In the beginning of 1997 the MMX instructions were introduced in the Pentium processors, so multitasking systems wouldn’t save the newly introduced MMX registers. To work around that problem, Intel mapped the MMX registers to the FPU register.
The consequence is that you can’t mix MMX and floating point operations. After using MMX operations and before using floating point operations, you have to call the routine EMMS of the MMX unit. This routine restores the FPU registers.
Careful: The compiler doesn’t warn if you mix floating point and MMX operations, so be careful.
The MMX instructions are optimized for multimedia operations (what else?). So it isn’t possible to perform all possible operations: some operations give a type mismatch, see section 5.4 for the supported MMX operations.
An important restriction is that MMX operations aren’t range or overflow checked, even when you turn range and overflow checking on. This is due to the nature of MMX operations.
The MMX unit must always be used when doing MMX operations because the exit code of this unit clears the MMX unit. If it wouldn’t do that, other program will crash. A consequence of this is that you can’t use MMX operations in the exit code of your units or programs, since they would interfere with the exit code of the MMX unit. The compiler can’t check this, so you are responsible for this!