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Unformatted text preview: origins of this trend were in the minicomputers developed during the 1970s. These computers had relatively slow main memories coupled to processors built using many simple integrated circuits. The processors were controlled by microcode ROMs (Read Only Memories) that were faster than main memory, so it made sense to implement frequently used operations as microcode sequences rather than them requiring several instructions to be fetched from main memory. Throughout the 1970s microprocessors were advancing in their capabilities. These single chip processors were dependent on state-of-the-art semiconductor technology to achieve the highest possible number of transistors on a single chip, so their development took place within the semiconductor industry rather than within the computer industry. As a result, microprocessor designs displayed a lack of original thought at the architectural level, particularly with respect to the demands of the technology that was used in their implementation. Their designers, at best, took ideas from the minicomputer industry where the implementation technology was very different. In particular, the microcode ROM which was needed for all the complex routines absorbed an unreasonable proportion of the area of a single chip, leaving little room for other performance-enhancing features. This approach led to the single-chip Complex Instruction Set Computers (CISCs) of the late 1970s, which were microprocessors with minicomputer instruction sets that were severely compromised by the limited available silicon resource. Into this world of increasingly complex instruction sets the Reduced Instruction Set Computer (RISC) was born. The RISC concept was a major influence on the design of the ARM processor; indeed, RISC was the ARM's middle name. But before we look at either RISC or the ARM in more detail we need a bit more background on what processors do and how they can be designed to do it quickly. If reducing the semantic gap between the processor instruction set and the high-level language is not the right w...
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- Spring '09