CISC and RISC instructions and their handling


CISC and RISC instructions and their handling

The first CPU's had a so called Complex Instruction Set Computer (CISC). This means that the
computer can understand many and complex instructions. The X86 instruction set, with its
varying length from 8 to 120 bit, was originally developed for the 8086 with its mere 29000
transistors.
Reduced Instruction Set Computer (RISC): The RISC instructions are brief and the same length (for example 32 bit long, as in Pentium Pro), and they process much faster than CISC
instructions. Therefore, RISC is used in all newer CPU's. However, the problem is that the
instructions arrive at the CPU in 8086 format. Thus, they must be decoded .

For every new CPU generation, the instruction set has been expanded. The 386 came with 26 new instructions, the 486 with 6 new instructions, and Pentium with 8 new instructions. These
changes mean that some programs require at least a 386 or a Pentium processor to work.

There is also a continuous optimizing of the instruction handling process. One is that the clock
frequency increases, as we will see later - the faster, the better. But what can the CPU do in
one clock tick. That is critical to its performance. For example, a 386 needed 6 clock ticks to
add a number to a sub total. A job which the 486 manages in only two clock ticks, because of
more effective instruction decoding, 5th and 6th generation CPU's can execute more than one
of those operations in one clock tick, since they contain more processing lines (pipelines),
which work parallel.

Floating-point unit - FPU

The first CPU's could only work with whole numbers. Therefore, it was necessary to add a
mathematical co-processor (FPU), when better math power was needed. Later, this FPU was built into the CPU:



It is said that Intel's CPU's have by far the best FPU units. Processors from AMD and Cyrix
definitely have a reputation for providing sub standard performance in this area. But, you may
not utilize the FPU. That depends on the applications (user programs) you are using. Common
office programs do not use the floating point operations, which the FPU can handle. However,
3D graphics programs like AutoCad do. Therefore, if you use your PC in advanced design
applications, the FPU performance becomes significant. For most users, it is only of limited
importance.

Many brand names

As mentioned, there are CPUs of many brand names (IBM, Texas, Cyrix, AMD), and often they make models, which overlap two generations. This can make it difficult to keep of track of CPU's. Here is an attempt to identify the various CPU's according to generation:



Now, let us see how CPU speed has been improved through generations.




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