GENERAL FEATURES
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By eliminating the access to external bus, operations with the
internal cache can be completed in a single cycle. 80386 at least needs
two cycles for an operation. To further increase the rate of data transfer
inside the CPU, the internal bus of the cache memory is increased to 128
bits, which is four times of the external bus. Since, in most of the time,
the CPU is using the internal cache, the large bus size substantially
improves the overall performance.
When the CPU writes data to the main memory, the data is first
stored in a write buffer. There are four write buffers. When the
external bus is idle, data will be sent to the main memory. If all buffers
are filled, it can start write operation in burst mode. Since the internal
cache is updated immediately, the CPU need not suspend its operation
and there is no need to wait for the external device to update the main
memory.
Many often-used instructions are executed in a clock cycle and
some instructions are modified to take fewer cycles than in 80386. On
the contrary, 80386 may take two to three more cycles for the same
instruction. The CPU contains an advanced instruction pipeline structure
and a 32-byte code queue to speed up the execution.
80486 includes all the functions of 80386 and is able to support
sophisticated software and operation systems which are widely employed
now. It is able to operate in real mode, protected mode and virtual 8086
mode.
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