Intel 80386 manual Highlights

HIGHLIGHTS

porating all of these, the 80386 delivers the highest perfonnance of any currently available microprocessor.

The 80386 is implemented in Intel's CHMOS III, a semiconductor process that combines the high frequency of HMOS with the modest power requirements of CMOS. Using 1.5 microngeome- tries and two metal layers, the 80386 packs over 275,000 transistors into a single chip. Both 12 and 16 MHz versions of the 80386 are initially available; running without wait states, the 16 MHz part can achieve sustained execution rates of 34 million instructions per second.

Internally, the 80386 is partitioned into six units that operate autonomously and in parallel with each other, synchronizing as necessary. All the internal buses that connect these units are 32 bits wide. By pipelining its functional units, the 80386 can overlap the execution of different stages of one instruction and can process multiple instruc- tions simultaneously. Thus, while one instruction is executed, another is decoded, and a third is fetched from memory.

In addition to pipelining all instructions, the 80386 applies dedicated hardware to important operations. The 80386's multiplyj divide unit can perform 32-bit multiplication in 941 clocks, depending on the number of significant digits; it can divide 32-bit operands in 38 clocks (unsigned) or 43 clocks (signed). The 80386's barrel shifter can shift 1-64 bits in a single clock.

Many 32-bit applications, such as reprogram- mabIe multiuser computers, need the logical-to- physical address translation and protection pro- vided by a memory management unit, or MMU. Other applications, for example, embedded real- time control systems, do not. Most 32-bit micro- processor architectures respond to this dichotomy by implementing the memory management unit in an optional chip. The 80386 MMU, by contrast, is incorporated on the processor chip as two of the processor's pipelined functional units. The operating system controls the operation of the MMU, allowing a real-time system, for

example, to forgo page translation. Implementing memory management on-chip produces better perfonnance for applications that use the MMU and no performance penalty for those that do not. This achievement is made possible by shorter signal propagation delays, use of the half-clock cyles that are available on-chip, and parallel operation.

Another facility that is crucial to some appli- cations and irrelevant to others is "number crunching," particularly single- and double- precision floating point arithmetic. Floating point operands are large, and the useful set of operations on them is quite complex; many thousands of transistors are required to imple- ment a standard set of floating point operations such as those defined by IEEE standard 754. Consequently, the 80386 provides hardware sup- port for numerics in a separate numeric coproces- sor chip. In fact, either of two chips, the 80287 Numeric Coprocessor or the higher-perfonnance 80387, can be connected to the 80386. The numeric coprocessors are invisible to applica- tion software; they effectively extend the 80386 architecture with IEEE 754-compatible regis- ters, data types, and instructions. The combi- nation of an 80386 and an 80387 can execute 1.8 million Whetstones per second.

A 32-bit processor running at 16 Mhz can outrun all but the fastest memories, making memory access time a potential performance bottleneck. The 80386 bus has been designed to make the best use of both very fast static RAMs and less expensive dynamic RAMs. For accesses to fast memory, such as caches, the 80386 provides a two-clock address-to-data bus cycle. (80386 caches can be any size from a minimum useful capacity of 4 kilobytes to the entire physical address space.) Accesses to slower memories (or Ij 0 devices) can utilize the 80386's address pipelining facility to extend the effective address- to-data time to three clocks, while maintaining two-clock throughput to the processor. Because of its internal pipelining of address translation with instruction execution, the 80386 generally

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