3.9 Directive OPTION WIN64

Directive OPTION WIN64 allows to set parameters for the Win64 output format if this format (see -win64 cmdline option) is selected. For other output formats, this option has no effect. The syntax for the directive is:

OPTION WIN64: switches

accepted values for switches are:

Store Register Arguments [ bit 0 ]:

- 0: the "home locations" (also sometimes called "shadow space") of the first 4 register parameters are uninitialized. This is the default setting.

- 1: register contents of the PROC's first 4 parameters (RCX, RDX, R8 and R9 ) will be copied to the "home locations" within a PROC's prologue.

INVOKE Stack Space Reservation [bit 1]:

- 0: for each INVOKE the stack is adjusted to reserve space for the parameters required for the call. After the call, the space is released again. This is the default setting.

- 1: the maximum stack space required by all INVOKEs inside a procedure is computed by the assembler and reserved once on the procedure's entry. It's released when the procedure is exited. If INVOKEs are to be used outside of procedures, the stack space has to be reserved manually!
Note: an assembly time variable, @ReservedStack, is created internally when this option is set. It will reflect the value computed by the assembler. It should also be mentioned that when this option is on, and a procedure contains no INVOKEs at all, then nevertheless the minimal amount of 4*8 bytes is reserved on the stack.
Warning: You should have understood exactly what this option does BEFORE you're using it. Using PUSH/POP instruction pairs to "save" values across an INVOKE is VERBOTEN if this option is on.

16-byte Alignment for Local Stack Variables [bit 2]:

0: standard 8-byte alignment for local variables.

1: 16-byte alignment for local variables. This setting is useful if you want to load or store XMM registers with instructions that expect aligned memory references ( i.e. MOVAPS ). Note that variables with size < 16 are not affected.