Why Applications Are Operating-System Specific
This YouTube video explains why an executable file from Windows won’t run on a Mac, even if both use the same CPU architecture (like Intel x86 before Apple’s switch to ARM). The core reason isn’t just the CPU architecture, but the operating system’s significant differences.
Key points:
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System Calls: Programs rely heavily on system calls—requests to the operating system for services (e.g., reading a file). Different operating systems have different sets of system calls, even if they use the same CPU. Even when system calls have similar names, their functionality or required parameters can differ significantly.
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Kernel Mode vs. User Mode: The CPU operates in two modes. User programs run in a restricted user mode, requiring system calls to access hardware. The OS runs in kernel mode (privileged), handling those requests. This separation is crucial for security.
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System Call Implementation Details: Many low-level details impact compatibility, even if the system calls are seemingly identical:
- System call numbers and tables: Each system call is assigned a number. The OS uses this number to look up the corresponding function in a table. Differences in numbering or the table’s location can cause issues.
- Parameter passing: The methods for passing parameters (registers vs. memory locations, stack vs. specific memory regions, address width) vary between operating systems.
- Application Binary Interface (ABI): This set of conventions defines how different binary code components interact. Incompatibilities here cause problems.
- Executable file format: The structure of the executable file (e.g., PE format on Windows, ELF on Linux) differs between operating systems.
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Runtime Environments: Programs using runtime environments (like Java or Python) might run across multiple platforms if the runtime is installed, but this doesn’t resolve all compatibility issues, especially if the application has modules compiled to machine code alongside runtime-dependent ones.
In short, while the CPU executes machine code, the operating system’s significant role in mediating hardware access through system calls and its various implementation details create major compatibility barriers between different operating systems. Even small differences in these low-level details can lead to program crashes or unpredictable behavior.