AH/BH/CH/DH are so seldom useful that it's not really worth adding support for using them to the compiler. They only exist for 4 of the 16 registers, and they o

AH/BH/CH/DH are so seldom useful that it's not really worth adding support for using them to the compiler. They only exist for 4 of the 16 registers, and they only let you address one random byte of the 8 bytes in that register. You get a bit slower code and a more complicated register allocator in order to save, what, a few bytes in the entire program?