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Computer Architecture

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IT Systems, Summer Term 2024
Dr. Jens Lechtenbörger (License Information)

1. Introduction

1.1. Today’s Core Question

  • How do the previously built chips fit together in a computer architecture?

1.2. Learning Objectives

  • Explain von Neumann architecture and discuss its principles, in general and in relation to Hack
  • Discuss implications of Moore’s law
  • Build, test, and analyze Hack computer
    • Trace execution of programs (e.g., program counter, register and memory contents)

1.3. Retrieval Practice


2. Von Neumann Architecture

2.1. Sketch of Von Neumann Architecture

von Neumann Architecture

von Neumann Architecture” by Kapooht under CC BY-SA 3.0; converted from Wikimedia Commons

  • Note: ROM is not part of von Neumann architecture
    • Actually, Harvard architecture is variant of Hack with separate memories for data and instructions
    • Details are not important for us

2.2. Von Neumann Principles

  • Major principles

    • Stored program concept

      • Instructions stored in memory, just as data, modifiable
      • General-purpose, programmable
    • Single CPU

    • Von Neumann Bottleneck

      • Fast CPU fetches every instruction and data over single bus from slow memory
      • Memory wall according to (Wulf and McKee 1995)
        • CPU speeds increase much more than RAM speeds
        • System performance bounded by memory speed
      • Caching and multi-threading as mitigation
        • Threads to be explained in OS part

2.3. Fetch-Decode-Execute Cycle

  • Use program counter to fetch instruction from memory

  • Control unit decodes bits of instruction to determine operation
  • CPU executes operation
    • Maybe load data
    • Perform (ALU) operation
    • Maybe write result
    • Update program counter

3. Moore’s Law

  • Published in (Moore 1965)

    • “The complexity for minimum component costs has increased at a rate of roughly a factor of two per year […]. Certainly over the short term this rate can be expected to continue, if not to increase. Over the longer term, the rate of increase is a bit more uncertain, although there is no reason to believe it will not remain nearly constant for at least 10 years. That means by 1975, the number of components per integrated circuit for minimum cost will be 65,000.

      I believe that such a large circuit can be built on a single wafer.”

    • Later corrected to “double every two years”
      • Exponential improvements

3.1. Sample Numbers

Moore's Law

Moore's Law” by Hannah Ritchie and Max Roser under CC BY 4.0; from Wikimedia Commons

3.2. Future Perspectives

  • Chip’s “speed” doubled every two years

    • Due to continued miniaturization

      • Smaller transistors need less power, switch faster

      • Until limits of physics reached

    • Options according to (Lundstrom and Alam 2022) (beyond class)

      • 2D nanoelectronics, 3D terascale integration, functional integration

3.3. Parallel Programming

  • For decades, individual CPUs became more powerful
    • Now, CPUs contain more cores
    • Need parallel programming to make programs faster

  • OS Topics
    • Processes and threads
      • Processes are programs in execution
        • Each process can have multiple threads of execution
        • Each core can execute a different thread
        • Needs to be programmed
    • Concurrency and mutual exclusion
      • Protect shared data structures (e.g., via locks)
        • Otherwise, data structures will be corrupted (cf. locking and update anomalies in database systems)
      • Programmers must learn this

4. Hack Computer

4.1. Hack Input/Output Devices

  • Recall: Memory-mapped I/O

    Hack Computer

    Hack Computer” by user52174 under CC BY-SA 4.0; from StackExchange

    • Screen

      • Built-in chip Screen
        • Acts like RAM8K (with in[16], address[13], load, out[16])
        • Bit patterns control pixels on screen
    • Keyboard

      • Built-in chip Keyboard
        • No input, only output out[16], reflects currently pressed key

4.2. Hack Data Memory

  • Specification in Memory.hdl

    CHIP Memory {
        IN in[16], load, address[15];
        OUT out[16];
    • Output out with contents of memory location address
    • If load then store in at address
    • Different address ranges
      • 0-16383: Access to RAM16K
      • 16384-24575: Access to Screen
      • 24576: Access to Keyboard
      • Larger values are invalid

4.3. Hack Instruction Memory

4.4. Hack CPU Chip

  • Specification via machine language and CPU.hdl

    • Details of machine language include:

      • Three registers: A, D, PC
      • Implicit memory location M (addressed by A register)
      CHIP CPU {
          IN  inM[16],         // M value input  (M = contents of Memory[A])
      	instruction[16], // Instruction for execution
      	reset;           // Signals whether to re-start the current
      			 // program (reset=1) or continue executing
      			 // the current program (reset=0).
          OUT outM[16],        // M value output
      	writeM,          // Write into M?
      	addressM[15],    // Address in data memory (for M)
      	pc[15];          // address of next instruction
      PARTS: ...}

4.4.1. ALU with Registers

Operation Bits of Hack C-Instructions

4.4.2. Proposed CPU Implementation

  • Incomplete picture
    • Several inputs “c” denote control inputs
      • E.g., load bits for registers, selector for Mux
      • Computed by additional control logic, revisited in class
        • Control unit of von Neumann architecture
        • Decoding of instruction

4.5. Hack Computer Chip

  • Specification in Computer.hdl

    • Hack computer, including CPU, Memory (with RAM16K, Screen, Keyboard), ROM32K
    • Input reset allows to (re-) start execution of program in ROM
  • Build in Project 5

5. Conclusions

5.1. Summary

  • Von Neumann architecture as blueprint for programmable, general-purpose computers
    • Bottleneck from memory access, memory wall
    • Hack as variant with ROM
  • Moore’s law predicts exponential growth of computer power
    • Physical limits ahead
    • Parallel programming skills required
  • Project 5: Hack computer, end of Part 1 of IT Systems
    • Few components: CPU, data memory, instruction memory
      • CPU executes machine instructions
      • Memory-mapped I/O for screen and keyboard

5.2. Outlook

  • In part 2, Operating Systems, we revisit I/O
  • Programmed I/O (polling) vs interrupt-driven I/O
    • You “polled” when accessing the keyboard in Hack
      • Poll: “Has a key been pressed yet?”
        • If not, wait in loop → Waste of CPU time
    • Interrupt: Additional input from I/O device to CPU
      • Via pin or bus
        • Notification to CPU: “Hey, someone pressed key X here. Please act.”
        • If no key, no unnecessary processing time
        • However, interrupt overhead; to be discussed

5.3. Beyond Class

5.4. Self-Study-Tasks

  • Taking all other parts for granted, implement Computer.hdl
    • Part of Project 5
    • (In class, we proceed backwards, with Memory and CPU)


Lundstrom, Mark S., and Muhammad A. Alam. 2022. “Moore’s Law: The Journey Ahead.” Science 378 (6621): 722–23.
Moore, Gordon E. 1965. “Cramming More Components onto Integrated Circuits.” Electronics 38 (8).
Neumann, John von. 1945. “First Draft of a Report on the EDVAC.” University of Pennsylvania.
Nisan, Noam, and Shimon Schocken. 2005. The Elements of Computing Systems: Building a Modern Computer from First Principles. The MIT Press.
Wulf, Wm. A., and Sally A. McKee. 1995. “Hitting the Memory Wall: Implications of the Obvious.” Sigarch Comput. Archit. News 23 (1): 20–24.

License Information

Source files are available on GitLab (check out embedded submodules) under free licenses. Icons of custom controls are by @fontawesome, released under CC BY 4.0.

Except where otherwise noted, the work “Computer Architecture”, © 2024 Jens Lechtenbörger, is published under the Creative Commons license CC BY-SA 4.0.