Transcript Class 11.2 Conditional Branches.pptx

Conditional Branches
• What distinguishes a computer from a simple
calculator is its ability to make decisions
• Decisions are made using the if statement,
sometimes combined with “go to” statements
and “labels”.
• MIPS assembly language includes many decisionmaking instructions. Two of them are BEQ and
BNE.
Branch on Equal
if ($u == $v) then jump to “Label”
beq
u,v,Label
 if the bit patterns in register $u == register $v
 PC <-- address of Label
 after a delay of one machine cycle.
 else
 Continue the sequence of instructions without
jumping.
bit patterns – unsigned or two’s complement ?
“If” structure
 IF $8 equals to $9 then go to CONT.
 Otherwise do the block of statements
 Then again go through the CONT
“If” structure on assembly
 The bit patterns in two registers are compared.
 If the bit patterns are the same, the PC is
changed to the branch address.
 There is a branch delay following the
instruction (just as for a jump instruction).
...
beq
sll
...
...
...
cont: add
# load values into $8 and $9
$8,$9,cont
$0,$0,0
$10,$10,$11
#
#
#
#
#
#
branch if equal
branch delay slot
conditionally
executed
statements
always executed
Two – way decision
...
equal:
cont:
beq
sll
...
...
...
j
sll
...
...
...
add
$8,$9,equal
$0,$0,0
?
cont
$0,$0,0
$10,$10,$11
#
#
#
#
#
#
#
load values into
$8 and $9
branch if equal
branch delay slot
false branch
# branch delay slot
#
# true branch
#
# always executed
Branch on Not Equal
if ($u != $v) then jump to “Label”
bne
u, v, Label
 if register $u != register $v
 PC
<-- address of Label
 after a delay of one machine cycle.
 else
 Continue the sequence of instructions without jumping.
Absolute Value calculation
# Get A
lui
lw
sll
$10,0x1000
$8,0($10)
$0,$0,0
# base register
# Load A
# no-op
# Is A Negative?
srl
$9,$8,31
beq
$0,$9,done
sll
$0,$0,0
#
#
#
Shift sign bit
sign bit == zero
no-op
# Store –A
sub
sw
#
#
negate A
save it
#
target of the branch
$8,$0,$8
$8,0($10)
done: sll
$0,$0,0
.data
A:
.word
-1
BEQ instruction format
beq
$0, $9, done
beq
$0
$9
0
opcode
oprnd
dest
0
0
3
offset(2’s comp.)
000100 00000 01001 0000 0000 0000 0011
0001 0000 0000 1001 0000 0000 0000 0011
1
0
0
9
0
0
0
3
New PC = PC + Off*4
= 0x00400010+4 + 0x0003*4
= 0x00400014 + 0x0000000C = 0x00400020
[0x00400000]
[0x00400004]
[0x00400008]
[0x0040000c]
PC+4 [0x00400010]
[0x00400014]
+12 [0x00400018]
[0x0040001c]
done:[0x00400020]
0x3c0a1000
0x8d480000
0x00000000
0x00084fc2
0x10090003
0x00000000
0x00084022
0xad480000
0x00000000
lui $10, 4096
lw $8, 0($10)
nop
srl $9, $8, 31
beq $0,$9,12 [done-0x00400010]
nop
sub $8, $0, $8
sw $8, 0($10)
nop
Addressing Modes
4. BEQ and BNE use PCrelative addressing,
where the branch address
is the sum of the PC and a
constant in the instruction
 Addressing mode – One
of several addressing
regimes delimited by
their varied use of
operands and or
addresses.