Microcode: Difference between revisions

Revision as of 08:31, 19 February 2024

Revision as of 00:18, 25 February 2024

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The result of this discovery was what is today known as the [[RISC]] concept. The complex microcode engine and its associated ROM is reduced or eliminated completely, and those circuits instead dedicated to things like additional registers or a wider ALU, which increases the performance of every program. When complex sequences of instructions are needed, this is left to the compiler, which is the entire purpose of using a compiler in the first place. The basic concept was soon picked up by university researchers in California, where simulations suggested such designs would trivially outperform even the fastest conventional designs. It was one such project, at the [[University of California, Berkeley]], that introduced the term RISC.

The industry responded to the concept of RISC with both confusion and hostility, including a famous dismissive article by the VAX team at Digital.<ref name=comments>{{cite journal |url=https://dl.acm.org/doi/pdf/10.1145/641914.641918 |title=Comments on "The Case for the Reduced Instruction Computer" |first1=Douglas |last1=Clark |first2=William |last2=Strecker |date=September 1980 |journal=ACM|volume=8 |issue=6 |pages=34–38 |doi=10.1145/641914.641918 |s2cid=14939489 }}</ref> A major point of contention was that implementing the instructions outside of the processor meant it would spend much more time reading those instructions from memory, thereby slowing overall performance no matter how fast the CPU itself ran.<ref name=comments/> Proponents pointed out that simulations clearly showed the number of instructions was not much greater, especially when considering compiled code.<ref name=risc/>

The debate raged until the first commercial RISC designs emerged in the second half of the 1980s, which easily outperformed the most complex designs from other companies. By the late 1980s it was over; even DEC was abandoning microcode for their [[DEC Alpha]] designs, and CISC processors switched to using hardwired circuitry, rather than microcode, to perform many functions. For example, the [[Intel 80486]] uses hardwired circuitry to fetch and decode instructions, using microcode only to execute instructions; register-register move and arithmetic instructions required only one microinstruction, allowing them to be completed in one clock cycle.<ref>{{cite conference|url=https://ieeexplore.ieee.org/document/63682|title=The execution pipeline of the Intel i486 CPU|book-title= Digest of Papers Compcon Spring '90. Thirty-Fifth IEEE Computer Society International Conference on Intellectual Leverage|publisher=[[IEEE]]|isbn=0-8186-2028-5|___location=San Francisco, CA|doi=10.1109/CMPCON.1990.63682}}</ref> The [[Pentium Pro]]'s fetch and decode hardware fetches instructions and decodes them into series of micro-operations that are passed on to the execution unit, which schedules and executes the micro-operations, possibly doing so [[out-of-order execution|out-of-order]]. Complex instructions are implemented by microcode that consists of pre-defined sequences of micro-operations.<ref>{{cite web|url=http://stffrdhrn.github.io/content/2019/Intel_PentiumPro.pdf|title=Pentium Pro Processor At 150, 166, 180, and 200 MHz|publisher=[[Intel]]|date=November 1995|type=Datasheet}}</ref>

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There may also be a [[memory address register]] and a [[memory data register]], used to access the main [[computer storage]]. Together, these elements form an "[[execution unit]]". Most modern [[Central processing unit|CPUs]] have several execution units. Even simple computers usually have one unit to read and write memory, and another to execute user code. These elements could often be brought together as a single chip. This chip comes in a fixed width that would form a "slice" through the execution unit. These are known as "[[bit slicing|bit slice]]" chips. The [[AMD Am2900]] family is one of the best known examples of bit slice elements.<ref>{{cite book

|title=Computer Architecture and Organization |last=Hayes |first=John P. |isbn=0-07-027363-4 |year=1978 |page=300|publisher=McGraw-Hill }}</ref> The parts of the execution units and the whole execution units are interconnected by a bundle of wires called a [[Computer bus|bus]].

Programmers develop microprograms, using basic software tools. A [[microassembler]] allows a programmer to define the table of bits symbolically. Because of its close relationship to the underlying architecture, "microcode has several properties that make it difficult to generate using a compiler."<ref name=Kent2813 /> A [[simulator]] program is intended to execute the bits in the same way as the electronics, and allows much more freedom to debug the microprogram. After the microprogram is finalized, and extensively tested, it is sometimes used as the input to a computer program that constructs logic to produce the same data.{{citation needed|date=February 2018}} This program is similar to those used to optimize a [[programmable logic device|programmable logic array]]. Even without fully optimal logic, heuristically optimized logic can vastly reduce the number of transistors from the number needed for a [[read-only memory]] (ROM) control store. This reduces the cost to produce, and the electricity used by, a CPU.