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Monday, November 8, 2010

MIPS:

Conditional branches

(beq, bne, slt, slti,  sltu( chck unsigned number) )

MIPS
if ( i == j )
  f = g + h;
else
  g = g - h;
assume:
f = $s0, g = $s1,  h = $s2
i = $s3,  j = $s4

#---------------
bne   $s3, $s4, ELSE
add   $s0, $s1, $2
j    Exit     # skip else

ELSE:
sub   $s1, $s1, $s2

Exit:
if ( i < j )
  f = g + h;
else
 g = g - h;
slt $t0, $s3, $s4    # i < j, $t0 = 0, else $t0=1

bne $t0, 1, Else
add    $s0, $s1, $s2
j    Exit      # skip else part

Else:
sub $s1, $s1, $s2

Exit:



While Loop


MIPS
while ( save[i] == k )
  i += 1;
assume:  i = $s3, k = $s5 , save[0] = $s6

#------------------------
(convert lw $t0, 4i($s6) )

Loop:
sll    $s3, $s3, 2        #  i = 4i
add  $t1, $s3, $s6    # 4i($s6)
lw    $t0, 0($t1)         #  lw $t0, 4i($s6)

bne $t0, $s5, Exit
addi $s3,$s3, 1
j  Loop                      # loop again

Exit:




Case / Switch Statement



switch (k)
{
 case 0:  f = i + j; break;
 case 1:  f = g + h; break;
 case 2:  f = i - j;  break;
 case 3:  f = g - h; break;

}

---------------------------------------
assume:  f = $s0, g = $s1, h = $s2, i = $s3,
j = $s4, k = $s5
$t2 = 4
$t4 contains address of an array, jumptable

slt $t0, $s5, $0   # k < 0; $t0 = 1; else $t0 =0
beq $t0, 1, Exit  # k < 0, go to L1
beq $s5, 0, L0   # k = 0, go to L2
beq $s5, 1, L1
beq $s5, 2, L2
beq $s5, 3, L3
slt  $t0, 3, $s5  # k > 3, $t0 = 1; else $t0 = 0
beq $t0, 1, Exit

L0:
  add $s0, $s3, $s4
  j Exit
L1:
  add $s0, $s1, $s2
  j Exit
L2:
  sub $s0, $3, $s4
  j Exit
L3:
  sub $s0, $s1,$s2
  j Exit

Exit:



Procedures



MIPS
int leaf_example ( int g, int h, int i, int j )
{
   int f;
   f = ( g + h )  - ( i + j );
   return f;
}

f, g, h , i,  j are local variables in the procedure.

any declaired variable in procedure use
$s0  - $s7

addi $sp, $sp, -12
sw    $t1,  8($sp)
sw    $t0,  4($sp)
sw    $s0, 0($sp)

add $t0, $a0, $a1
add $t1, $a2, $a3
sub $s0, $t0, $t1
add $v0, $s0, $0

lw  $s0, 0($sp)
lw  $t0,  4($sp)
lw  $t1,  8($sp)
addi $sp, $sp, 12

jr $ra
Does it work??? If I don’t store in the stack?

add $v0, $a0, $a1
add $t0, $a2, $a3
sub $v0, $v0, $t0

jr $ra

is it right????
#--------------------------------
addi $sp, $sp, -8
sw $t0, 4($sp)
sw $t1, 0($sp)

add $t0, $a0, $a1
add $t1, $a2, $a3
sub $v0, $t0, $t1

lw $t1, 0($sp)
lw $t0, 4($sp)
addi $sp, $sp,8

jr $ra



Nested Procedures


int fact ( int n )
{   
   if ( n < 1 ) return (1);
   else
          return ( N * fact ( n - 1));
}
#----------------------
fact:
addi   $sp, $sp, -8
sw     $ra, 4($sp)    # save the return address
sw     $a0, 0($sp)   # save the argument n

slti   $t0, $a0, 1      # n < 1; $t0 = 1; else $t0 = 0
beq  $t0, $0, L1     # n >= 1 go to L1

addi  $v0, $0, 1      # return 1
addi  $sp, $sp, 8
jr    $ra

L1:
addi  $a0, $a0, -1    # n >= 1; argument gets ( n - 1 )
jal fact
lw     $a0, 0($sp)
lw     $v0, 4($sp)
addi $sp, $sp,8

mul   $v0, $a0, $v0    # return n * fact ( n - 1 )

jr     $ra
int fact ( int n )
{   
   if ( n < 1 ) return (1);
   else
          return ( N * fact ( n - 1));
}
2 values need to be stored:
n, return value

fact:
addi $sp, $sp, -8
sw $a0, 4($sp)
sw $ra, 0($sp)

slt $t0, $a0, 1        # $a0 < 1 , $t0 = 1; else $t0 = 0
beq $t0, $0, L1      # n >= 1
# n < 1
add $v0, $0, 1       # return 1  
addi $sp, 8($sp)   #
jr $ra

L1:  # (n >= 1)
addi $a0, $a0, -1   # n--
jal  fact

lw $ra, 0($sp)
lw $a0, 4($sp)
addi $sp, $sp, -8
mul $v0, $a0, $v0   # N * fact (n -1)

jr $ra







void strcpy ( char x[], char y[] )
{
  int i;
  i = 0;
  while ( x[i] = y[i] != 0 )
  i = i + 1;
}

$a0 = address of x
$a1 = address of y

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