Intro to Assembly

• In assembly language numeric opcodes are replaced with nemonics
• Op codes are replaced with
  • Register labels
  • Address labels

• 	.data
  array: .space 400
  
  	.text
  	addi	$t1,$zero,0     		# t0 will be the lcv
  	addi 	$t2,$zero, 1
  #
  # for i=0;i<100;i++
  #     a[i] = 1
  #
  top:	beq     $t0, 400, step2
  	sw	$t2, array($t0)
  	addi	$t0, $t0, 4
  	b 	top
  
  step2:	
  
  end:	li	$v0, 10
  	syscall
  
      

• The job of the assembler is to (at least)
  • Convert the nemonics into opcodes
  • Resolve labels
• These two tasks themselves make the assembler IMMENSELY valuable.
• There are some other tasks.
• Look at the green sheet, do you see a b instruction?
  • Many assemblers allow for pseudo instruction
  • These are instructions which the assembler supports but are not part of the ISA.
  • The assembler must translate these pseudo instructions into real instructions.
  • Look at what happens to beq $t0, 400, step2
• Users are allowed to define macros
  • these are essentially user defined pseudo instructions
  • The assembler is responsible for dealing with these.
• Users often like (need, must) declare and set up data
  • The assembler deals with this as well.
  • Assembler directives are used to tell the assembler that we are in different parts of the program
  • .data, .text
  • They are also used to accomplish tasks (.align, .extern)
  • Or to identify data types (.word, .ascii, .space)
• Often we wish to include an external library
  • you know, to do things like print, or even exit properly
  • The linker takes the output of the assembler and produces an executable file.
  • But often this is a single command (or button push)