The Intel 486, otherwise known as the 80486, i486 or just 486, was the first tightly pipelined x86 design. Introduced in 1989, it was the first x86 chip that used more than a million transistors, due to a large on-chip cache and an integrated floating point unit. It represents a second generation of 32-bit x86 designs, following the original 32-bit x86 processor, the Intel 80386, and a fourth generation of binary compatible CPUs since the 8086.
(The i486 was so named, without the usual 80-prefix, because of a court ruling that prohibited trademarking numbers like 80486. Later, with the Pentium, Intel dropped number-based naming altogether.)
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 throughput. 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.
The 486 has a 32-bit data bus and a 32-bit address bus. This required either four matched 30-pin (8-bit) SIMMs or one 72-pin (32-bit) 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. 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.
There are several suffixes and variants including:
|Model||Specified max clock||Voltage||L1-Cache||Introduced|
|i486DX (P4)||20,25,33 MHz; 50 MHz||5V||8 KiB WT||April 1989; April 1989; May 1990; June 1991|
|i486SL||20,25,33 MHz||5V or 3.3V||8 KiB WT||Nov 1992|
|i486DXL||?||?||8 KiB WT||?|
|i486SX (P23)||16,20,25 MHz (33 MHz)||5V||8 KiB WT||September 1991 (September 1992)|
|i486DX2 (P24)||40/20, 50/25 MHz (66/33 MHz)||5V||8 KiB WT||March 1992 (August 1992)|
|i486DX-S (P4S)||33 MHz; 50 MHz||5V or 3.3V||8 KiB WT||June 1993|
|i486DX2-S (P24S)||40/20, 50/25 MHz (66/33 MHz)||5V or 3.3V||8 KiB WT||June 1993|
|i486SX-S (P23S)||25,33 MHz||5V or 3.3V||8 KiB WT||June 1993|
|i486SXL||?||?||8 KiB WT||?|
|i486SX2||50/25, 66/33 MHz||5V||8 KiB WT||March 1994|
|IntelDX4 (P24C)||75/25, 100/33 MHz||3.3V||16 KiB WT||March 1994|
|IntelDX4WB||100/33 MHz||3.3V||16 KiB WB||October 1994|
|i486DX2WB (P24D)||50/25, 66/33 MHz||5V||8 KiB WB||October 1994|
|i486DX2 (P24LM)||30/90 MHz; 33/100MHz||2.5-2.9V||8 KiB WT||1994|
WT = Write-Through cache strategy, WB = Write-Back cache strategy
The specified maximum internal clock frequency (on Intels versions) ranged from 16 to 100 MHz. The 16MHz i486SX model was used by Dell Computers but sometimes ridiculed for the fact that it was handily beaten by many 386 systems. One of the few 486 models specified for a 50 MHz bus (486DX-50) initially had overheating problems and was moved to the 0.8 micrometre fabrication process. However, problems continued when installed in local bus systems due to the high bus speed, making it rather unpopular with mainstream consumers as local bus video was considered a requirement at the time. It was soon eclipsed by the clock-doubled i486DX2 which instead ran the CPU logic at twice the external bus speed. However, the 486DX-50 remained popular with users of EISA systems. More powerful 486 iterations such as the OverDrive and DX4 were less popular (the latter available as an OEM part only), as they came out after Intel had released the next generation Pentium. Certain steppings of the DX4 also officially supported 50MHz bus operation but was a seldom used feature.
AMD produced several models of the 486 using the 40MHz bus (486DX-40, 486DX/2-80 & 486DX/4-120) not available from Intel, as well a 90MHz part using a proprietary 30MHz for OEM use only. The fastest running 486 CPU, the Am5x86,ran at 133MHz was released by AMD in 1995. 150MHz and 160MHz parts were planned but never officially released.
Early 486 machines were equipped with several ISA (Industry Standard Architecture) aka "AT-Bus" slots and sometimes one or two 8-bit-only "PC/XT-Bus" 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 slower (at the time) custom VLSI designs. This could give significant performance gains (such as for old video cards moved from a 386 or 286 computer, for example). However, operation beyond 8 or 10MHz could lead to stability problems, particularly in systems equipped with SCSI and/or sound cards.
Some motherboards came equipped with a 32-bit version of the ISA-standard called EISA (Extended Industry Standard Architecture), and offered a number of attractive features such as increased bandwidth, extended addressing and IRQ sharing. However, EISA cards were very expensive and later relegated to the server and workstation arena, being rather quickly supplanted by the simpler but faster (and often buggy) VLB (VESA Local Bus) on consumer desktops. PCI (Peripheral Component Interconnect) then began gradually replacing VLB during the final one or two years of the 486's sale life.
Mature 486 boards were normally equipped with both PCI- and ISA-slots, and sometimes a single VLB-slot as well. In this configuration VLB or PCI though-put sometimes suffered greatly depending on how the buses were bridged. In this setup the VL-Bus slot 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 sometimes had a divider setting available via the BIOS. This could be set to 1/1 or 1/2, sometimes even 2/3 (for 50MHz CPU clocks). Some boards limited the PCI clock to the specified maximum of 33 MHz (certain 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).
Windows 95 signaled the end of the 486 era due to its high memory requirements (16MB to perform as well as Windows 3.x with just 8MB). Many 486 users at that time were running eight 1MB 30-pin SIMMs leaving no available slots for expansion. As 4meg 30-pin SIMMs were still very expensive at that time, it made more sense to buy a Pentium rather than spend a premium on upgrading a system that was nearing the end of its service life. The 486 were used as budget machine for people who could not afford the latest computers until around 2001 when Windows 95 was not supported and Windows 98, ME, NT 4.0, 2000, NT 4.0 and XP required more powerful computers.