Lecture 4

Lecture 4

 

• Main memory is one long single memory array of cells
  
  • The cells are bytes
    
  • These are collapsed into words
    
    • Instructions are stored in words
      
    • Words take 4 bytes of main memory
      
• Memory access
  
  • Data from memory to register
    
    • lw a, r(b)
      
      • Where 'b' is the base address
        
      • 'r' is the relative address
        
        • Relative address MUST be a constant, cannot be a variable
          
      • 'a' is the absolute (actual) address of register
        
  • Data to memory from register
    
    • sw a, r(b)
      
      • 'b' is base address
        
      • 'r' is relative address
        
        • Relative address MUST be a constant, cannot be a variable
          
      • 'a' is absolute address from register STORE WORD to register from main memory
        
• Compile Assignment and using main memory
  
  • Change c program into MIPS
    
    • g = h + A[8]
      
  • MIPS program ( in Arial font)
    
    • lw $t0, 32($s3)
      
      • A[8]'s relative address is 32 ( 4*8 )
        
    • add $s1, $s2, $t0
      
  • multiplying with mips
    
    • using previous example how do i get 4*i (without just knowing it)?
      
      • add $t1, $s4, $s4
        
        • find four times i
          
      • add $t1, $t1, $t1
        
        • find absolute address
          
      • add $t1, $s3, $t1
        
        • t1 ß base + relative addresses
          
          • $s3 is the base address
            
      • Lw $t0, 0($t1)
        
        • Load
          
      • Add $s1, $s2, $t0
        
        • Add h + A[i]
          
  • Beq reg1, reg2, label
    
    • Operands (register1, register2, label)
      
    • If (r1=r2) then go to L1
      
  • Bne reg1, reg2, label
    
    • Operands; (register, register2, label)
      
    • If r1 != r2 then go to L2
      

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If ( i == j ) then

    F = g + h

    Else

    F = g – h

 

If ( i != j ) then go to else

    F = g + h;

    Go to exit;

Else

    F = g – h;

Exit

MIPS

bne $s3, $s4, Else

add $s0, $s1, $s2

j Exit #jump to exit

Else: sub $s0, $s1, $s2

Exit:

Loops

Example:

Loop: g=g + A[i]

I=i+j;

If (i != h) go to loop;

 

Loop

1. Get address of A[i]
   
2. Load A[i]
   
3. G ßg+A[i]
   
4. I ßi+j
   
5. If (i != h) go to loop
   

Steps 1-3 are the body, 4 is the modift, 5 is the stopping rule

 

MIPS

Loop: add $t1, $s3, $s3

Add $t1, $t1, $t1

Add $t1, $s5, $t1    #where $s5 is the base address of the array

Lw $t0, 0($t1)

Add $s1, $s1, $t0

Add $s3, $s3, $s4

Bne $s3, $s2, loop

Exit: