Page 114 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

4.1 Introductory Example and Assembler Printout 91

Table 4.2. Addressing Modes

Notes: 1: Prefix "<" or postfix .B forces Page 0. 2t Prefix ">"
or postfix .W forces Direct. 3: Can substitute Y, SP, or PC for X.
4: Can substitute Y or SP for X. 5: Can substitute B or D for A
address or label ALPHA, the addressing modes in Table 4.2 can now all use symbolic
addresses in place of numbers in the previous examples.
The assembler understands the use of addition, multiplication, and the like, using
symbolic addresses in expressions. If ALPHA is location 100 and the operand field
contains ALPHA+1, the assembler will put in the value 101. In simplest terms, an
expression is just the usual algebraic combination of labels, numbers, and C language
operations +, -, *, /, %, «, », &, I , ~, !, <, >, <=, >=. !=, ==. Pascal operators = and
o are also recognized. Parenthesis are allowed, and precedence and evaluation are exactly
as they are in C. Some examples of expressions are:
JUMP JUMP* ( 8 -I- TAB) ( (RATE-2 ) * 17 ) -TEMP

Comment Field
In the comment field, the programmer can insert short comments stating the purpose of
each instruction. The comment must begin with a semicolon (;). In other assemblers, the
comments begin one or more blanks after the operand field and are printed in the
assembler listing but are otherwise ignored by the assembler.

In summary, writing an assembly-language program is a lot easier than writing
machine code by hand. You can use symbolic addresses, letting the assembler determine
where to put them and letting the assembler make sure that the instructions have the
right operand values. You do have to conform to the rales of the language, however, and
you have to spell the mnemonics exactly the way the assembler wants to recognize
them. Although it would be nice to be able to just talk to the computer and tell it what
you want it to do using conversational English, an assembler can barely understand the
mnemonics for the instructions if you write them correctly and carefully. Nevertheless,
writing assembly-language programs is easier than writing hexadecimal machine code.