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Registers A and B are 8-bit and register D is 16-bit. Registers A cascaded with register B is equivalent to register D. When register A is modified the upper 8 bits of register D are modified. When register B is modified the lower 8 bits of register D are modified. Registers A and B are the destination for 8-bit arithmetic/logic operations. Register D is the destination register for many 16-bit operations.
Index registers IX and IY can be used as pointers to memory locations. Because the 68HC12 has a 16-bit address bus, the index registers are 16-bit as well.
The program counter points to the program memory. Again since the address bus is 16 bits the program counter is 16-bit as well. The stack pointer references memory and is 16-bit. The flag register is 8-bit. The index registers, the stack pointer and the flag register are discussed in detail later in the course.