The Elements of Computing Systems: Building a Modern Computer from First Principles by Noam Nisan, Shimon Schocken Page A
Authors: Noam Nisan, Shimon Schocken
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the Computer chip (built in chapter 5’s project) into the hardware simulator, then load the .hack file generated by the assembler into the computer’s ROM chip.
5
Computer Architecture
Form ever follows function.
—Louis Sullivan (1856—1924), architect
Form IS function.
—Ludwig Mies van der Rohe (1886—1969), architect
This chapter is the pinnacle of the “hardware” part of our journey. We are now ready to take all the chips that we built in chapters 1-3 and integrate them into a general-purpose computer capable of running stored programs written in the machine language presented in chapter 4. The specific computer we will build, called Hack, has two important virtues. On the one hand, Hack is a simple machine that can be constructed in just a few hours, using previously built chips and the hardware simulator supplied with the book. On the other hand, Hack is sufficiently powerful to illustrate the key operating principles and hardware elements of any digital computer. Therefore, building it will give you an excellent understanding of how modern computers work at the low hardware and software levels.
Following an introduction of the stored program concept, section 5.1 gives a detailed description of the von Neumann architecture —a central dogma in computer science underlying the design of almost all modern computers. The Hack platform is one example of a von Neumann machine, and section 5.2 gives its exact hardware specification. Section 5.3 describes how the Hack platform can be implemented from available chips, in particular the ALU built in chapter 2 and the registers and memory systems built in chapter 3.
The computer that will emerge from this construction will be as simple as possible, but not simpler. This means that it will have the minimal configuration necessary to run interesting programs and deliver a reasonable performance. The comparison of this machine to typical computers is taken up in section 5.4, which emphasizes the critical role that optimization plays in the design of industrial-strength computers, but not in this chapter. As usual, the simplicity of our approach has a purpose: All the chips mentioned in the chapter, culminating in the Hack computer itself, can be built and tested on a personal computer running our hardware simulator, following the technical instructions given in section 5.5. The result will be a minimal yet surprisingly powerful computer.
5.1 Background
5.1.1 The Stored Program Concept
Compared to all the other machines around us, the most unique feature of the digital computer is its amazing versatility. Here is a machine with finite hardware that can perform a practically infinite array of tasks, from interactive games to word processing to scientific calculations. This remarkable flexibility—a boon that we have come to take for granted—is the fruit of a brilliant idea called the stored program concept. Formulated independently by several mathematicians in the 1930s, the stored program concept is still considered the most profound invention in, if not the very foundation of, modern computer science.
Like many scientific breakthroughs, the basic idea is rather simple. The computer is based on a fixed hardware platform, capable of executing a fixed repertoire of instructions. At the same time, these instructions can be used and combined like building blocks, yielding arbitrarily sophisticated programs. Moreover, the logic of these programs is not embedded in the hardware, as it was in mechanical computers predating 1930. Instead, the program’s code is stored and manipulated in the computer memory, just like data, becoming what is known as “software.” Since the computer’s operation manifests itself to the user through the currently executing software, the same hardware platform can be made to behave completely differently each time it is loaded with a different program.
5.1.2 The von Neumann Architecture
The stored program concept is a
5
Computer Architecture
Form ever follows function.
—Louis Sullivan (1856—1924), architect
Form IS function.
—Ludwig Mies van der Rohe (1886—1969), architect
This chapter is the pinnacle of the “hardware” part of our journey. We are now ready to take all the chips that we built in chapters 1-3 and integrate them into a general-purpose computer capable of running stored programs written in the machine language presented in chapter 4. The specific computer we will build, called Hack, has two important virtues. On the one hand, Hack is a simple machine that can be constructed in just a few hours, using previously built chips and the hardware simulator supplied with the book. On the other hand, Hack is sufficiently powerful to illustrate the key operating principles and hardware elements of any digital computer. Therefore, building it will give you an excellent understanding of how modern computers work at the low hardware and software levels.
Following an introduction of the stored program concept, section 5.1 gives a detailed description of the von Neumann architecture —a central dogma in computer science underlying the design of almost all modern computers. The Hack platform is one example of a von Neumann machine, and section 5.2 gives its exact hardware specification. Section 5.3 describes how the Hack platform can be implemented from available chips, in particular the ALU built in chapter 2 and the registers and memory systems built in chapter 3.
The computer that will emerge from this construction will be as simple as possible, but not simpler. This means that it will have the minimal configuration necessary to run interesting programs and deliver a reasonable performance. The comparison of this machine to typical computers is taken up in section 5.4, which emphasizes the critical role that optimization plays in the design of industrial-strength computers, but not in this chapter. As usual, the simplicity of our approach has a purpose: All the chips mentioned in the chapter, culminating in the Hack computer itself, can be built and tested on a personal computer running our hardware simulator, following the technical instructions given in section 5.5. The result will be a minimal yet surprisingly powerful computer.
5.1 Background
5.1.1 The Stored Program Concept
Compared to all the other machines around us, the most unique feature of the digital computer is its amazing versatility. Here is a machine with finite hardware that can perform a practically infinite array of tasks, from interactive games to word processing to scientific calculations. This remarkable flexibility—a boon that we have come to take for granted—is the fruit of a brilliant idea called the stored program concept. Formulated independently by several mathematicians in the 1930s, the stored program concept is still considered the most profound invention in, if not the very foundation of, modern computer science.
Like many scientific breakthroughs, the basic idea is rather simple. The computer is based on a fixed hardware platform, capable of executing a fixed repertoire of instructions. At the same time, these instructions can be used and combined like building blocks, yielding arbitrarily sophisticated programs. Moreover, the logic of these programs is not embedded in the hardware, as it was in mechanical computers predating 1930. Instead, the program’s code is stored and manipulated in the computer memory, just like data, becoming what is known as “software.” Since the computer’s operation manifests itself to the user through the currently executing software, the same hardware platform can be made to behave completely differently each time it is loaded with a different program.
5.1.2 The von Neumann Architecture
The stored program concept is a
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