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Historical Notes on CISC and RISC: Difference between revisions
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As the designers began to develop more effective architectural principles based on the success and failures of earlier efforts, hardware became smaller and less expensive, which in turn allowed for greater freedom in the CPU design, so the complexity of the systems grew; by 1966, the PDP-6, a typical design of the time, had sixteen 36-bit general-purpose registers, plus a stack register and frame register. The instruction sets became more complex as well; since a significant percentage of programming even into the 1970s was done in assembly language, it was thought that the instruction sets should be 'rich', that is, they should have a special instruction for nearly every common operation the programmer would want to use.
This culminated with the 32-bit VAX-11 series of minicomputers and mainframes, which had 256 primary instructions, including ones for polynomial evaluation, trigonometric functions, and CRC
To provide these instructions, designers began to use 'microcode', which was a set of what could be called firmware-encoded macro instructions that would be handled by the CPU itself
About the time the VAX was being designed, four other things were happening that would change this attitude. First, high-level languages were becoming the primary method of programming, meaning that the baroque instruction sets of the then-current CPUs
Second, assembly language programmers were noting that in the majority of programs, whether in assembly or in a compiled language, only a handful of instructions were being used: few programs needed a CRC instruction, and hardly any programmers were familiar enough with the entire VAX instruction set to know that the instruction existed and how to use it. A side aspect of this is that the more complex instruction sets were increasingly difficult to learn and to teach.
Third, computer hardware design had advanced to the point where graduate courses in CPU design were being offered; such courses naturally enough stuck to very simple and regular designs, which they found actually outperformed comparable complex systems
Finally, the advent of single chip microprocessors meant that it would soon be possible to mass-produce inexpensive computers for a fraction of the cost of minis and mainframes. Early on, these were limited by the capabilities of the hardware in much the same ways the first generation of CPUs were (and some of the design
This last part had some major repercussions, especially regarding some of the design decisions as microprocessors became more powerful. Chief amongst these was the decision by Intel to try and extend the 8-bit 8080 design into a new 16-bit design, the 8086; they bent over backwards to make the CPU as familiar to older programmers as possible, with the result that the design ended up with some unnecessary complications and limitations, especially in how it addresses larger sections of memory. Also, because the designers had given the system only a small number of registers, most of the operations
The fact that this processor was selected for the IBM PC, which would soon become the dominant platform, meant that these weaknesses were of critical importance to millions of users. This was further exacerbated when they extended the design still further with the 80286, which now needed a separate 'protected mode' to access its full abilities while retaining 'normal mode' for backwards compatibility. Some of the design flaws were resolved in the next design, the 80386, but at the cost of exponentially increasing complexity both of the chip itself and of assembly programming for it.
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