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

LABEL OPCODE OPERANDS COMMENTS

• 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

Labels

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

Loop ADD R1,R1,#-1
     Brp LOOP

Comments

• 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

Pseudo-operations

• 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