TECHNOLOGY 

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KembaraXtra- Computer Science - User Mode Bubble and System Calls
I. User Mode Limitations
  • Definition: Code running in user mode has restricted system access.
  • Capabilities:
    • Read/Write to its own virtual memory.
    • Perform mathematical and logical operations.
    • Control program flow of its own code.
  • Limitations:
    • Cannot access physical memory addresses (including memory-mapped I/O).
    • Cannot directly perform I/O operations (e.g., printing, keyboard input, graphics, sound, network communication, file access).
  • "User Mode Bubble": Analogy representing user mode's isolation from direct hardware interaction.
  • Practical Effect: User mode code can do work but requires assistance to share results.
II. System Calls: Bridging the Gap
  • Definition: A request from user mode code for kernel mode code to perform a privileged operation on its behalf.
  • Purpose: Allows user mode applications to interact with the outside world (perform I/O).
  • Mechanism:
    1. User mode code requests a specific operation (e.g., reading from a file).
    2. The kernel (with device drivers) performs the operation.
    3. The kernel returns the results to the user mode process.
  • Kernel's Role:
  • Acts as an intermediary between user mode code and hardware resources.
  • Provides an abstraction layer, hiding hardware details.
  • Constraints: The kernel enforces security and access control policies (e.g., preventing unauthorized file access).
III. System Call Implementation
  • CPU Instructions: Specific instructions facilitate system calls.
    • ARM: SVC (Supervisor Call).
    • x86: SYSCALL, SYSENTER.
  • System Call Numbers: Each system call is identified by a unique number.
    • Example (Linux ARM): write (file writing) is number 4.
  • Process:
i.Load the system call number into a specific processor register.
ii.Put parameters into other registers.
iii.Execute the system call instruction.
IV. System Calls in Practice
  • Abstraction: Developers typically don't make system calls directly.
  • OS APIs and Standard Libraries: Operating systems and programming languages provide interfaces for system calls (e.g., open, CreateFileA).
  • Transparency: Programmers write code at a higher level and may not be aware of the underlying system calls.
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