The term Computer Architecture may refer to the entire hardware level of the computer. However, it is often used to refer to the design and implementation of the digital processor part of the computer hardware, and we focus on the computer processor architecture in this chapter.
The central processing unit (CPU, or processor) is the part of the computer that executes program instructions on program data. Program instructions and data are stored in the computer’s Random Access Memory (RAM). A particular digital processor implements a specific Instruction Set Architecture (ISA), which defines the set of instructions and their binary encoding, the set of CPU registers, and the effects of executing instructions on the state of the processor. There are many different ISAs, including: SPARC, IA32, MIPS, ARM, ARC, PowerPC, and x86 ISAs (which include IA32 and x86-64). Microarchitectures define the circuitry of an implementation of a specific ISA. Microarchitecture implementations of the same ISA can differ as long as they implement the ISA definition. For example, Intel and AMD produce different microprocessor implementations of IA32 ISA.
Some ISAs define a Reduced Instruction Set Computer (RISC), and others define a Complex Instruction Set Computer (CISC). RISC ISAs have a small set of basic instructions that execute quickly; each instruction executes in about a single processor clock cycle, and compilers combine sequences of several basic RISC instructions to implement higher-level functionality. In contrast, CISC ISA’s instructions provide higher-level functionality than RISC instructions. CISC architectures also define a larger set of instructions than RISC, support more complicated addressing modes (ways to express the memory locations of program data), and support variable length instructions. A single CISC instruction may perform a sequence of low-level functionality and may take several processor clock cycles to execute. This same functionality would require multiple instructions on a RISC architecture.
All modern processors, regardless of their ISA, adhere to the von Neumann Architecture model. The general-purpose design of the von Neumann architecture allows it to execute any type of program. It uses a stored-program model, meaning that the program instructions reside in computer memory along with program data, and both are inputs to the processor.
This chapter introduces the von Neumann architecture and the ancestry and components that underpin modern computer architecture. We build an example digital processor (CPU) based on the von Neumann architecture model, design a CPU from digital circuits that are constructed from logic gate building blocks, and demonstrate how the CPU executes program instructions.
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