Instructions: Language of the Computer

• We begin a quick trip through the MIPS instruction set.
• We will be writing some programs using MARS
• I expect that you will learn a portion of this language, and will be able to write simple programs in it.
• Read the chapter. At least 2.1 through 2.14.
• There are three sources for understanding the instruction set
  • The green card lists the Core instruction set.
  • Page 64 lists these instructions in a more programmer friendly way.
  • A.10, especially beginning on page A-51 trough A-80 provides a detailed overview of each instruction.
• Let's look at several instructions on each.
  • ADD

    • Name       Mnemonic   Format   Operation              OPCODE/FUNCT
      Add        add         R      R[rd]= R[rt] + R[rs]    0/2016
      
      Note: May cause overflow exception
      	    

    • Opcode	RS	RT	RD	shamt	funct
      31    26	25  21	20  16	15  11	10    6	5    0

    • Category   Inst  Example             Meaning        Comment
      Arithmetic add   add $s1, $s2, $s3   $s1 = $s2+$s3  Three register operation
      	    

  • LW

    • Name       Mnemonic   Format   Operation              OPCODE/FUNCT
      Load Word  lw          I      R[rt] = M[R[rs]+SignExtendImm]   2316
      
      Note: SignExtImm = { 16{immediate[15],immediate}
      	    

    • Opcode	RS	RT	immediate
      31    26	25  21	20  16	15           0

    • Category   Inst  Example             Meaning        Comment
      Data Trans lw    lw $s1, 20($s2)     $s1=M[$s2+20]  Word from mem to register
      	    

  • J

    • Name    Mnemonic   Format   Operation              OPCODE/FUNCT
      Jump    J          J       PC = JumpAddr            2
      
      Note: JumpAddr = {PC+4[31:28],address,2'b0}
                  

    • This one takes just a little more work.
      • It uses the top 4 bits of the pc
      • All 26 bits of the address
      • And is padded with two zeros on the bottom end.

    • Opcode	address
      31    26	25       0

    • Category   Inst  Example             Meaning        Comment
      Uncond B   jump  jump 2500           to to 10000    Jump to target address
      

    • Assume the top four bits of the PC are 0000
    • The instruction becomes 0000 2500₁₀ 00
    • or 000010011100010000₂ = 10000₁₀
• There are pseudo instructions
  • Several examples include move rd, rs and li $rs, imm
• So here is a small program in MIPS assembly.

  .data
  a:	.word 100
  
  .text		
  .globl main
  
  main:
  	lw	$t1, a
  	add	$t2, $t1, $t1
  	
  	li	$v0, 1
  	move	$a0, $t2
  	syscall
  	j	exit
  		
  exit:
  	li	$v0, 10
  	syscall
      

• or the source code