Listing 1: “Hello World!” in LC-3.
The above listing is a typical hello world program written in LC-3 assembly language. The program
outputs “Hello World!” to the console and quits. We will now look at the composition of this
program.
Lines 1 and 2 of the program are comments. LC-3 uses the semi-colon to denote the beginning
of a comment, the same way C++ uses “//” to start a comment on a line. As you probably already
know, comments are very helpful in programming in high-level languages such as C++ or Java. You
will find that they are even more necessary when writing assembly programs. For example in C++,
the subtraction of two numbers would only take one statement, while in LC-3 subtraction usually
takes three instructions, creating a need for further clarity through commenting.
Line 3 contains the .ORIG pseudo-op. A pseudo-op is an instruction that you can use when
writing LC-3 assembly programs, but there is no corresponding instruction in LC-3’s instruction
set. All pseudo-ops start with a period. The best way to think of pseudo-ops are the same way you
would think of preprocessing directives in C++. In C++, the #include statement is really not a C++
statement, but it is a directive that helps a C++ complier do its job. The .ORIG pseudo-op, with its
numeric parameter, tells the assembler where to place the code in memory.
Memory in LC-3 can be thought of as one large 16-bit array. This array can hold LC-3 instructions or it can hold data values that those instructions will manipulate. The standard place for code
to begin at is memory location x3000. Note that the “x” in front of the number indicates it is in
hexadecimal. This means that the “.ORIG x3000” statement will put “LEA R0, HW” in memory
location x3000, “PUTS” will go into memory location x3001, “HALT” into memory location x3002,
and so on until the entire program has been placed into memory. All LC-3 programs begin with the
.ORIG pseudo-op.
Lines 4 and 5 are LC-3 instructions. The first instruction, loads the address of the “Hello World!”
Revision: 1.17, January 20, 2007 viiProgramming in LC-3
string and the next instruction prints the string to the console. It is not important to know how these
instructions actually work right now, as they will be covered in the labs.
Line 6 is the HALT instruction. This instruction tells the LC-3 simulator to stop running the
program. You should put this in the spot where you want to end your program.
Line 7 is another pseudo-op .STRINGZ. After the main program code section, that was ended
by HALT, you can use the pseudo-ops, .STRINGZ, .FILL, and .BLKW to save space for data that
you would like to manipulate in the program. This is a similar idea to declaring variables in C++.
The .STRINGZ pseudo-op in this program saves space in memory for the “Hello World!” string.
Line 8 contains the .END pseudo-op. This tells the assembler that there is no more code to assemble. This should be the very last instruction in your assembly code file. .END can be sometimes
confused with the HALT instruction. HALT tells the simulator to stop a program that is running.
.END indicates where the assembler should stop assembling your code into a program.
Syntax of an LC-3 Instruction
Each LC-3 instruction appears on line of its own and can have up to four parts. These parts in order
are the label, the opcode, the operands, and the comment.
Each instruction can start with a label, which can be used for a variety of reasons. One reason
is that it makes it easier to reference a data variable. In the hello world example, line 7 contains
the label “HW.” The program uses this label to reference the “Hello World!” string. Labels are also
used for branching, which are similar to labels and goto’s in C++. Labels are optional and if an
instruction does not have a label, usually empty space is left where one would be.
The second part of an instruction is the opcode. This indicates to the assembler what kind of
instruction it will be. For example in line 4, LEA indicates that the instruction is a load effective
address instruction. Another example would be ADD, to indicate that the instruction is an addition
instruction. The opcode is mandatory for any instruction.
Operands are required by most instructions. These operands indicate what data the instruction
will be manipulating. The operands are usually registers, labels, or immediate values. Some instructions like HALT do not require operands. If an instruction uses more than one operand like LEA in
the example program, then they are separated by commas.
Lastly an instruction can also have a comment attached to it, which is optional. The operand
section of an instruction is separated from the comment section by a semicolon.
LC-3 Memory
LC-3 memory consists of 216 locations, each b
