Can I get a more in-depth guide about the inline assembler?

[ZILtoid1991 via Digitalmars-d-learn]

Here's the assembly code for my alpha-blending routine:
ubyte[4] src = *cast(ubyte[4]*)(palette.ptr + 4 * *c);
ubyte[4] *p = cast(ubyte[4]*)(workpad + (offsetX + x)*4 + 
offsetY);
asm{	//moving the values to their destinations
movd	MM0, p;
movd	MM1, src;
movq	MM5, alpha;
movq	MM7, alphaMMXmul_const1;
movq	MM6, alphaMMXmul_const2;
									punpcklbw	MM2, MM0;
punpcklbw	MM3, MM1;

paddw	MM6, MM5;	//1 + alpha
psubw	MM7, MM5;	//256 - alpha

pmulhuw	MM2, MM6;	//src * (1 + alpha)
pmulhuw MM3, MM7;	//dest * (256 - alpha)
paddw	MM3, MM2;	//(src * (1 + alpha)) + (dest * (256 - alpha))
psrlw	MM3, 8;		//(src * (1 + alpha)) + (dest * (256 - alpha)) / 
256
									//moving the result to its place;
									packuswb	MM4, MM3;
movd	p, MM4;
emms;
}

The two constants being referred here:
static immutable ushort[4] alphaMMXmul_const1 = [256,256,256,256];
static immutable ushort[4] alphaMMXmul_const2 = [1,1,1,1];

alpha is a ushort[4] containing the alpha value four times.

After some debugging, I found out that the p pointer becomes null 
at the end instead of pointing to a value. I have no experience 
with using in-line assemblers (although I made a few Hello World 
programs for MS-Dos with a stand-alone assembler), so I don't 
know when and how the compiler will interpret the types from D.