LC-3 Pseudo-Ops (Assembler Directives) pp. 182 – 183, Introduction to Computing Systems, Patt & Patel

The pseudo-op or assembler directive is strictly a message to the assembler to help in the assembly process. Once the assembler handles the message, the pseudo-op is discarded. The LC-3 assembler contains 5 pseudo-ops: .ORIG, .FILL, .BLKW, .STRINGZ, and .END. The dot is the first character of these pseudo-ops.

.ORIG

.ORIG tells the assembler where in memory to place the LC-3 program. Ex:

.ORIG x3000

.FILL

.FILL tells the assembler to set aside the next location in the program and initialize it with the value of the operand. A label on the left binds a name to the location. <This is a constant> Ex:

SIX .FILL 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 opearand of the .BLKW pseudo-op. A label on the left of .BLKW binds a label to the memory location. <This is a variable to store a value> Ex:

NUMBER .BLKW 1

.STRINGZ

.STRINGZ tells the assembler to initialize a sequence of n + 1 memory locations. The argument is a sequence of n characters, inside double quotation marks. The first n words of memory are initialized with the zer-extended ASCII codes of the corresponding characters in the string. The final word of memory is initialized to 0. The last character, x0000, provides a convenient sentinel for processing the string of ASCII codes. Ex:

.ORIG x3010

HI .STRINGZ “Hey!”

Would store the following values in the indicated locations:

x3010: x0048 ; the letter H

x3011: x0065 ; e

x3012: x0079 ; y

x3013: x0021 ; !

x3014 x0000 ; sentinel value

.END

.END tells the assembler where the program ends. Any characters that come after .END will not be used by the assembler. Note: .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. Ex:

.END