;++
;
; Module Name:
;
;   cs.asm
;
;--

.CODE

;++
;
; PVOID
; MwGenericCall(
;   _In_ ULONG SyscallNumber,
;   _In_ ULONG ArgumentCount,
;   _In_ va_list ArgumentList
;   )
;
;--

    MwGenericCall PROC PUBLIC

        ;
        ; Save r12 to the shadow space.
        ; This register will be held as a temporary
        ; stack pointer for the syscall.
        ;

        mov     qword ptr [rsp + 8], r12

        ;
        ; eax = SyscallNumber
        ; r10 = ArgumentCount
        ; r11 = ArgumentList
        ; r12 = ArgumentList - sizeof(PVOID)
        ;       (space for return address)
        ;

        mov     eax,  ecx
        mov     r10d, edx
        mov     r11,  r8
        lea     r12,  qword ptr [r11 - 8]


        ;
        ; Assign first 4 arguments (rcx, rdx, r8, r9)
        ; from the ArgumentList.
        ;

        cmp     r10d, 1
        cmovae  rcx, qword ptr [r11 +  0]

        cmp     r10d, 2
        cmovae  rdx, qword ptr [r11 +  8]

        cmp     r10d, 3
        cmovae  r8,  qword ptr [r11 + 16]

        cmp     r10d, 4
        cmovae  r9,  qword ptr [r11 + 24]

        ;
        ; The syscall handler expects the first
        ; parameter in the r10 register.
        ;

        mov     r10, rcx

        ;
        ; Stack pivot, syscall, restore stack.
        ;

        xchg    r12, rsp
        syscall
        xchg    r12, rsp

        ;
        ; Restore r12 register from the shadow space.
        ;

        mov     r12, qword ptr [rsp + 8]

        ret

    MwGenericCall ENDP


END