.ALLOCSTACK Directive

[D.F.]

Can somebody explain what .ALLOCSTACK directive do?

I don't understand Microsoft desciption:

Generates a UWOP_ALLOC_SMALL or a UWOP_ALLOC_LARGE with the specified size for the current offset in the prologue. .ALLOCSTACK allows ml64.exe users to specify how a frame function unwinds and is only allowed within the prologue, which extends from the PROC FRAME declaration to the .ENDPROLOG directive. These directives do not generate code; they only generate .xdata and .pdata. .ALLOCSTACK should be preceded by instructions that actually implement the actions to be unwound. It is a good practice to wrap both the unwind directives and the code they are meant to unwind in a macro to ensure agreement.

[ToutEnMasm]

http://msdn.microsoft.com/en-us/library/ck9asaa9.aspx