LC3 Assembler Labels

1
LC3 Assembler Labels

• Labels are case sensitive
• Lables are 1 to 10 alphabetic or numeric
  characters.
• Lables are normally used as the target of a
  branch statement, BRp AGAIN
• Labels are used to name a location for a value
  that is loaded or stored.
• Any statement can have a label but there is no
  need unless the location is referenced.

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LC3 Assembler Comments

• Comments are for people who read the program or
  to help remind you of something.
• A semicolon indicates that the rest of the line
  is a comment.
• A comment may follow a statement or it may be the
  first character on a line.
• Too many comments clutter up a program
• Use a comment(s) at the start of your programs to
  give your name, assignment , email

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LC3 Assembler Pseudo-ops

• A better name might be assembler directives
• Once the assembler handles the directive or
  message, then it is discarded.
• The LC3 has 5 Pseudo-Ops
• .ORIG
• .FILL
• .BLKW
• .STRINGZ
• .END

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LC3 .ORIG

• .ORIG tells the assembler where in memory to
  start the program. Many people use .ORIG x3000
• It is usually better to take an easy location
  since .ORIG x3C4D will work but it makes it
  harder to follow the memory locations.
• A program can only have one .ORIG statement and
  not 2. If you want to write a program to load in
  different parts of memory, then write it in two
  parts.

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LC3 .FILL

• .FILL sets aside a memory location and fills it
  with the value of the operand. Dont confuse this
  with define in C.
• .FILL is useful for constants or to name memory
  locations.
• SEVEN .FILL 7 this will store a 7 in the
  location and name it SEVEN
• TEMP .FILL 0 this will name the location TEMP
  and store a 0 in it.

6
LC3 .BLKW

• .BLKW reserves a Block of Words or sequential
  memory locations. The operand indicates the
  number to reserve.
• Used to reserve some locations where data will be
  stored during execution of the program.
• Could also be used to store a table of data.

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LC3 - .STRINGZ

• Stores a string of characters.
• Uses n1 memory locations and each location
  stores one character in zero-extended ascii code.
  
• Terminates string with x0000
• Characters to be stored are enclosed with double
  quotations.
• HELLO .STRINGZ Hello World!

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LC3 .END

• Indicates where the program ends.
• Anything after .END is ignored.
• .END does not stop the program, it only tells the
  assembler that this is the end of the source
  program.

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LC3 - Examples

• ADD1to5.asm
• Mult6by4.asm
• GET11-7.asm
• shift-left-4.asm
• Loop Counter
• Two-Part Program
• first.asm
• second.asm