The Elements of Computing Systems: Building a Modern Computer from First Principles by Noam Nisan, Shimon Schocken Page B
Authors: Noam Nisan, Shimon Schocken
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key element of many abstract and practical computer models, most notably the universal Turing machine (1936) and the von Neumann machine (1945). The Turing machine—an abstract artifact describing a deceptively simple computer—is used mainly to analyze the logical foundations of computer systems. In contrast, the von Neumann machine is a practical architecture and the conceptual blueprint of almost all computer platforms today.
Figure 5.1 The von Neumann architecture (conceptual). At this level of detail, this model describes the architecture of almost all digital computers. The program that operates the computer resides in its memory, in accordance with the “stored program” concept.
The von Neumann architecture is based on a central processing unit (CPU), interacting with a memory device, receiving data from some input device, and sending data to some output device (figure 5.1). At the heart of this architecture lies the stored program concept: The computer’s memory stores not only the data that the computer manipulates, but also the very instructions that tell the computer what to do. Let us explore this architecture in some detail.
5.1.3 Memory
The memory of a von Neumann machine holds two types of information: data items and programming instructions. The two types of information are usually treated differently, and in some computers they are stored in separate memory units. In spite of their different functions though, both types of information are represented as binary numbers that are stored in the same generic random-access structure: a continuous array of cells of some fixed width, also called words or locations, each having a unique address. Hence, an individual word (representing either a data item or an instruction) is specified by supplying its address.
Data Memory High-level programs manipulate abstract artifacts like variables, arrays, and objects. When translated into machine language, these data abstractions become series of binary numbers, stored in the computer’s data memory. Once an individual word has been selected from the data memory by specifying its address, it can be either read or written to. In the former case, we retrieve the word’s value. In the latter case, we store a new value into the selected location, erasing the old value.
Instruction Memory When translated into machine language, each high-level command becomes a series of binary words, representing machine language instructions. These instructions are stored in the computer’s instruction memory. In each step of the computer’s operation, the CPU fetches (i.e., reads) a word from the instruction memory, decodes it, executes the specified instruction, and figures out which instruction to execute next. Thus, changing the contents of the instruction memory has the effect of completely changing the computer’s operation.
The instructions that reside in the instruction memory are written in an agreed-upon formalism called machine language. In some computers, the specification of each operation and the codes representing its operands are represented in a single-word instruction. Other computers split this specification over several words.
5.1.4 Central Processing Unit
The CPU—the centerpiece of the computer’s architecture—is in charge of executing the instructions of the currently loaded program. These instructions tell the CPU to carry out various calculations, to read and write values from and into the memory, and to conditionally jump to execute other instructions in the program. The CPU executes these tasks using three main hardware elements: an Arithmetic-Logic Unit (ALU), a set of registers, and a control unit.
Arithmetic Logic Unit The ALU is built to perform all the low-level arithmetic and logical operations featured by the computer. For instance, a typical ALU can add two numbers, test whether a number is positive, manipulate the bits in a word of data, and so on.
Registers The CPU
Figure 5.1 The von Neumann architecture (conceptual). At this level of detail, this model describes the architecture of almost all digital computers. The program that operates the computer resides in its memory, in accordance with the “stored program” concept.
The von Neumann architecture is based on a central processing unit (CPU), interacting with a memory device, receiving data from some input device, and sending data to some output device (figure 5.1). At the heart of this architecture lies the stored program concept: The computer’s memory stores not only the data that the computer manipulates, but also the very instructions that tell the computer what to do. Let us explore this architecture in some detail.
5.1.3 Memory
The memory of a von Neumann machine holds two types of information: data items and programming instructions. The two types of information are usually treated differently, and in some computers they are stored in separate memory units. In spite of their different functions though, both types of information are represented as binary numbers that are stored in the same generic random-access structure: a continuous array of cells of some fixed width, also called words or locations, each having a unique address. Hence, an individual word (representing either a data item or an instruction) is specified by supplying its address.
Data Memory High-level programs manipulate abstract artifacts like variables, arrays, and objects. When translated into machine language, these data abstractions become series of binary numbers, stored in the computer’s data memory. Once an individual word has been selected from the data memory by specifying its address, it can be either read or written to. In the former case, we retrieve the word’s value. In the latter case, we store a new value into the selected location, erasing the old value.
Instruction Memory When translated into machine language, each high-level command becomes a series of binary words, representing machine language instructions. These instructions are stored in the computer’s instruction memory. In each step of the computer’s operation, the CPU fetches (i.e., reads) a word from the instruction memory, decodes it, executes the specified instruction, and figures out which instruction to execute next. Thus, changing the contents of the instruction memory has the effect of completely changing the computer’s operation.
The instructions that reside in the instruction memory are written in an agreed-upon formalism called machine language. In some computers, the specification of each operation and the codes representing its operands are represented in a single-word instruction. Other computers split this specification over several words.
5.1.4 Central Processing Unit
The CPU—the centerpiece of the computer’s architecture—is in charge of executing the instructions of the currently loaded program. These instructions tell the CPU to carry out various calculations, to read and write values from and into the memory, and to conditionally jump to execute other instructions in the program. The CPU executes these tasks using three main hardware elements: an Arithmetic-Logic Unit (ALU), a set of registers, and a control unit.
Arithmetic Logic Unit The ALU is built to perform all the low-level arithmetic and logical operations featured by the computer. For instance, a typical ALU can add two numbers, test whether a number is positive, manipulate the bits in a word of data, and so on.
Registers The CPU
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