---
tags:
  - low-level
---
- How data structures & computational routines are accessed in machine code ([Code Types](Code%20Types.md))
	- Machine code therefore hardware-dependent
- API defines this structure in source code
- Adherence usually responsibility of
	- [Compilers](Compilers.md)
	- OS
	- Library author

# Components

- Processor instruction set
	- Register file structure
	- Stack organisation
	- Memory access types
- Size, layouts and alignments of basic data types
- [Calling Conventions](Calling%20Conventions.md)
	- How function arguments are passed
		- Stack or register
		- Which registers for which function param
		- Parameter order
	- Return values retrieved
- How app makes sys calls to OS
	- If ABI specifies direct sys calls over procedure calls to sys call stubs then sys call numbers
- Binary format of object files, program files
	- For complete OS ABIs

# Complete ABI
- Allows program from one OS to run on any other without change
	- Provided all shared libraries and prerequisites etc

# Embedded ABI
- File format, data types, register usage, stack frame organisation, function parameter passing conventions
	- For embedded OS
- [Compilers](Compilers.md) create object code compatible with code from other [compilers](Compilers.md)
	- Link libraries from different [compilers](Compilers.md)