Page 112 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

4,! Introductory Example and Assembler Printout 89

program into machine code, and you should understand how the assembler works.
Although you may not be able to understand how to write an assembler, you will be
prepared from now on to use an assembler as a tool to help you write your programs,

4.1 Introductory Example and Assembler Printout

We now consider a simple example to introduce you to assembly-language programs.
Consider a program that obtains the maximum of a sequence of numbers. We will
assume that this sequence consists of 16-bit unsigned numbers stored consecutively in
memory, high byte first for each number. This data structure is called a vector or (one-
dimensional) array. The name of the vector will be the location of the first byte of the
vector, so that the high byte of the ith number in the vector (i = 0, 1, 2, . . .) can be
found by adding 2*i to the vector name. Suppose then that Z is a vector of four 16-bit
two's-complement numbers beginning in location $86a with N stored in location $868.
The ith number will be denoted Z(i) for i = 0 through N - 1. We want a program that
finds the maximum of these numbers, putting it in locations $868 and $869.
One possible program for this, following the style of previous examples, is shown
in Figure 4.1. We have arbitrarily started the program at address $89C.
Looking at the preceding program, we certainly would like to use just the
mnemonics column with the variable addresses and the labels for the branches and let the
assembler generate the other two columns, that is, do what we have been doing by hand.
We would also like to be able to use labels, also called symbolic addresses (or just
symbols) for the memory locations that hold the values of variables. The meaning of
symbolic addresses is explored in greater detail in the next chapter. We use them in this
section to get the main idea (they are used before dissecting them carefully). The use of
symbolic addresses allows program segment (2) to be replaced by program segment (3),

LDX #$86A (2)
STD $868

LDX #Z (3)
STD RESULT

Figure 4.1. Program MAX