5.18. Jal, Jr Subroutine Instructions
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5.18 jal, jr: Subroutine instructions
Subroutines
A program often needs to perform the same operation for different data values. Ex: Determining the maximum of two value
temperature from Fahrenheit to Celsius, etc. Instead of duplicating the instruction sequence for an operation multiple times
can use a subroutine. A subroutine is a sequence of instructions that performs a specic operation that can be called from
a program. A subroutine call causes the subroutine's statements to execute.
PARTICIPATION
ACTIVITY 5.18.1: Subroutine for computing maximum of two values.
A brief note from your instructor:
Sections 5.18-5.21 provide a good conceptual description of how MIPS subroutines work, using temporary registers ($t)
and parameters. However, practically speaking their translation is not accurate because temporary registers by denition
saved across procedure calls, and MIPS provides $a registers for parameters.
Therefore I recommend skimming this section but will not require completing the activities. I will be demonstrating in cla
way of translating subroutines to MIPS, using these concepts.
# Compute max of DM[5000] & DM[5004]
addi $t6, $zero, 5000
lw $t0, 0($t6)
addi $t6, $zero, 5004
lw $t1, 0($t6)
# Compute max of DM[5000] & DM[5004]
addi $t6, $zero, 5000
lw $t0, 0($t6)
addi $t6, $zero, 5004
lw $t1, 0($t6)
Start 2x speed
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PARTICIPATION
ACTIVITY 5.18.2: Subroutines.
Refer to the animation above.
1) What label indicates the rst instruction
of the subroutine for computing the
maximum value?
slt $t3, $t0, $t1
bne $t3, $zero, Max1IsT1
add $t2, $zero, $t0
j Max1End
Max1IsT1: add $t2, $zero, $t1
Max1End: addi $t6, $zero, 5008
sw $t2, 0($t6)
# Compute max of DM[5012] & DM[5016]
addi $t6, $zero, 5012
lw $t0, 0($t6)
addi $t6, $zero, 5016
lw $t1, 0($t6)
slt $t3, $t0, $t1
bne $t3, $zero, Max2IsT1
add $t2, $zero, $t0
j Max2End
Max2IsT1: add $t2, $zero, $t1
Max2End: addi $t6, $zero, 5020
sw $t2, 0($t6)
No subroutine with redundant code CompMax subroutine computes max of $t0
and $t1, writing max to $t2.
CompMax:
slt $t3, $t0, $t1
bne $t3, $zero, MaxIsT1
add $t2, $zero, $t0
j MaxEnd
MaxIsT1: add $t2, $zero, $t1
MaxEnd: Return from subroutine
addi $t6, $zero, 5008
sw $t2, 0($t6)
# Compute max of DM[5012] & DM[5016]
addi $t6, $zero, 5012
lw $t0, 0($t6)
addi $t6, $zero, 5016
lw $t1, 0($t6)
addi $t6, $zero, 5020
sw $t2, 0($t6)
...
Call CompMax subroutine (not shown)
Call CompMax subroutine (not shown)
CompMax
MaxEnd
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2) How many redundant instructions in the
original code were moved to the
subroutine.
3) A subroutine's instructions must be
duplicated each time the subroutine is
called.
4) A subroutine may have up to 1024
instructions.
Jump and link and jump register instructions
The jump and link (jal) instruction stores the address of the next instruction in register $ra, and then jumps to the instructio
specied location. Ex: jal CalcCube stores the address of the instruction after the jal instruction in $ra, and continues exe
instruction at CalcCube; CalcCube is the label for the rst instruction of the subroutine. The $ra register (or return address r
the instruction address to which a subroutine returns after executing. The jump register (jr) instruction jumps to the instruc
address held in a register. Ex: jr $ra jumps to the instruction at the address held in register $ra. A programmer uses jal to
subroutine, and jr to return from a subroutine.
PARTICIPATION
ACTIVITY 5.18.3: Subroutine call using jal and jr instructions.
5
10
True
False
True
False
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PARTICIPATION
ACTIVITY 5.18.4: jal and jr instructions.
1) Write a jump and link instruction to call
a subroutine named CalcTip.
2) If the jal instruction below is located in
instruction memory at address 200,
what value is written to register $ra?
jal DetSpeed
$zero
$ra
$t6
$t1
$t0
Register file
5004
5000
Data memory DM
0
12
16
20
60
64
68
3
...
20
5000
# Compute cube of 3
addi $t0, $zero, 3
jal CalcCube # Call CalcCube
sw $t1, 0($t6)
...
