Using and Preserving Registers in Inline Assembly

Microsoft Specific

In general, you should not assume that a register will have a given value when an __asm block begins. Register values are not guaranteed to be preserved across separate __asm blocks. If you end a block of inline code and begin another, you cannot rely on the registers in the second block to retain their values from the first block. An __asm block inherits whatever register values result from the normal flow of control.

If you use the __fastcall calling convention, the compiler passes function arguments in registers instead of on the stack. This can create problems in functions with __asm blocks because a function has no way to tell which parameter is in which register. If the function happens to receive a parameter in EAX and immediately stores something else in EAX, the original parameter is lost. In addition, you must preserve the ECX register in any function declared with __fastcall.

To avoid such register conflicts, don't use the __fastcall convention for functions that contain an __asm block. If you specify the __fastcall convention globally with the /Gr compiler option, declare every function containing an __asm block with __cdecl or __stdcall. (The __cdecl attribute tells the compiler to use the C calling convention for that function.) If you are not compiling with /Gr, avoid declaring the function with the __fastcall attribute.

When using __asm to write assembly language in C/C++ functions, you don't need to preserve the EAX, EBX, ECX, EDX, ESI, or EDI registers. For example, in the POWER2.C example in Writing Functions with Inline Assembly, the power2 function doesn't preserve the value in the EAX register. However, using these registers will affect code quality because the register allocator cannot use them to store values across __asm blocks. In addition, by using EBX, ESI or EDI in inline assembly code, you force the compiler to save and restore those registers in the function prologue and epilogue.

You should preserve other registers you use (such as DS, SS, SP, BP, and flags registers) for the scope of the __asm block. You should preserve the ESP and EBP registers unless you have some reason to change them (to switch stacks, for example). Also see Optimizing Inline Assembly.

Some SSE types require eight-byte stack alignment, forcing the compiler to emit dynamic stack-alignment code. To be able to access both the local variables and the function parameters after the alignment, the compiler maintains two frame pointers. If the compiler performs frame pointer omission (FPO), it will use EBP and ESP. If the compiler does not perform FPO, it will use EBX and EBP. To ensure code runs correctly, do not modify EBX in asm code if the function requires dynamic stack alignment as it could modify the frame pointer. Either move the eight-byte aligned types out of the function, or avoid using EBX.


If your inline assembly code changes the direction flag using the STD or CLD instructions, you must restore the flag to its original value.

END Microsoft Specific

See also

Inline Assembler