Designing Embedded Hardware, 2nd Edition

Coding in Assembly

Assembly-language programming is not difficult, particularly with the smaller microcontroller and microprocessor architectures. The main difficulty many people seem to have is knowing how to tackle a given programming problem. As with any other language, there are always many ways in which a program may be written. There is rarely a single "right way" (although there may be a "most efficient way"). It is simply a matter of breaking down the problem into a series of tiny steps.

For example, let's say we want a 68HC11 processor to add three numbers together (0x10, 0x1F, and 0x0C) and store the result at address 0x0027. This problem is easily broken down into four steps. We take the first number (step one), add the second number (step two), add the third number (step three), and store the result (step four). We want to do some arithmetic, so we choose an accumulator to hold our numbers, since accumulators are registers designed for this type of operation. We start by loading an accumulator with the first number:

LDAA   #$10   ; load first number into the A accumulator

The A accumulator now contains the number 0x10 (16 in decimal). Remember that the "#" in the instruction means that the addressing mode is immediate. In other words, the processor is to load the actual number 0x10, not use the operand as a pointer. We then want to add the number 0x1F. So our next instruction is:

ADDA   #$1F   ; add the second number to the A accumulator

This causes 0x1F to be added to accumulator ...