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Internally, the HC11 instruction set is upward compatible with the 6800, with the addition of a Y index register. (Instructions using the Y register have opcodes prefixed with the byte 0x18). It has two eight-bit accumulators, A and B, two sixteen-bit index registers, X and Y, a condition code register, a 16-bit stack pointer, and a program counter. In addition, there is an 8 x 8-bit multiply (A x B), with full 16-bit result, and Fractional/Integer 16-bit by 16-bit Divide instructions. A range of 16-bit instructions treat the A and B registers as a combined 16-bit D register for comparison (X and Y registers may also be compared to 16-bit memory operands), addition, subtraction and shift operations, or can add the B accumulator to the X or Y index registers. Bit test operations have also been added, performing a logical AND function between operands, setting the correct conditions codes, but not modifying the operands.