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Unformatted text preview: CMOS implementations based on alternative transistor circuits. There are many ways to satisfy the basic requirements of logic design using the complementary transistors available on a CMOS chip, and new transistor circuits are published regularly. For further information consult a text on logic design; a suitable reference is suggested in the 'Bibliography' on page 410. 1.3 MU0 - a simple processor
A simple form of processor can be built from a few basic components: a program counter (PC) register that is used to hold the address of the current instruction; a single register called an accumulator (ACC) that holds a data value while it is worked upon; an arithmetic-logic unit (ALU) that can perform a number of operations on binary operands, such as add, subtract, increment, and so on; an instruction register (IR) that holds the current instruction while it is executed; instruction decode and control logic that employs the above components to achieve the desired results from each instruction. 8 An Introduction to Processor Design This limited set of components allows a restricted set of instructions to be implemented. Such a design has been employed at the University of Manchester for many years to illustrate the principles of processor design. Manchester-designed machines are often referred to by the names MUn for 1 < n < 6, so this simple machine is known as MU0. It is a design developed only for teaching and was not one of the large-scale machines built at the university as research vehicles, though it is similar to the very first Manchester machine and has been implemented in various forms by undergraduate students. The MU0 instruction set MU0 is a 16-bit machine with a 12-bit address space, so it can address up to 8 Kbytes of memory arranged as 4,096 individually addressable 16-bit locations. Instructions are 16 bits long, with a 4-bit operation code (or opcode) and a 12-bit address field (S) as shown in Figure 1.4. The simplest instruction set uses only eight of the 16 available opcodes and is summarized in Table 1.1. An instruction such as 'ACC...
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- Spring '09