Computer Systems Fundamentals

Lecture Plan Overview

• This lecture covers Information and Context, Data versus Information, Program Translation, Program Execution, Caching and Storage, and the Role of the Operating System.

Understanding the System

• All computers share the same basic components, which include hardware and software.
• Key Hardware components include the CPU/Processor, RAM, Memory Disks, and Storage.
• Key Software components consist of the Operating System, Programs, System Utilities, and Applications.
• All components are interdependent; a missing or malfunctioning component renders the computer useless.
• The focus is on understanding software, which requires comprehensive knowledge of underlying components.

Manipulation of information

• Programs are designed to produce and manipulate information.
• Information is composed of bits combined with context i.e. Information = Bits + Context

Programs Translation

• Translation must reconcile program & hardware requirements.
• Translation addresses the difference between program needs (potentially large numbers) hardware limitations (32-bit operations).
• A single program instruction can translate into multiple machine instructions.
• Program code relies on libraries for many functions.

Program Execution

• How programs are executed depends on the hardware Architecture/Organization.

Programs Environment

• Program behavior is affected by the Operating System and the Memory Architecture and Caching mechanisms.

Information: Bits + Context

• Data by itself lacks inherent meaning, needing interpretation or context to derive meaning.
• Data shares the same meaning but represented differently, such as in ASCII, Hex, or Binary.

Data vs. Information

• The difference is illustrated using #include <stdio.h>
• Data represents raw, uninterpreted content, while information is a more interpreted representation of this data
• e.g. C code represents the information, while the hexadecimal representation is the raw data

Same Data, Different Meanings

• Considers different ways of interpreting data such as C test, 8-bit integers, and 32-bit integers.

Context Matters

• To make data meaningful, context to interpret the data is needed.
• "hello.c" contains Text, 8-bit integers, or 32-bit integers etc
• Context applies to all information in a system, including Files, Memory, and the CPU.
• Everything within a system is subject to interpretation based on context.
• To effectively manipulate a system, one must understand how data is represented and interpreted.

Program Translation

• Program translation is a fundamental aspect of systems.
• Programs undergo multiple translations to be compatible with a system.
• Java programs are translated as: Java program (.java) → byte code (.class) → machine code (running on JVM)
• C programs are translated as: C program (.c) → assembly (.s) → object code (.o) → executable
• At each translation, the program is altered.
• Each stage relies on the previous stage and the translator used.

The C Compilation Chain

• C Code -> Preprocessor -> Modified source program -> Compiler hello.s -> Assembler hello.o -> Linker hello

Optimizing Performance

• Writing efficient code is a key goal.
• Effective coding enhances performance, e.g., using switch statements over if statements, and considering loop and indexing choices.

Avoiding Security Holes

• Understanding how programs are compiled helps avoid security bugs.
• Recognizing potential issues such as stack/buffer overflows and uninitialized memory is crucial.

Understanding Linking Errors

• Understanding linking errors is essential.
• This includes dealing with multiple definitions of global variables/functions, variable/function definitions in header files, and linking order issues.

Execution on the Computer

• Program execution relies on hardware components such as the CPU (Processor), Instruction Set, Word size (16-bit, 32-bit, 64-bit), and Implementation of the CPU.
• RAM is used to determine the amount of memory and speed, and caching.
• Bus interconnect manages word-size, speed, and coherency protocols.

Fetch-Decode-Execute

• The CPU operates in an infinite loop, executing instructions.
• Processes involve fetching the next instruction from memory.
• The process involves decoding and executing instructions.
• Execution may involve reading from or writing to memory, modifying registers/CPU state, or changing program locations.

Instructions Speed

• Instructions that access memory are slower: accessing memory requires data transfers across the bus, slowing down the CPU.
• Efficiency involves reducing memory accesses and using cache-friendly methods.

Cache Speed

• Caches can speed up execution by storing commonly accessed data, reducing access to main memory.
• Caches are smaller and closer to the CPU but are more expensive.

Memory Hierarchy

• Inserting faster storage between the processor and a larger, slower storage is a general idea.
• Each level of storage acts as a cache for the level below it.
• To write efficient programs, we need to understand the location of data, when data is used and where the data is cached.

Operating System Role

• Managing computer hardware, including CPU, I/O devices, and RAM.
• Providing an execution environment for programs.
• Acting as an intermediary between the computer, users, and programs.
• Protecting hardware, programs, and users from each other,
• The OS provides abstractions for accessing computer resources.

Resources and Abstractions

• Abstractions provided by the operating system include the CPU, managed with Processes.
• RAM and Secondary Storage (eg HD) are managed as Memory Address Space and Files respectively.
• Network and Peripheral Devices as Files.

Key Abstractions

• Processes are instances of running programs, associated with memory space and potentially open files.
• Memory (Address) Space defines the range of accessible memory locations uniquely identified by addresses.
• A File is a data stream identified by a name in the operating system's file system or file descriptor.

Fundamental Abstractions: Processes

• The OS's abstraction of a running program & run concurrently.
• Instructions interleaved with those of other processes.

Fundamental Abstractions: Virtual Memory

• Abstraction allowing each process the exclusive use of memory.
• Virtual address space seen by each process with specified purposes with areas well defined: Kernel memory, User stack, memory region and Run-time heap.

Fundamental Abstractions: Files

• A file is a sequence of bytes. Every I/O device – displays, keyboards, and even the network are files
• Files provide applications with a uniform view across many types of I/O devices.

Process Composition

• CPU context: General registers, program counter and special purpose registers
• Memory space: Stack for local variables/return addresses, Heap for dynamically allocated memory, Data for global/static variables, and Code for the program's instructions.

Thread of Execution

• A thread is an execution of program code.
• Location stored in PC, which points to the current instruction.
• Threads of execution depends on only the CPU context and memory

Key Take-Aways

• Information = data + context.
• Data can be represented and interpreted in multiple ways.
• Programs must be translated before execution.
• Translation affects execution quality from hardware and the OS
• Memory/storage access impacts execution cost; systems minimize access costs.
• The OS manages the system, providing execution mechanisms, abstractions and environments.