I486

The Intel 486, officially named i486 and also known as 80486, is a higher-performance follow-up to the Intel 386 microprocessor. The i486 was introduced in 1989 and was the first tightly pipelined x86 design as well as the first x86 chip to use more than a million transistors, due to a large on-chip cache and an integrated floating-point unit. It represents a fourth generation of binary compatible CPUs since the original 8086 of 1978.
A 50 MHz i486 executes around 40 million instructions per second on average and is able to reach 50 MIPS peak performance, approximately twice as fast as the i386 or 80286 per clock cycle, thanks to its five-stage pipeline with all stages bound to a single cycle. The on chip enhanced FPU unit was also significantly faster than the 80387 per cycle.
Background.
The i486 was announced at Spring Comdex in April 1989. At the announcement, Intel stated that samples would be available in the third quarter of 1989 and production quantities would ship in the fourth quarter of 1989. The first i486-based PCs were announced in late 1989, but some advised that people wait until 1990 to purchase an i486 PC because there were early reports of bugs and software incompatibilities.
Improvements.
The instruction set of the i486 is very similar to its predecessor, the i386, with the addition of only a few extra instructions, such as CMPXCHG which implements a compare-and-swap atomic operation and XADD, a fetch-and-add atomic operation returning the original value (unlike a standard ADD which returns flags only).
From a performance point of view, the architecture of the i486 is a vast improvement over the i386. It has an on-chip unified instruction and data cache, an on-chip floating-point unit (FPU) and an enhanced bus interface unit. Due to the tight pipelining, sequences of simple instructions (such as ALU reg,reg and ALU reg,im) could sustain a single-clock-cycle throughput (one instruction completed every clock). These improvements yielded a rough doubling in integer ALU performance over the 386 at the same clock rate. A 16-MHz i486 therefore had a performance similar to a 33-MHz i386, and the older design had to reach 50 MHz to be comparable with a 25-MHz i486 part.
Differences between i386 and i486.
Just as in the i386, a simple flat 4 GB memory model could be implemented by setting all "segment selector" registers to a neutral value in protected mode, or setting (the same) "segment registers" to zero in real mode, and using only the 32-bit "offset registers" (x86-terminology for general CPU registers used as address registers) as a linear 32-bit virtual address bypassing the segmentation logic. Virtual addresses were then normally mapped onto physical addresses by the paging system except when it was disabled. ("Real" mode had no "virtual" addresses.) Just as with the i386, circumventing memory segmentation could substantially improve performance in some operating systems and applications.
On a typical PC motherboard, either four matched 30-pin (8-bit) SIMMs or one 72-pin (32-bit) SIMM per bank were required to fit the i486's 32-bit data bus. The address bus used 30-bits (A31..A2) complemented by four byte-select pins (instead of A0,A1) to allow for any 8/16/32-bit selection. This meant that the limit of directly addressable physical memory was 4 gigabytes as well (230 "32-bit" words = 232 "8-bit" words).
Models.
There are several suffixes and variants. (see Table). Other variants include:
The specified maximal internal clock frequency (on Intel's versions) ranged from 16 to 100 MHz. The 16 MHz i486SX model was used by Dell Computers.
One of the few i486 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 the 486DX-50 was 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, though it remained popular with users of EISA systems. The 486DX-50 was soon eclipsed by the clock-doubled i486DX2, which although running the internal CPU logic at twice the external bus speed (50 MHz), was nevertheless slower due to the external bus running at only 25 MHz. The i486DX2 at 66 MHz (with 33 MHz external bus) was faster than the 486DX-50, overall.
More powerful i486 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 P5 Pentium processor family. Certain steppings of the DX4 also officially supported 50 MHz bus operation, but it was a seldom-used feature.
Other makers of 486-like CPUs.
Processors compatible with the i486 have been produced by other companies such as IBM, Texas Instruments, AMD, Cyrix, UMC, and STMicroelectronics (formerly SGS-Thomson). Some were clones (identical at the microarchitectural level), others were clean room implementations of the Intel instruction set. (IBM's multiple-source requirement is one of the reasons behind its x86 manufacturing since the 80286.) The i486 was, however, covered by many of Intel's patents covering new R&D as well as that of the prior i386. 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.
AMD produced several clones of the i486 using a 40 MHz bus (486DX-40, 486DX/2-80, and 486DX/4-120) which had no equivalent available from Intel, as well as a part specified for 90 MHz, using a 30 MHz external clock, that was sold only to OEMs. The fastest running i486-compatible CPU, the Am5x86, ran at 133 MHz and was released by AMD in 1995. 150 MHz and 160 MHz parts were planned but never officially released.
Cyrix made a variety of i486-compatible processors, positioned at the cost-sensitive desktop and low-power (laptop) markets. Unlike AMD's 486 clones, the Cyrix processors were the result of clean-room reverse engineering. Cyrix's early offerings included the 486DLC and 486SLC, two hybrid chips which plugged into 386DX or SX sockets respectively, and offered 1 KB of cache (versus 8 KB for the then-current Intel/AMD parts). Cyrix also made "real" 486 processors, which plugged into the i486's socket and offered 2 or 8 KB of cache. Clock-for-clock, the Cyrix-made chips were generally slower than their Intel/AMD equivalents, though later products with 8 KB caches were more competitive, if late to market.
