Page 30 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes END does not stop the program during execution. In fact, .END does not even
exist at the time of execution. It is simply a delimiter; it marks the end of the
source program.
2.1.4 Comments

Comments are messages intended only for human consumption. They have no effect on the
translation process and indeed are not acted on by the LC-3b Assembler. They are identified in
the program by semicolons. A semicolon signifies that the rest of the line is a comment and is to
be ignored by the assembler. If the semicolon is the first non-blank character on the line, the
entire line is ignored. If the semicolon follows the operands of an instruction, then only the
comment is ignored by the assembler.
The purpose of comments is to make the program more comprehensible to the human reader.
They help explain a nonintuitive aspect of an instruction or a set of instructions. In line 0A, the
comment “Clear R3; it will contain the product” lets the reader know that the instruction on line
0A is initializing R3 prior to accumulating the product of the two numbers. While the purpose of
line 0A may be obvious to the programmer today, it may not be the case two years from now,
after the programmer has written an additional 30,000 lines of code and cannot remember why
he/she wrote AND R, R3, 0. It may also be the case that two years from now, the programmer no
longer works for the company and the company needs to modify the program in response to a
product update. If the task is assigned to someone who has never seen the code before, comments
go a long way to helping comprehension.
It is important to make comments that provide additional insight and not just restate the obvious.
There are two reasons for this. First, comments that restate the obvious are a waste of everyone’s
time. Second, they tend to obscure the comments that say something important because they add
clutter to the program. For example, in line 0F, the comment “Decrement R1” would be a bad
idea. It would provide no additional insight to the instruction, and it would add clutter to the
page.
Another purpose of comments, and also the judicious use of extra blank spaces to a line, is to
make the visual presentation of a program easier to understand. So, for example, comments are
used to separate pieces of the program from each other to make the program more readable. That
is, lines of code that work together to compute a single result are placed on successive lines, while
pieces of a program that produce separate results are separated from each other. For example,
note that lines 0E through 10 are separated from the rest of the code by lines 0D and 11. There is
nothing on lines 0D and 11 other than the semicolons. Extra spaces that are ignored by the assembler
provide an opportunity to align elements of a program for easier readability. For example, all the
opcodes start in the same column on the page.

Prepare a program and use some of the instructions.

2.1.5 Pseudo-ops (Assembler Directives)
The LC-3b assembler is a program that takes as input a string of characters representing a computer
program written in LC-3b assembly language, and translates it into a program in the ISA of the
LC-3b. Pseudo-ops are helpful to the assembler in performing that task. Actually, a more formal
name for a pseudo-op is assembler directive. They are called pseudo-ops because they do not refer
to operations that will be performed by the program during execution. Rather, the pseudo-op is
strictly a message to the assembler to help the assembler in the assembly process. Once the assembler
handles the message, the pseudo-op is discarded. The LC-3b assembler contains five pseudo-ops:

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