According to the Intel documentation, all MMX registers require parentheses around the register number. These were compulsory in early versions of PowerBASIC, but the parentheses are now optional.
! PXOR mm(7), mm(7) ' PB/DLL 5.0, PB/CC 1.0 format
! PXOR mm7, mm7 ' PB/DLL/WIN 6.0+, PB/CC 2.0+ format
The Inline Assembler
Data types in registers
Saving Registers at the Procedure level
Tricks in preserving registers