From Machine Language To Assembly Language


Assembler is a program that turns symbols into binary machine instructions.

$Lc-_3$ Assembly language Syntax

  • Each line of a program is one of the following:
    • an instruction
    • an assembler directive (or pseudo-op)
    • a comment
  • Whitespace (between symbols) and case are ignored.
  • Comments (beginning with “;”) are also ign

$Lc-_3$ Assembly Language Instruction


  • optional: Label, Comments
  • mandatory: Opcode, Operands

Opcode and Operands

  • Opcodes:
    • reserved symbols that correspond to LC-3 instructions
    • example: ADD, AND, LD, LDR
  • Operands:
    • Registers:$R_n$
    • Numbers: #(DEC) or x(HEX)
    • Label: symbolic name oof memory location


placed at the beginning of the line assigns a symbolic name to the address corresponding to line

Loop ADD R1,R1,#-1
     Brp LOOP


  • Anything after a semincolon is comments
  • Ignored by assembler
  • Used by humans to document/ understand programs
  • Tips for useful comments:
    • avoid restating the obvious, as “decrement R1”
    • provide additional insight, as in “accumulate product in R6”
    • use comments to separate pieces of program

Assembler Directives


  • Do not refer to operations executed by program
  • used by assembler
  • look like instruction, but “opcode” starts whith dots
Opcode Operand Meaning
.ORIG address starting address of program
.END end of program  
.BLKW n allocate n words of storage
.FILL n allocate one word,initializa with value n
.STRINGZ n-character string allocate n+1 locations, initialize w/characters and null terminator