 2.5x the clock speed, was never released).
Maximum specified internal clock rates ranged from 16 to 133 MHz (16, 20, 25, 33, 40, 50, 66, 75, 100, 120, and 133 MHz). The only 486 specified for a 50 MHz bus had reliability problems with many boards and components because of this high bus speed, making it rather unpopular. It was soon replaced by the clock-doubled i486DX2 which instead ran the CPU logic at twice the external bus speed. A slower i486DX2-50 was also sold, with a bus speed of 25MHz. More powerful 486 iterations such as the OverDrive and DX4 were less popular, as they came out after Intel had released the next generation Pentium.
[edit] Gaming
The 486DX2 66 MHz was the most widespread high-end 486 version. For many players of video games during the early and mid 1990s, towards the end of the MS-DOS gaming era, it was often coupled with 8 - 16 MB RAM and a VLB video card. This configuration was capable of running every title available for several years after its release, making it a "sweet spot" in CPU performance and longevity. The introduction of 3D graphics spelled the end of the 486's reign, because of its heavy use of floating point calculations and the need for faster cache and more memory bandwidth. Developers also began to target the Pentium almost exclusively with assembly optimizations (e.g. Quake).
[edit] Competitive alternatives
486 compatible processors have been produced by other companies such as IBM, Texas Instruments, AMD, Cyrix, UMC, and Chips and Technologies. Some are near duplicates in terms of specifications and performance, some are not. The 486 was, however, covered by many of Intel's 386 patents as well as some of its own. Intel and IBM have broad cross-licenses of these patents, and AMD was granted rights to the relevant patents in the 1995 settlement of a lawsuit between the companies.[3]
The fastest running 486 CPU, the Am5x86, was released by AMD in 1995.
[edit] Motherboards and buses
Early 486 machines were equipped with several ISA-slots (Industry Standard Architecture) which was noting but alternatively named 16-bit AT-bus compatible slots (plus one or two 8-bit-only PC/XT slots). Many motherboards enabled overclocking of these up from the default 6 or 8 MHz to perhaps 16.5 or 20 MHz (half the i486 bus clock) in a number of steps, often from within the BIOS setup. Especially older peripheral cards normally worked well at such speeds as they often used standard MSI chips instead of (the at the time) slower custom VLSI designs. This could give significant performance gains (such as for old video cards moved from a 386 or 286 computer, for example).
Some motherboards came equipped with a 32 bit version of the ISA-standard called EISA (Extended Industry Standard Architecture). However, EISA were rather quickly supplanted by the simpler but faster VLB (VESA Local Bus) intended for fast video cards and hard drive controllers. PCI (Peripheral Component Interconnect) then began gradually replacing VLB.
Mature 486 boards were normally equipped with both PCI- and ISA-slots, and sometimes a single VLB-slot as well. In this configuration VLB sometimes suffered performance-wise and was usually only compatible with video cards (VLB-IDE, multi I/O, or SCSI cards often had problems on motherboards with PCI slots). The VL-Bus operated at the same clock speed as the i486-bus (basically being a local 486-bus). The PCI bus also usually depended on the i486 clock but had a divider. This could be set to 1/1, but some boards limited the PCI clock to the specified maximum of 33 MHz (some network cards assumed 33MHz and would produce erroneous bit-rates in an overclocked slot). The ISA clock was either generated by a separate clock generator or by a divider of the VLB/PCI clock.
One of the earliest complete systems to use the 80486 chip was the Apricot VX FT, produced by United Kingdom hardware manufacturer Apricot Computers. Even overseas in the United States it drew attention as "The World's First 486" in a popular September 1989 issue of Byte magazine (shown right).
Later 486 boards also supported Plug-And-Play, the Microsoft technology that began as a part of Windows 95 designed to make component installation easier for consumers.
[edit] References
- ^ Microprocessor Hall of Fame. Intel. Retrieved on 2007-08-11.
[edit] See also
- List of Intel microprocessors
- Motorola 68040, although not compatible, often positioned as the Motorola equivalent to the Intel 80486 in terms of performance and features.
- "80486 System Architecture" published by Mindshare (pdf)
[edit] Notes
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
[edit] External links
|
|||||||||||||||||
