Transcript View File - University of Engineering and Technology, Taxila

Computer Architecture &
Organization
Instructions: Language of Computer
Engr. Umbreen Sabir
Computer Engineering Department,
University of Engg. & Technology Taxila.
1
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Instructions for Making Decisions
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Distinguishes computer from simple calculator.
–
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Decision making Instructions are if with goto and label.
MIPS decision making statements are:
–
Branch if equal-> beq register1, register2, L1
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–
Branch not equal-> bne register1, register2, L1
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2
Go to label L1, if register1==register2
Go to label L1, if register1!=register2
These two statements are traditionally called
Conditional Branches.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
MIPS Control Flow Instructions
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MIPS conditional branch instructions:
bne $s0, $s1, Lbl #go to Lbl if $s0$s1
beq $s0, $s1, Lbl #go to Lbl if $s0=$s1

Ex:
Lbl1:
3
if (i==j) h = i + j;
bne $s0, $s1, Lbl1
add $s3, $s0, $s1
…
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
MIPS Control Flow Instructions
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Instruction Format (I format):
op
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4
rs
rt
16 bit offset
How is the branch destination address
specified?
Keep it pending till next chapter….
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Compiling if-then-else into
Conditional Branches:
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In the code below f, g, h, I and j are in registers
$s0 through $s4 .
C code is
– if ( i==j ) f= g + h; else f=g-h;
Its assembly notation is:
–
–
–
–
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Bne $s3, $s4, Else # go to Else if I != j
add $s0, $s1, $s2
# f= g + h (skipped if i != j).
j Exit
# go to Exit.
Else:sub $s0, $s1, #s2 # f= g – h (skipped if i == j).
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Loops
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Decisions are also important for iterating
a computation.
–
–
6
Found in loops.
Refer to next slide for an example.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Example # 1
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C statements are:
– While ( save[i] ==k)
–
i += 1;

Assume i and k are in $s3 and $s5 and base of
save is in $6.
We must multiply i by 4 due to byte addressing
problem.
We use shift left logical since shifting left by 2
bits multiply by 4.
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11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Example # 1 cont.
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Assembly code will be:
–
–
–
–
–
–
–
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Loop:sll $t1, $s3, 2 # Temp reg $t1 = 4 * i
Add $t1, $t1, $s6 # $t1= address of save[i]
Lw $t0, 0( $t1) # Temp reg $t0 = save[i]
Bne $t0, $s5, Exit# go to Exit if save[i] != k
Add $s3, $s3, 1
# i = i + 1;
J Loop
# go to Loop
Exit:
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Basic Block
9
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A sequence of instructions without branches,
except possibly at the end, and without
branch targets or branch labels, except
possibly at the beginning.
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One of the early phases of compilation is
breaking the program into basic blocks.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
More Branch Instructions

We have beq, bne, but what about other kinds of
branches (e.g., branch-if-less-than)? For this, we
need yet another instruction, slt.
Set on less than instruction:
slt $t0,$s0,$s1 # if $s0 < $s1 then
# $t0 = 1
else
# $t0 = 0
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Instruction format (R format):

op
10
rs
rt
11/02/2009
rd
funct
CA&O Lecture 03 by Engr. Umbreen Sabir
More Branch Instructions cont.
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11
Constant operands are also used in comparisons.
Register $zero always has 0, we can compare to
zero.
To compare to other values, instruction is:
Set on less than immediate:
slti $t0,$s2,10 # $t0 = 1 if $s2< 10
MIPS doesn’t include branch on less than because it
is too complicated.
Either it would stretch the clock cycle time or it would
take extra clock cycles/instruction.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
More Branch Instructions cont.

Can use slt, beq, bne, and the fixed value of 0 in
register $zero to create all relative conditions.
– less than
blt $s1, $s2, Label
slt
bne
–
–
–
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$at, $s1, $s2
$at, $zero, Label
#$at set to 1 if
# $s1 < $s2
less than or equal to ble $s1, $s2, Label
greater than
bgt $s1, $s2, Label
great than or equal to bge $s1, $s2, Label
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Case/switch Statement
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13
Case/switch can be implemented through
a sequence of conditional tests.
More effectively done by jump address
table.
For this MIPS include a jump register
instruction (jr), meaning an unconditional
jump to the address specified in a
register.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Case/switch Statement

MIPS also has an unconditional branch
instruction or jump instruction:
j label
#go to label.
 Instruction Format (J Format):
op
14
26-bit address
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Supporting Procedures in
Computer Hardware


Procedure: A stored subroutine that performs a
specific task based on the parameters with which it
is provided.
Procedure execution step
–
–
–
–
–
–
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Place Parameters.
Transfer control to the procedure.
Acquire storage resources
Perform desired task.
Place result value.
Return control back.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Supporting Procedures in
Computer Hardware cont.

MIPS register conventions for procedures:
–
–
–
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MIPS instruction for procedures is jump and
link instruction (jal).
–
16
$a0 - $a3: argument registers
$v0 - $v1: two registers to return values.
$ra – return address register
It jumps to an address and simultaneously saves
the address of following instruction in return
register $ra.
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Instructions for Accessing
Procedures
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MIPS procedure call instruction:
jal ProcedureAddress #jump n link

Machine format (J format):
op
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Then can do procedure return with a
jr $ra
#return
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Instruction format (R format):
op
17
26 bit address
rs
funct
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
Next Lecture and Reminders
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Next lecture
–
MIPS ISA
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18
Assignment Due – 13 Feb 2009
11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir
END OF LECTURE 3
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11/02/2009
CA&O Lecture 03 by Engr. Umbreen Sabir