The Elements of Computing Systems: Building a Modern Computer from First Principles (17 page)
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Authors: Noam Nisan,Shimon Schocken
BOOK: The Elements of Computing Systems: Building a Modern Computer from First Principles
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I/O-Handling Program
(Fill.asm): This program runs an infinite loop that listens to the keyboard input. When a key is pressed (any key), the program blackens the screen, namely, writes “black” in every pixel. When no key is pressed, the screen should be cleared. You may choose to blacken and clear the screen in any spatial order, as long as pressing a key continuously for long enough will result in a fully blackened screen and not pressing any key for long enough will result in a cleared screen. This program has a test script (Fill.tst) but no compare file—it should be checked by visibly inspecting the simulated screen.
I/O-Handling Program
(Fill.asm): This program runs an infinite loop that listens to the keyboard input. When a key is pressed (any key), the program blackens the screen, namely, writes “black” in every pixel. When no key is pressed, the screen should be cleared. You may choose to blacken and clear the screen in any spatial order, as long as pressing a key continuously for long enough will result in a fully blackened screen and not pressing any key for long enough will result in a cleared screen. This program has a test script (Fill.tst) but no compare file—it should be checked by visibly inspecting the simulated screen.
Steps
We recommend proceeding as follows:
We recommend proceeding as follows:
0. The assembler and CPU emulator programs needed for this project are available in the tools directory of the book’s software suite. Before using them, go through the assembler tutorial and the CPU emulator tutorial.
1. Use a plain text editor to write the first program in assembly, and save it as projects/04/mult/Mult.asm.
2. Use the supplied assembler (in either batch or interactive mode) to translate your program. If you get syntax errors, go to step 1. If there are no syntax errors, the assembler will produce a file called projects/04/mult/Mult.hack, containing binary machine instructions.
3. Use the supplied CPU emulator to test the resulting Mult.hack code. This can be done either interactively, or batch-style using the supplied Mult.tst script. If you get run-time errors, go to step 1.
4. Repeat stages 1-3 for the second program (Fill.asm), using the projects/04/ fill directory.
Figure 4.7
The visual assembler supplied with the book.
The visual assembler supplied with the book.
Debugging Tip
The Hack language is case sensitive. A common error occurs when one writes, say, @foo and @Foo in different parts of the program, thinking that both commands refer to the same variable. In fact, the assembler treats these symbols as two completely different identifiers.
The Hack language is case sensitive. A common error occurs when one writes, say, @foo and @Foo in different parts of the program, thinking that both commands refer to the same variable. In fact, the assembler treats these symbols as two completely different identifiers.
The Supplied Assembler
The book’s software suite includes a Hack assembler that can be used in either command mode or GUI mode. The latter mode of operation allows observing the translation process in a visual and step-wise fashion, as shown in figure 4.7.
The book’s software suite includes a Hack assembler that can be used in either command mode or GUI mode. The latter mode of operation allows observing the translation process in a visual and step-wise fashion, as shown in figure 4.7.
The machine language programs produced by the assembler can be tested in two different ways. First, one can run the .hack program in the CPU emulator. Alternatively, one can run the same program directly on the hardware, by loading it into the computer’s instruction memory using the hardware simulator. Since we will finish building the hardware platform only in the next chapter, the former option makes more sense at this stage.
Figure 4.8
The CPU emulator supplied with the book. The loaded program can be displayed either in symbolic notation (as shown in this screen shot) or in binary code. The screen and the keyboard are not used by this particular program.
The CPU emulator supplied with the book. The loaded program can be displayed either in symbolic notation (as shown in this screen shot) or in binary code. The screen and the keyboard are not used by this particular program.
The Supplied CPU Emulator
This program simulates the Hack computer platform. It allows loading a Hack program into the simulated ROM and visually observing its execution on the simulated hardware, as shown in figure 4.8.
This program simulates the Hack computer platform. It allows loading a Hack program into the simulated ROM and visually observing its execution on the simulated hardware, as shown in figure 4.8.
For ease of use, the CPU emulator enables loading binary .hack files as well as symbolic .asm files. In the latter case, the emulator translates the assembly program into binary code on the fly. This utility seems to render the supplied assembler unnecessary, but this is not the case. First, the supplied assembler shows the translation process visually, for instructive purposes. Second, the binary files generated by the assembler can be executed directly on the hardware platform. To do so, load 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.
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 Background5.1.1 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.
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 ArchitectureThe stored program concept is a 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 (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 MemoryThe 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.
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.
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.
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