.SAVEREG
Generates either a UWOP_SAVE_NONVOL or a UWOP_SAVE_NONVOL_FAR unwind code entry for the specified register (reg) and offset (offset) using the current prologue offset. MASM will choose the most efficient encoding.
Syntax
.SAVEREG reg, offset
Remarks
.SAVEREG 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. .SAVEREG 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.
For more information, see MASM for x64 (ml64.exe).