Chapter 3: Microprocessors^59
The 386DX
The first of the 386 processors introduced by Intel was the 386DX. The 386 was a true 32-bit
processor and included 32-bit internal registers, a 32-bit internal data bus, and a 32-bit
externaldatabus.ItwasbuiltwithanewtechnologycalledVLSI(VeryLargeScaleIntegra-
tion) with 275,000 transistors. The 386 used less power than its predecessors, including
the 8086, because it was constructed of CMOS (Complementary Metal Oxide Semicon-
ductor)materials, a way of manufacturing transistors that reduces the amount of power
required when idle.
The 386DX could address u pto 4 gigabits of system memory, but its built-in virtual
memory management (VMM) system allowed VMM-enabled software access a virtual
memory store of the equivalent of 64 terabytes (TB) of memory (aterabyteis a trillion
bytes of memory).
The 386SX
Intel also released a lower-cost version of the 386 processor called the 386SX. The primary
difference between the 386SX and the 386DX was that the SX model had only a 16-bit
external data bus and a 24-bit address bus, which made it backward compatible to the 286
processor. The 386SX was released to fill a market need, which was a lower-priced pro-
cessor with the power of the 386DX at the cost of the 286.
Figure 3-12. The Intel 80286 microprocessor. Photo courtesy of Intel Corporation