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Faculty of Computer Science
Subroutines (Part 1)
The 68K Stack
CMPUT 229
© 2006
Department of Computing Science
A stack-based machine
Problem: Compute (A+B)(C-D) in a stack-based
machine.
D
B
C
C
C-D
A
A
A+B
A+B
A+B
A+B
(A+B)(C-D)
Push A
Push B
Add
Push C
Push D
Add
Multiply
CMPUT 229
Clements, pp. 264
© 2006
Department of Computing Science
A memory stack-based machine
Problem: Compute (A+B)(C-D) in a stack-based
machine.
CMPUT 229
Clements, pp. 264
© 2006
Department of Computing Science
Pushing D0 into the stack
Stack pointer (A7) always points to element at the top of the stack.
- Decrement A7 before a push
- Increment A7 after a pull
CMPUT 229
Clements, pp. 265
© 2006
Department of Computing Science
MOVEM
MOVEM saves and restores group of registers.
CMPUT 229
Clements, pp. 265
© 2006
Department of Computing Science
Subroutine Calling Conventions
Parameter Passing
– Where?
• On registers
• On the stack frame
– How?
• By value
• By reference
Register Preservation Conventions
– Which registers are preserved by a function call?
CMPUT 229
© 2006
Department of Computing Science
Register-Saving Convention for CMPUT 229
After Apple Computer’s C convention:
Register
Preserved by Function Call?
D0-D2
No
D3-D7
Yes
A0
No
A1
No
A2-A6
Yes
A7
Stack Pointer
CMPUT 229
© 2006
Department of Computing Science
BSR and JSR
There are two instructions to call a subroutine:
– BSR (Branch to Subroutine)
• Is relative to the current address
– Range of - 32 kbytes to + 32 kbytes
– Allows position-independent code
– JSR (Jump to Subroutine)
• The address is absolute
– Range is not limited
– Code is position dependent
CMPUT 229
© 2006
Department of Computing Science
Passing Parameter On The Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
State of the Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
State of the Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
State of the Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
State of the Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
State of the Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
State of the Stack
CMPUT 229
Clements, pp. 273
© 2006
Department of Computing Science
PEA - Push Effective Address
PEA pushes the address specified into the stack.
PEA
X
Is equivalent to
MOVE.L
#X, -(A7)
CMPUT 229
© 2006
Department of Computing Science
Example: Add Two Numbers
MK68K assembly:
ORG
LEA
PEA
PEA
PEA
BSR
MOVE.W
LEA
STOP
*
AddUp
MOVEA.L
MOVEA.L
MOVE.W
MOVE.W
ADD
MOVEA.L
MOVE.W
RTS
*
ORG
X
DC.W
Y
DC.W
Z
DC.W
CMPUT 229
$400
$1000, A7
X
Y
Z
AddUp
Z, D2
12(A7),A7
#$2700
Set up stack pointer
Push address of variable X
Push address of variable Y
Push address of variable Z (the result)
Call adder routine
Read result (a dummy operation)
Clean up stack
12(A7), A0
8(A7), A1
(A0),D2
(A1),D3
D2, D3
4(A7),A3
D3,(A3)
Get address of parameter X
Get address of parameter Y
Get value of X
Get value of Y
Add them
Get address of parameter Z
Put result in variable Z
$500
1
2
1
© 2006
Department of Computing Science
Example: Add Two Numbers (Parameter
Passing)
MK68K assembly:
*
AddUp
ORG
LEA
PEA
PEA
PEA
BSR
MOVE.W
LEA
STOP
$400
$1000, A7
X
Y
Z
AddUp
Z, D2
12(A7),A7
#$2700
Set up stack pointer
Push address of variable X
Push address of variable Y
Push address of variable Z (the result)
Call adder routine
Read result (a dummy operation)
Clean up stack
MOVEA.L
MOVEA.L
MOVE.W
MOVE.W
ADD
MOVEA.L
MOVE.W
RTS
12(A7), A0
8(A7), A1
(A0),D2
(A1),D3
D2, D3
4(A7),A3
D3,(A3)
Get address of parameter X
Get address of parameter Y
Get value of X
Get value of Y
Add them
Get address of parameter Z
Put result in variable Z
ORG
DC.W
DC.W
DC.W
$500
1
2
1
*
X
Y
Z
CMPUT 229
© 2006
Department of Computing Science
Example: Add Two Numbers
MK68K assembly:
ORG
$400
LEA
$1000, A7
PEA
X
PEA
Y
PEA
Z
BSR
AddUp
MOVE.W Z, D2
LEA
12(A7),A7
STOP
#$2700
*
MOVEA.L 12(A7), A0
MOVEA.L 8(A7), A1
MOVE.W (A0),D2
MOVE.W (A1),D3
ADD
D2, D3
MOVEA.L 4(A7),A3
MOVE.W D3,(A3)
RTS
*
ORG
X
DC.W
Y
DC.W
Z
DC.W
CMPUT 229
Set up stack pointer
Push address of variable X
Push address of variable Y
Push address of variable Z (the result)
Call adder routine
Read result (a dummy operation)
Clean up stack
Get address of parameter X
Get address of parameter Y
Get value of X
Get value of Y
Add them
Get address of parameter Z
Put result in variable Z
$500
1
2
1
© 2006
Parameter Passing
By Reference
Clements, pp. 278
Parameter Passing
By Reference
Clements, pp. 278
Parameter Passing
By Reference
Clements, pp. 278
Parameter Passing
By Reference
Clements, pp. 278
Parameter Passing
By Reference
Clements, pp. 278
Department of Computing Science
Procedure Call
1 Place parameters in a place where the procedure can access
them.
