msfastcall

Msfastcall is a calling convention used on 32-bit x86 Windows, primarily associated with Microsoft toolchains and some assemblers. It is a variant of the fastcall family designed to improve call efficiency by placing the initial arguments in registers rather than on the stack.

Under msfastcall, the first two parameters are passed in CPU registers, typically ECX and EDX. Any additional arguments are passed on the stack in right-to-left order, similar to other 32-bit conventions for stack-passed parameters. The exact rules for which parameter types are placed in registers can vary slightly between compilers, but the general approach is to reduce stack usage and improve speed for common call patterns.

The responsibility for cleaning up the stack falls to the callee in msfastcall. The function being called must remove the stack-based arguments from the stack frame before returning. In addition, callee-saved registers must be preserved across calls, with registers such as EBX, ESI, EDI, and EBP typically required to be preserved by the callee.

Compatibility and usage notes: msfastcall is largely associated with older Windows toolchains and 32-bit code. It is not a universal standard and is not portable to non-Microsoft compilers in the same way as more widely adopted conventions. Modern 64-bit Windows uses a different Application Binary Interface, and many developers rely on the broader, standardized x86-64 calling conventions. For code portability, it is common to limit reliance on msfastcall to specific project targets and compiler versions, referring to the respective compiler documentation for exact semantics.