Intel 80386 Totally Explained

Everything about 80386 totally explained

The Intel 80386, otherwise known as the Intel386, i386 or just 386, is a microprocessor which has been used as the central processing unit (CPU) of many personal computers since 1986. During its design phase the processor was code-named simply P3, the third-generation processor in the x86 line. The 80386 operated at about 5 million instructions per second (MIPS) to 11.4 MIPS for the 33 MHz model. (External Link

) It was the first x86 processor to have a 32-bit architecture, with a basic programming model that has remained virtually unchanged for over twenty years and remains completely backward compatible. Successively newer implementations of this same architecture have become literally several hundred times faster than the original i386 chip during these years.
It was designed with several very impressive features:

run multiple concurrent multi-user multitasking operating systems ( see here)
page 4GB of memory from 64TB of virtual memory
hardware debugging support
Virtual 8086 mode

Designed and manufactured by Intel, the i386 processor was taped-out (1000 nm) in October of 1985. Intel decided against producing the chip before that date, as the cost of production would have been uneconomical. Full-function chips were first delivered to customers in 1986. Mainboards for 386-based computer systems were highly elaborate and expensive to produce, but were rationalized upon the 386's mainstream adoption. The first personal computer to make use of the 386 was designed and manufactured by Compaq (External Link

), and Andy Grove, Intel's CEO at the time, made the decision to single-source the processor, a decision that was ultimately crucial to both the processor's and Intel's success in the market.
The range of processors compatible with the 80386 is often collectively termed x86 or the i386 architecture; today, Intel prefers the name IA-32 however.
In May 2006 Intel announced that production of the 386 would cease at the end of September 2007. (External Link

) Although it had long been obsolete as a personal computer CPU, Intel, and others, had continued to manufacture the chip for embedded systems, including aerospace technology.

Architecture

The processor was a significant evolution in a long line of processors that stretched back to the Intel 8008. The predecessor of the 80386 was the Intel 80286, a 16-bit processor with a segment-based memory management and protection system. The 80386 added a 32-bit architecture and a paging translation unit, which made it much easier to implement operating systems which used virtual memory.
The 80386 featured three operating modes: real mode, protected mode and virtual mode. The protected mode allowed the use of all the possibilities of the 286 and the protected mode extension of the 386, especially addressing up to 4 GB of memory. Finally, the virtual 8086 mode (or VM86) made it possible to run one or more real mode programs in a protected environment. Though Intel would shortly introduce the 80486 and eventually the Intel Pentium line of processors, the support in the 386 for the 32-bit flat memory model would arguably be the most important feature change for the x86 processor family until AMD released x86-64 in 2003.

i386SX

Later in the i386's production run, Intel introduced the i386SX (internally codenamed P9), which was meant to be a low cost version of the 386 line (a similar situation to the introduction of the 8088 chip, used in the original IBM PC, which was a lower cost version of the 8086). The SX series of chips was 32-bit internally, but had a 16-bit external data bus and a 24 bit address bus; therefore, the processor could only address 16 MB of memory (similar to a Motorola 68000). The original i386 was subsequently renamed the i386DX to avoid confusion. Neither CPU included a math coprocessor (most mainboards included a socket for an i387), though the naming would cause some head-scratching later when the i486 came in a DX variant that did include floating-point capability (which was physically present but disabled in early i486SXs).

i386SL

The i386SL was introduced as an alternative processor for laptop computers. The processor offered several power management options (for example SMM), as well as different "sleep" modes to conserve battery power. It also contained support for an external cache of 16 to 64 KB. The extra functions caused this variant to have over 3 times as many transistors as the i386DX. The i386SL was first available at 20 MHz clockspeed (External Link

), with the 25 MHz model later added. (External Link

)

Embedded variants

An embedded version of the i386SX was produced as the i376. This was later superseded by the i386EX, which was used in space applications such as the Hubble Space Telescope.

Business importance

The first PC company to design and manufacture a PC based on the 386 was Compaq, rather than IBM, which had been dominant until that time. IBM was offered use of the processor, but relied instead on earlier processors to which it had manufacturing rights. The success of the Compaq 386 PC played an important role in legitimizing the PC "clone" industry, and to establishing Intel (and Microsoft) as central component suppliers to it.
From a business perspective, the i386 was significant because it was the first significant microprocessor to be single-sourced – it was available only from Intel Corp. Prior to this, the difficulty of making chips and the uncertainty of reliable supply required that any mass-market semiconductor be multi-sourced, that is, made by two or more manufacturers, the second and subsequent ones manufacturing under license from the designer. Single-sourcing the i386 allowed Intel greater control over its development and substantially greater profits in later years. However, AMD introduced its compatible Am386 processor in March 1991 after overcoming legal obstacles, thus ending Intel's monopoly on 32-bit 386-compatible processors. Later Cyrix offered Cx486SLC and Cx486DLC processors, pin-compatible with 80386SX and 80386DX respectively.

Early Problems

Early in production, Intel found a bug that could cause a system to unexpectedly halt when running 32-bit software. Not all of the processors already manufactured were affected, so Intel tested its inventory. Processors that were found to be bug-free were marked with a double-sigma (ΣΣ), whilst affected processors were marked "16 BIT S/W ONLY" and sold as otherwise good-parts, since at the time 32 bit software wasn't relevant for most users. Such chips are now extremely rare.

Upgrades

Intel offered a modified version of its later 80486DX in 80386 packaging, branded as the Intel RapidCAD, to provide an upgrade path for 80386 users. The upgrade worked as a pair of chips that replaced both the 386 and 387, although since the 80486DX contained an FPU itself the chip that replaced the 387 served no purpose except to appear like a coprocessor, so that the system board would be configured correctly. The CAD branding can be explained by the massive increase in floating point performance offered, whilst integer performance increase was around 30%.

Further Information

Get more info on '80386'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://intel_80386.totallyexplained.com">Intel 80386 Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.