=====SIMD signum function=====

//by Richard Russell, 30th December 2014 (based on a [[http://blogs.msdn.com/b/oldnewthing/archive/2014/12/29/10583275.aspx|blog post]] by Raymond Chen)//\\ \\  The **signum** function returns **-1** for all negative numbers, **+1** for all positive non-zero numbers and **zero** for an input of zero; it corresponds to the **SGN()** function in BBC BASIC. There is no direct equivalent in assembly language, but the following code snippets illustrate how it may be calculated on multiple data values using MMX and SSE2 code. In all cases the **input** is in mm0 or xmm0 and the **output** is in mm1 or xmm1:\\ \\ **Signum of eight 8-bit values**\\ 
        pxor    mm1, mm1
        pxor    mm2, mm2
        pcmpgtb mm1, mm0 ; mm1 = pcmpgt(0, x)
        pcmpgtb mm0, mm2 ; mm0 = pcmpgt(x, 0)
        psubb   mm1, mm0 ; mm1 = signum
\\ **Signum of four 16-bit values**\\ 
        pxor    mm1, mm1
        pxor    mm2, mm2
        pcmpgtw mm1, mm0 ; mm1 = pcmpgt(0, x)
        pcmpgtw mm0, mm2 ; mm0 = pcmpgt(x, 0)
        psubw   mm1, mm0 ; mm1 = signum
\\ **Signum of two 32-bit values**\\ 
        pxor    mm1, mm1
        pxor    mm2, mm2
        pcmpgtd mm1, mm0 ; mm1 = pcmpgt(0, x)
        pcmpgtd mm0, mm2 ; mm0 = pcmpgt(x, 0)
        psubd   mm1, mm0 ; mm1 = signum
\\ **Signum of sixteen 8-bit values (requires ASMLIB2 library)**\\ 
        pxor    xmm1, xmm1
        pxor    xmm2, xmm2
        pcmpgtb xmm1, xmm0 ; xmm1 = pcmpgt(0, x)
        pcmpgtb xmm0, xmm2 ; xmm0 = pcmpgt(x, 0)
        psubb   xmm1, xmm0 ; xmm1 = signum
\\ **Signum of eight 16-bit values (requires ASMLIB2 library)**\\ 
        pxor    xmm1, xmm1
        pxor    xmm2, xmm2
        pcmpgtw xmm1, xmm0 ; xmm1 = pcmpgt(0, x)
        pcmpgtw xmm0, xmm2 ; xmm0 = pcmpgt(x, 0)
        psubw   xmm1, xmm0 ; xmm1 = signum
\\ **Signum of four 32-bit values (requires ASMLIB2 library)**\\ 
        pxor    xmm1, xmm1
        pxor    xmm2, xmm2
        pcmpgtd xmm1, xmm0 ; xmm1 = pcmpgt(0, x)
        pcmpgtd xmm0, xmm2 ; xmm0 = pcmpgt(x, 0)
        psubd   xmm1, xmm0 ; xmm1 = signum