Friday, April 17, 2009

CPU » 80386

The third x86 generation of x86 microprocessors, Intel 80386 (i386) was a 32-bit microprocessor backwards compatible with previous generations of 80x86 CPUs. Major new feature in the i386 CPU was 80386 protected mode - this mode fixed many shortcomings that existed in the 80286 processor and in the 80286 protected mode:

* The 80386 mode included complete set of 32-bit registers and 32-bit instructions.
* Although in this mode the CPU still used memory segment architecture similar to the one present in earlier x86 microprocessors, the size of memory segments was increased to 4 GB. This simplified development of 32-bit software, and in most cases applications could run without worrying about switching memory segments.
* It became possible to switch from protected mode back to real-mode without simulating processor reset.

Another new mode in the 80386 CPU was 8086 virtual mode. In this mode the CPU could run old 8086 applications while providing necessary protection of memory and other resources. Introduction of this mode and 80386 protected mode was very significant step. All current 32-bit x86-based operating systems use these modes to run legacy 16-bit and more modern 32-bit applications.

There were a few different versions of the 80386 CPUs:

* 80386DX - this CPU could work with 16-bit and 32-bit external buses.
* 80386SX - low cost version of the 80386. This processor had 16 bit external data bus and 24-bit external address bus.
* 80386SL - low-power microprocessor with power management features, with 16-bit external data bus and 24-bit external address bus. The processor included ISA bus controller, memory controller and cache controller.
* Embedded 80376 and 80386EX processors.

The Intel 80386 was produced at speeds up to 33 MHz, AMD produced even faster 40 MHz version.

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