SP0256 Program Counter
The microsequencer in the SP0256 conceptually maintains a 19-bit Program Counter. The upper 16 bits of the PC form a byte address. On control transfers (branches and returns), the upper 16 bits of the PC get set to the byte address of the target, and the lower 3 bits initialize to 0.
All serial speech ROMs keep a shadow copy of the PC as a function of the SP0256 Serial Bus Protocol.