Conditional and Unconditional
Operation
The instructions to make
decisions are called branch instructions. There are two kind of branch
instructions: conditional instructions and unconditional instructions. Compared
with high-level programming languages, the conditional branch is similar to an if statement,
while unconditional branch (or just branch) to goto statement.
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Conditional
branch
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branch on equal
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beq $1,$2,100
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if ($1 == $2) go to
PC+4+100
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Equal test; PC
relative branch
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branch on not equal
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bne $1,$2,100
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if ($1 != $2) go to
PC+4+100
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Not equal test; PC
relative
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set on less than
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slt $1,$2,$3
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if ($2 < $3) $1
= 1; else $1 = 0
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Compare less than;
2`s complement
|
|
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set less than
immediate
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slti $1,$2,100
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if ($2 < 100) $1
= 1; else $1 = 0
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Compare <
constant; 2`s complement
|
|
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set less than
unsigned
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sltu $1,$2,$3
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if ($2 < $3) $1
= 1; else $1 = 0
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Compare less than;
natural number
|
|
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set less than
immediate unsigned
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sltiu $1,$2,100
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if ($2 < 100) $1
= 1; else $1 = 0
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Compare constant;
natural number
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Example :
Compiling an if statement
into a conditional branch
The if statement of C
language
|
f
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g
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h
|
i
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j
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$s0
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$s1
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$s2
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$s3
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$s4
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if (i==j) f = g + h; else f = g - h;
can be compiled to
bne
$s3, $s4, Else # go to Else if
i!= j
add
$s0, $s1, $s2 # f = g + h
j
Exit # go to Exit
Exit :
lw
$t0, 0($t1) # $t0 = A[i]
add
$s1, $s1, $t0 # $s1 = g = g +
A[i]
add
$s3, $s3, $s4 # i = i + j
bne
$s3, $s2, ForLoop # go to ForLoop
if i != h
Compiling the while loop
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i
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j
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k
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A[]
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$s3
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$s4
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$s5
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$s6
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while (A[i] == k) i = i + j;
can be compiled to :
Loop:
add
$t1, $s3, $s3
add
$t1, $t1, $t1 # $t1 = 4 * i
add
$t1, $t1, $s6
lw
$t0, 0($t1) # $t0 = A[i]
bne
$t0, $s5, Exit # go to Exit if
A[i] != k
add
$s3, $s3, $s4 # i = i + j
j
Loop # go to Loop
Exit:
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Unconditional jump
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jump
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j 10000
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goto 10000
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Jump to target address
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jump register
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j $31
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goto $31
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For switch, procedure return
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jump and link
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jal 10000
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$31 = PC + 4;
go to 10000
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For procedure call
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By : Ku Man Yi B031210161
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