MMX Misaligned

You are correct. The assembly code listing in the text box "Maintaining Stack Alignment contained an error and, as a result, aligns the stack to 32 bits. In order to align the stack to 64 bits, the alignment mask should be 0FFFFFFF8, as shown below.--Bev Bachmayer, Intel Corp.

Prologue:
push   ebp            ;save old frame ptr
mov    ebp, esp       ;make new frame ptr
sub    ebp, 4         ;make room for stack ptr
and    ebp, 0FFFFFFF8 ;align to 64-bits
mov
    [ebp], esp     ;save old stack ptr
mov    esp, ebp       ;copy alignment ptr
sub    esp, FRAMESIZE ;allocate space