# CalcCube subroutine
CalcCube:
mul $t1, $t0, $t0
mul $t1, $t1, $t0
jr $ra
927
27
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3) Using the $ra register, write an
instruction to return from a subroutine
named CalcTip.
4) Assume $ra holds 116. If the jr
instruction below is located in
instruction memory at address 200,
what is the address of the instruction
executed after jr $ra?
Arguments and return values
An argument is a value passed to a subroutine, that inuences the subroutine's operations. A return value is a value returne
subroutine. A simple subroutine may use specic registers for the argument and return value. Ex: The CalcCube subroutine
for the subroutine's argument and $t1 for the return value.
The assembly program below passes arguments to the CalcCube subroutine using $t0. The CalcCube subroutine returns t
$t1. The program rst passes 3 to the subroutine by writing 3 to register $t0. After executing the subroutine, $t1 holds the v
stored in data memory at address 5000. The program then passes 17 to the subroutine by writing 17 to $t0. The result of 4
stored to data memory at address 5004.
Figure 5.18.1: Passing arguments to multiple
CalcCube subroutine calls.
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# Initialize registers for DM addresses
addi $t5, $zero, 5000
addi $t6, $zero, 5004
# Compute cube of 3
addi $t0, $zero, 3 # Pass argument of 3
jal CalcCube # Call CalcCube
sw $t1, 0($t5) # Store result to DM[5000]
# Compute cube of 17
addi $t0, $zero, 17 # Pass argument of 17
jal CalcCube # Call CalcCube
sw $t1, 0($t6) # Store result to DM[5004]
j Done
# CalcCube subroutine.
# $t0 is subroutine argument
# $t1 is subroutine return value
CalcCube:
mul $t1, $t0, $t0
mul $t1, $t1, $t0
jr $ra # Return from subroutine
Done:
PARTICIPATION
ACTIVITY 5.18.5: Subroutine arguments and return values.
The CalcEq subroutine below evaluates the equation: x * (y - z). Values for x, y, and z are
passed to the subroutine as arguments.
CalcEq:
sub $t5, $t1, $t2
mul $t3, $t5, $t0
jr $ra
1) Which register is used for x?
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2) Which register is used for the argument
y?
3) Which register is used for the argument
z?
4) Which register is used for the return
value?
PARTICIPATION
ACTIVITY 5.18.6: Create a subroutine.
Using the CompMax subroutine, complete the assembly program to compute the maximum of
the three values in DM[5000], DM[5004], and DM[5008], storing the result in DM[5020].
1. Load $t0 and $t1 with DM[5000] and DM[5004], and call the CompMax subroutine.
2. Copy the result, which is held in $t2, into $t0. Load $t1 with DM[5008]. Call the CompMax
subroutine.
3. Store the result, which is held in $t2, to DM[5020]
$t0
$t1
$t2
$t0
$t1
$t2
$t2
$t3
$ra
$t3
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Table 5.18.1: Instruction summary: jal, jr.
Instruction Format Description Example
jal jal JLabel
Jump and link: Stores the address of the next
instruction in register $ra, and continues execution
with the instruction at JLabel.
jal CalcTip
jr jr $a Jump register: Causes execution to continue with
the instruction at address $a. jr $t3
CHALLENGE
ENTER SIMULATION
STEP
RUN
More options
Assembly Registers
$zero 0
$t0
$t1
$t2
$t3
$t4
$t5
$t6
0
0
0
0
0
0
0
+
Da
5000
5004
5008
5020
+
# FIXME: Compute maximum of DM[5000],
# DM[5004], and DM[5008]
j Done
CompMax:
slt $t3, $t0, $t1
bne $t3, $zero, MaxIsT1
add $t2, $zero, $t0
j MaxEnd
MaxIsT1: add $t2, $zero, $t1
MaxEnd: jr $ra
Done:
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ACTIVITY 5.18.1: Call and create subroutines.
1
2
Start
Pass DM[$t3] to the YearlySalary subroutine, and store the return value to DM[$t4].
$t1 is the subroutine argument.
$t2 is the subroutine return value.
lw
$t1
, 0(
$t1
)
lw
$t1
, 0(
$t1
)
lw
$t1
, 0(
$t1
)
lw
$t1
, 0(
$t1
)
YearlySalary:
addi
$t2
,
$zero
,
2000
mul
$t2
,
$t1
,
$t2
jr
$ra
Done:
2
Registers
$t1 0
$t2 0
$t3 5000
$t4 5004
5
5
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