Understanding Computer Systems

• To program a system, understanding the system itself is crucial. All computers share the same basic components: hardware (CPU, RAM, Disks) and software (Operating System, Programs, System Utilities, Applications). These components are interdependent, and software development requires understanding all components.

Key Concepts for Understanding Programs

• How to manipulate information
• How programs are translated
• How programs are executed
• How programs are affected by their environment

Information: Bits + Context

• Data alone has no meaning; its interpretation or context gives it meaning. Data can have the same meaning but different representations.
• Data could be in ASCII, Hex, or Binary
• Context is essential for interpreting data, so consider hello.c which can contain text, 8-bit integers, or 32-bit integers. Interpretation applies to files, memory, and the CPU. Manipulating a system requires understanding data representation and interpretation within it.

Program Translation

• Programs undergo multiple translation stages before execution, such as Java (.java to .class to machine code) and C (.c to .s to .o to executable). Each translation stage transforms the program. Each stage depends on the previous stage and the translator, affecting the final version.

Importance of Understanding Program Translation

• Optimizing Program Performance: Efficient code is desired and understanding program translation helps with making appropriate code choices, such as when to use switch over if statements.
• Avoiding Security Holes: Understanding compilation helps prevent stack/buffer overflow errors and usage of uninitialized memory.
• Understanding Linking Errors: Helps resolve issues like multiple definitions of global variables/functions, variable/function definitions in header files, and linking order problems.

Program Execution

• Program execution depends on the hardware, including the CPU (instruction set, word size, implementation), RAM (amount, speed, caching), and bus (word-size, speed, coherency protocols). The CPU performs an infinite fetch-decode-execute loop. Executing an instruction involves memory access, modifying registers/CPU state, and jumping to other program locations.

Performance Considerations for Program Execution

• Instructions that access memory are slower
• Data must transfer accross the bus
• Typically two words of data are trasnfered per read or write
• Memory is slower than the CPU, so the CPU typically waits for the memory
• Programs can be made faster by; reducing memory accesses, making memory access cache-friendly
• Computation can be sped up by speeding up memory access. A cache is a smaller, faster memory between the storage being accessed (main memory) and the accessing component (CPU).
• Caches store commonly accessed data, reducing main memory accesses, which are smaller and more expensive.

The Memory Hierarchy

• Faster storage between the processor and larger, slower storage improves efficiency. Each storage level acts as a cache for the level below it.
• Writing efficient programs requires understanding where data is stored, when it is used, and where it is cached. Which leads to understanding who manages the caches.

Role of the Operating System

• The OS manages computer hardware (CPU, I/O devices, RAM) and provides an execution environment for programs. It acts as an intermediary between the computer, users, and programs, protecting hardware, programs, and users. The OS also provides abstractions for accessing computer resources.

Key Takeaways

• Information equals Data plus Context.
• Data can be represented and interpreted differently.
• Programs must be translated before execution.
• Execution depends on hardware, architecture, and the OS.
• Memory and storage access are dominant costs in execution.
• The OS manages the system, mechanisms, and the execution environment.

Understanding the system itself is crucial to program a system. All computers share the same basic components: hardware and software and these components are interdependent. Manipulating a system requires understanding data representation and interpretation within it.