The Motorola 68040, while not compatible with the i486, was often positioned as its equivalent in features and performance. Clock-for-clock basis the Motorola 68040 could significantly outperform the Intel 486 chip. However, the i486 had the ability to be clocked significantly faster without suffering from overheating problems. The Motorola 68040 performance lagged behind the later production i486 systems.
Motherboards and buses.
Early i486-based computers were equipped with several ISA slots (using an emulated PC/AT-bus) and sometimes one or two 8-bit-only slots (compatible with the PC/XT-bus). Many motherboards enabled overclocking of these up from the default 6 or 8 MHz to perhaps 16.7 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 10 MHz could sometimes lead to stability problems, at least in systems equipped with SCSI or sound cards.
Some motherboards came equipped with a 32-bit bus called EISA that was backward compatible with the ISA-standard. EISA offered a number of attractive features such as increased bandwidth, extended addressing, IRQ sharing, and card configuration through software (rather than through jumpers, DIP switches, etc.) However, EISA cards were expensive and therefore mostly employed in servers and workstations. Consumer desktops often used the simpler but faster VESA Local Bus (VLB), unfortunately somewhat prone to electrical and timing-based instability; typical consumer desktops had ISA slots combined with a single VLB slot for a video card. VLB was gradually replaced by PCI during the final years of the i486 period. Few Pentium class motherboards had VLB support as VLB was based directly on the i486 bus; it was no trivial matter adapting it to the quite different P5 Pentium-bus. ISA persisted through the P5 Pentium generation and was not completely displaced by PCI until the Pentium III era.
Late i486 boards were normally equipped with both PCI and ISA slots, and sometimes a single VLB slot as well. In this configuration VLB or PCI throughput suffered depending on how buses were bridged. Initially, the VLB slot in these systems was usually fully compatible only with video cards (quite fitting as "VESA" stands for "Video Electronics Standards Association"); VLB-IDE, multi I/O, or SCSI cards could have problems on motherboards with PCI slots. The VL-Bus operated at the same clock speed as the i486-bus (basically "being" a local bus) while 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 50 MHz CPU clocks). Some motherboards limited the PCI clock to the specified maximum of 33 MHz and certain network cards depended on this frequency for correct bit-rates. The ISA clock was typically generated by a divider of the CPU/VLB/PCI clock (as implied above).
One of the earliest complete systems to use the i486 chip was the Apricot VX FT, produced by British hardware manufacturer Apricot Computers. Even overseas in the United States it was popularized as "The World's First 486" in the September 1989 issue of "Byte" magazine (shown right).
Later i486 boards also supported Plug-And-Play, a specification designed by Microsoft that began as a part of Windows 95 to make component installation easier for consumers.
Gaming.
The 486DX2 66 MHz processor was popular on home-oriented PCs during the early to mid 1990s, toward the end of the MS-DOS gaming era. It was often coupled with a VESA Local Bus video card.
The introduction of 3D computer graphics spelled the end of the i486's reign, because 3D graphics make heavy use of floating-point calculations and require a faster CPU cache and more memory bandwidth. Developers began to target the P5 Pentium processor family almost exclusively with x86 assembly language optimizations (e.g., "Quake") which led to the usage of terms like "Pentium-compatible processor" for software requirements. Many of these games required the speed of the P5 Pentium processor family's double-pipelined architecture.
Obsolescence.
The AMD Am5x86, up to 133 MHz, and Cyrix Cx5x86, up to 120 MHz, were the last i486 processors that were often used in late generation i486 motherboards with PCI slots and 72-pin SIMMs that are designed to be able to run Windows 95, and also often used as upgrades for older 80486 motherboards. While the Cyrix Cx5x86 faded quite quickly when the Cyrix 6x86 took over, the AMD Am5x86 was important during the time when the AMD K5 was delayed.
Computers based on the i486 remained popular through the late 1990s, serving as low-end processors for entry-level PCs. Production for traditional desktop and laptop systems ceased in 1998, when Intel introduced the Celeron brand as an modern replacement for the aging chip, though it continued to be produced for embedded systems through the late 2000s.
In the general-purpose desktop computer role, i486-based machines remained in use into the early 2000s, especially as Windows 95, Windows 98, and Windows NT 4.0 were the latest Microsoft operating systems to officially support installation on an i486-based system. However, as Windows 95/98 and Windows NT 4.0 were eventually overtaken by newer operating systems, i486 systems likewise fell out of use. Still, a number of i486 machines remained in use, mostly for backward compatibility with older programs (most notably games), especially since many of them have problems running on newer operating systems. However, DOSBox is also available for current operating systems and provides emulation of the i486 instruction set, as well as full compatibility with most DOS-based programs.
Although the i486 was eventually overtaken by the Pentium for personal computer applications, Intel had continued production for use in embedded systems. In May 2006 Intel announced that production of the i486 would stop at the end of September 2007.