Page 31 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 2: Introduction to Assembly Language

.ORIG, .FILL, .BLKW, .STRINGZ, and .END. All are easily recognizable by the dot as their first Notes
character.
.ORIG

.ORIG tells the assembler where in memory to place the LC-3b program. In line 05, .ORIG x3050
says, start with location x3050. As a result, the LEA R2, NUMBER instruction will be put in
location x3050.
.FILL
.FILL tells the assembler to set aside the next location in the program and initialize it with the
value of the operand. In line 15, the ninth location in the resultant LC-3b program is initialized to
the value x0006.

.BLKW
.BLKW tells the assembler to set aside some number of sequential memory locations (i.e. a Block
of Words) in the program. The actual number is the operand of the .BLKW pseudo-op. In line 11,
the pseudo-op instructs the assembler to set aside one location in memory (and also to label it
NUMBER, incidentally).
The pseudo-op .BLKW is particularly useful when the actual value of the operand is not yet
known. For example, one might want to set aside a location in memory for storing a character
input from a keyboard. It will not be until the program is run that we will know the identity of
that keystroke.
The argument is a sequence of n characters, inside double quotation marks. The first n 1 bytes of
memory are initialized with the ASCII codes of the corresponding characters in the string, followed
by x00. A final byte x00 is added if necessary to end the string on a word boundary. The n 1st
character (x00) provides a convenient sentinel for processing the string of ASCII codes.

For example, the code fragment
.ORIG x3010
HELLO .STRINGZ “Hello, World!”
would result in the assembler initializing locations x3010 through x301D to the following values:
x3010: x48
x3011: x65
x3012: x6C
x3013: x6C
x3014: x6F
x3015: x2C
x3016: x20
x3017: x57
x3018: x6F
x3019: x72

x301A: x6C
x301B: x64
x301C: x21
x301D: x00

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