Page 100 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

3.5 Examples 77

Figure 3.18. Program Segment to Move a Block of Memory
overflow; that is, the result can be expressed with an 8-bit two's-complement number.
Note that accumulator B is initially loaded with the number of times that the loop is to
be executed. This loop counter (accumulator B in this case) is decremented by DBNE ,
which branches back to the location L if the (B) is greater than zero after it is
decremented. The loop from location L to the DBNE instruction is repeated N times,
where N is the initial value in accumulator B.
In the program in Figure 3.19, register A is used to accumulate the result, register B
holds the number of remaining bytes to be added, and the index register X contains the
address that points to the next byte to be added.
We extend the multiply-and-accumulate example of §2.7 to evaluate the expression:
2 i = 0 Bi * Q , where a vector of five signed 16-bit numbers Bj are at $914, and a
vector of five signed 16-bit numbers Q is at $9IE. This expression is called the inner
product of the vectors B and C. The two vectors B and C can have as many elements as
you want, but the two vectors have the same number of elements. It is widely used in
signal processing and data compression. See Figure 3.20. Note that EM ACS only uses
pointer addressing, so the index registers X and Y must be moved using LEA
instructions in order to pick up the elements Bi and Q. This procedure is very similar to
the WAV instruction but is for 16-bit elements while WAV is for 8-bit elements.
This section illustrates several examples of the great value of the synergetic
combination of autoincrement index addressing and counting using the DBNE instruction.
The combination of accumulator index addressing and counting using the DBNE
instruction, whose counter register is the accumulator used with the index, is also widely
used. We seern to run into such a combination in every other program that we write.

Figure 3.19. Program Segment to Add Vector Elements