2 Transfer control to procedure.
3
Acquire the storage resources needed for the procedure.
4 Perform the procedure’s task.
5 Place the result value in a place where the calling program can
access it.
6 Return control to the point of origin.
CMPUT 229
© 2006
Pat.-Hen. pp. 132
Department of Computing Science
The LINK instruction
AFTER
BEFORE
A5
A7(SP)
LINK
$ABCC
A5
$7FFC
A7(SP)
$7FF0
A5, #-12
$8000
Memory
Memory
$7FF8
$7FF8
$7FFC
$7FFC
SP
$7FF0
$7FF4
$7FF8
$7FFC
SP
$8000
$1234
Link creates a Local
“workspace” in the
stack to be used by a
subroutine.
$7FF0
$7FF4
$7FF8
A5
$7FFC
$ABCC
$8000
$1234
$8004
$8004
CMPUT 229
Clements, pp. 625
© 2006
Department of Computing Science
The UNLK instruction
AFTER
BEFORE
A5
UNLK
$7FFC
A7(SP)
A5
$ABCC
A7(SP)
$8000
A5
$7FF0
Memory
Memory
$7FF8
$7FF8
$7FFC
$7FFC
SP
Unlink collapses the
stack to release workspace
previously allocated by LINK.
$7FF0
$7FF4
$7FF8
A5
$7FFC
$ABCC
$8000
$1234
$7FF0
$7FF4
$7FF8
$7FFC
SP
$8000
$1234
$8004
$8004
CMPUT 229
Clements, pp. 639
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
Before subroutine starts
Procedure
Memory
A6
$1234
int leaf_example ( int g, int h, int i, int j)
{
int f;
$7FFC
$8000
A7(SP)
f = (g + h) - (i + j);
return f;
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 8(A6) , D0
ADD.L
12(A6), D0
MOVE.L 16(A6), D1
ADD.L
20(A6), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$8004 <ret adr>
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
D0 D0-D1
Write f into stack
Read f from stack into D0
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
After the LINK instruction
Procedure
Memory
A6
int leaf_example ( int g, int h, int i, int j)
{
int f;
$8000
A7
$7FFC
A6
$8000
$1234
$8004 <ret adr>
f = (g + h) - (i + j);
return f;
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 8(A6) , D0
ADD.L
12(A6), D0
MOVE.L 16(A6), D1
ADD.L
20(A6), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
D0 D0-D1
Write f into stack
Read f from stack into D0
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
Before UNLK instruction
Procedure
Memory
A6
int leaf_example ( int g, int h, int i, int j)
{
int f;
$8000
A7
$7FFC
f
A6
$8000
$1234
$8004 <ret adr>
f = (g + h) - (i + j);
return f;
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 8(A6) , D0
ADD.L
12(A6), D0
MOVE.L 16(A6), D1
ADD.L
20(A6), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
D0 D0-D1
Write f into stack
Read f from stack into D0
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
After UNLK instruction
Procedure
Memory
A6
$1234
int leaf_example ( int g, int h, int i, int j)
{
int f;
A7
f = (g + h) - (i + j);
return f;
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 8(A6) , D0
ADD.L
12(A6), D0
MOVE.L 16(A6), D1
ADD.L
20(A6), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$7FFC
f
$8000
$1234
$8004 <ret adr>
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
D0 D0-D1
Write f into stack
Read f from stack into D0
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
After RTS instruction
Procedure
Memory
A6
$1234
int leaf_example ( int g, int h, int i, int j)
{
int f;
$7FFC
f
$8000
$1234
$8004 <ret adr>
f = (g + h) - (i + j);
return f;
A7
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 8(A6) , D0
ADD.L
12(A6), D0
MOVE.L 16(A6), D1
ADD.L
20(A6), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
D0 D0-D1
Write f into stack
Read f from stack into D0
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
Procedure
Memory
int leaf_example ( int g, int h, int i, int j)
{
int f;
A7
f = (g + h) - (i + j);
return f;
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 8(A6) , D0
ADD.L
12(A6), D0
MOVE.L 16(A6), D1
ADD.L
20(A6), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$7FFC
f
$8000
$1234
$8004 <ret adr>
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
D0 D0-D1
Write f into stack
Optimization
Read f from stack into D0
© 2006
Department of Computing Science
A Procedure that Doesn’t Call Another
Procedure
Memory
int leaf_example ( int g, int h, int i, int j)
{
int f;
$7FFC
$8000
A7
f = (g + h) - (i + j);
return f;
}
MK68K assembly:
LINK.w
A6,#-4
MOVE.L 4(A7) , D0
ADD.L
8(A7), D0
MOVE.L 12(A7), D1
ADD.L
16(A7), D1
SUB.L
D1, D0
MOVE.L D0, -4(A6)
MOVE.L -4(A6), D0
UNLK
A6
RTS
CMPUT 229
$8004 <ret adr>
$8008
g
$800C
h
$8010
i
$8014
j
$8018
Make room in stack for 1 more item
Load g from stack into D0
D0 g+h
Load i into D1
D1 i+j
Further
D0 D0-D1
Write f into stack
Read f from stack into D0
Optimization
© 2006
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