Step 2 - Defining variables The first step is defining the variables that will be used in this code sample: clicking on the button with a key on it or choosing Edit|Variables from the menu (Ctrl-V) opens the Edit Variables window. The name of the variable is typed in the edit box at the top of the dialog, then clicking on the Add button will add the newly defined variable to the list of variables on the center of the window. When you select a variable from the list, you can optionally choose its type (Signed or Unsigned) and size (Byte, Word or DoubleWord).  A permanent variable is allocated to a MMX register at the beginning of the code and it is never deallocated: permanent variables are used to implement accumulators that must be preserved between iterations of the same loop. Allocating a permanent variable reduces the number of free MMX registers and it may reduce the degree of parallelism that can be achieved: a variable should never be declared as permanent if it does not need it. You can also force the allocation of a permanent variable to a specified MMX register. Now two variables (SourceImage1 and SourceImage2) that contain 8 unsigned bytes each have been defined, so press the OK button  to close the Edit Variables window.