Page 57 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

34 Chapter 2 The Instruction Set

2.2 Arithmetic Instructions

The computer is often used to compute numerical data, as the name implies, or to keep
books or control machinery. These operations need arithmetic instructions, which we
now study. However, you must recall that computers are designed and programs are
written to enhance static or dynamic efficiency. Rather than have the four basic
arithmetic instructions that you learned in grade school—-add, subtract, multiply, and
divide—computers have the instructions that occur most often in programs. Rather than
having the sophisticated square root as an instruction, for instance, we will see the often-
used increment instruction in a computer. Let us look at them. See Table 2.4.
We have already discussed the add instructions: ADCA, ADCB,ADDA, ADDB, and
ADDD. The corresponding subtraction instructions, SBCA, SBCB, SUBA, SUBB, and
SUBD, are the obvious counterparts of add instructions, where the carry condition code
bit holds the borrow. However, 16-bit add and subtract instructions with carry, ADCD and
SBCD, are missing; multiple-byte arithmetic must be done one byte at a time rather than
two bytes at a time. Comparisons are normally made by subtracting two numbers and
checking if the result is zero, negative, positive, or a combination of these. But using
the subtract instruction to compare a fixed number against many numbers requires that
the fixed number has to be reloaded in the register each time the subtraction is performed.
To streamline this process, compare instructions are included that do not change the
contents of the register used. These compare instructions are used to compare the
contents of registers A, B, D, X, Y, and SP with the contents of memory locations in
order to give values to the condition code bits C, V, N, and Z. Finally, note that DEC,
INC, and NEC are provided for often-used special cases of add and subtract instructions,
to improve efficiency.

Table 2.4. Add Instructions Using an Effective Address