GNU-EFI ABI and Calling Convention

Overview

During development of the Memory Map Viewer, an unexpected behaviour occurred when calling UEFI Boot Services directly.

The issue appeared to be related to the interaction between GNU-EFI, GCC generated code and the firmware calling convention on x86_64.

The direct function call:

SystemTable->BootServices->GetMemoryMap(...)

caused unreliable behaviour.

The same function call through the GNU-EFI helper:

uefi_call_wrapper(...)

worked correctly.

UEFI firmware functions are not ordinary C library functions.

They are provided by firmware and must follow the calling convention defined by the UEFI specification.

On x86_64 systems, differences in:

can lead to invalid function calls even when the C syntax appears correct.

The compiler generates a normal function call based on the active compiler ABI.

Example:

Status = SystemTable->BootServices->GetMemoryMap(...);

GNU-EFI provides:

uefi_call_wrapper(Function, Arguments, ...);

The wrapper ensures that the function call follows the expected UEFI calling convention.

The exact root cause was not fully investigated.

The current assumption is:

Further investigation would require:

This issue demonstrated that firmware development has additional constraints compared to normal application development.

A function prototype alone does not guarantee a valid function call.

When interacting with firmware interfaces, developers must consider:

This finding influenced the implementation style used throughout the project.

Firmware calls are treated carefully, especially when working with:

Understanding these details is essential for future boot loader development.