Byte and word issues
1 Appending “.b” to an instruction makes it a byte operation.
A byte instruction with a register destination clears the high 8 bits of the register to 0. Thus, the following would clear the top byte of the register, leaving the lower byte unchanged:
mov.b Rn,Rn
2 Adding ".w"(or nothing) to an instruction makes it a word operation, which is also the default behaviour.
3 Peripherals are divided into an 8-bit bank and a 16-bit bank.
The 8-bit peripherals must only be accessed using 8-bit instructions; using a 16-bit access produces garbage in the high byte.
The 16-bit peripherals must only be accessed at even addresses. Byte accesses to even addresses are legal, but not usually useful.
原文：http://www.cnblogs.com/mengdie/p/4486415.html