Subroutines (Part 1)

Faculty of Computer Science

CMPUT 229 © 2006

Subroutines (Part 1)

The 68K Stack

© 2006

Department of Computing Science

CMPUT 229

A stack-based machine

Problem: Compute (A+B)(C-D) in a stack-based

machine.

A

Push A

A

B

Push B

A+B

Add

A+B

C

Push C

A+B

C

D

Push D

A+B

C-D

Add

(A+B)(C-D)

Multiply

Clements, pp. 264

© 2006


CMPUT 229

A memory stack-based machine

Problem: Compute (A+B)(C-D) in a stack-based

machine.

Clements, pp. 264

© 2006


CMPUT 229

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

Clements, pp. 265

© 2006


CMPUT 229

MOVEM

MOVEM saves and restores group of registers.

Clements, pp. 265

© 2006


CMPUT 229

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?

© 2006


CMPUT 229

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

© 2006


CMPUT 229

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

© 2006


CMPUT 229

Passing Parameter On The Stack

Clements, pp. 273

© 2006


CMPUT 229

State of the Stack

Clements, pp. 273

© 2006


CMPUT 229

State of the Stack

Clements, pp. 273

© 2006


CMPUT 229

State of the Stack

Clements, pp. 273

© 2006


CMPUT 229

State of the Stack

Clements, pp. 273

© 2006


CMPUT 229

State of the Stack

Clements, pp. 273

© 2006


CMPUT 229

State of the Stack

Clements, pp. 273

© 2006


CMPUT 229

PEA - Push Effective Address

PEA pushes the address specified into the stack.

PEA X

Is equivalent to

MOVE.L #X, -(A7)

© 2006


CMPUT 229

Example: Add Two NumbersMK68K assembly:

ORG $400LEA $1000, A7 Set up stack pointerPEA X Push address of variable XPEA Y Push address of variable YPEA Z Push address of variable Z (the result)BSR AddUp Call adder routineMOVE.W Z, D2 Read result (a dummy operation)LEA 12(A7),A7 Clean up stackSTOP #$2700

*AddUp MOVEA.L 12(A7), A0 Get address of parameter X

MOVEA.L 8(A7), A1 Get address of parameter YMOVE.W (A0),D2 Get value of XMOVE.W (A1),D3 Get value of YADD D2, D3 Add themMOVEA.L 4(A7),A3 Get address of parameter ZMOVE.W D3,(A3) Put result in variable ZRTS

*ORG $500

X DC.W 1Y DC.W 2Z DC.W 1

© 2006


CMPUT 229

Example: Add Two Numbers (Parameter Passing) MK68K assembly:

ORG $400LEA $1000, A7 Set up stack pointerPEA X Push address of variable XPEA Y Push address of variable YPEA Z Push address of variable Z (the result)BSR AddUp Call adder routineMOVE.W Z, D2 Read result (a dummy operation)LEA 12(A7),A7 Clean up stackSTOP #$2700

*AddUp MOVEA.L 12(A7), A0 Get address of parameter X

MOVEA.L 8(A7), A1 Get address of parameter YMOVE.W (A0),D2 Get value of XMOVE.W (A1),D3 Get value of YADD D2, D3 Add themMOVEA.L 4(A7),A3 Get address of parameter ZMOVE.W D3,(A3) Put result in variable ZRTS

*ORG $500

X DC.W 1Y DC.W 2Z DC.W 1

© 2006


CMPUT 229

Example: Add Two NumbersMK68K assembly:ORG $400LEA $1000, A7 Set up stack pointerPEA X Push address of variable XPEA Y Push address of variable YPEA Z Push address of variable Z (the result)BSR AddUp Call adder routineMOVE.W Z, D2 Read result (a dummy operation)LEA 12(A7),A7 Clean up stackSTOP #$2700*MOVEA.L 12(A7), A0 Get address of parameter XMOVEA.L 8(A7), A1 Get address of parameter YMOVE.W (A0),D2 Get value of XMOVE.W (A1),D3 Get value of YADD D2, D3 Add themMOVEA.L 4(A7),A3 Get address of parameter ZMOVE.W D3,(A3) Put result in variable ZRTS*

ORG $500X DC.W 1Y DC.W 2Z DC.W 1

Parameter Passing By Reference

Clements, pp. 278


Clements, pp. 278


Clements, pp. 278


Clements, pp. 278


Clements, pp. 278

© 2006


CMPUT 229

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.

Pat.-Hen. pp. 132

© 2006


CMPUT 229

The LINK instruction

Clements, pp. 625

$1234$8000

$8004

$7FFC

$7FF8

$7FF4

$7FF0

$7FFC

$7FF8

Memory

SP

$8000A7(SP)

$ABCCA5

BEFORE

LINK A5, #-12

$ABCC

$1234$8000

$8004

$7FFC

$7FF8

$7FF4

$7FF0

$7FFC

$7FF8

Memory

SP

$7FF0A7(SP)

$7FFCA5

AFTER

A5

Link creates a Local

“workspace” in the

stack to be used by a

subroutine.

© 2006


CMPUT 229

The UNLK instruction

Clements, pp. 639

$1234$8000

$8004

$7FFC

$7FF8

$7FF4

$7FF0

$7FFC

$7FF8

Memory

SP

$8000A7(SP)

$ABCCA5

AFTER

UNLK A5

$ABCC

$1234$8000

$8004

$7FFC

$7FF8

$7FF4

$7FF0

$7FFC

$7FF8

Memory

SP

$7FF0A7(SP)

$7FFCA5

BEFORE

A5

Unlink collapses the

stack to release workspace

previously allocated by LINK.

© 2006


CMPUT 229

A Procedure that Doesn’t Call Another Procedureint leaf_example ( int g, int h, int i, int j) { int f;

f = (g + h) - (i + j); return f; }

MK68K assembly:LINK.w A6,#-4 Make room in stack for 1 more itemMOVE.L 8(A6) , D0 Load g from stack into D0ADD.L 12(A6), D0 D0 g+hMOVE.L 16(A6), D1 Load i into D1ADD.L 20(A6), D1 D1 i+j SUB.L D1, D0 D0 D0-D1MOVE.L D0, -4(A6) Write f into stackMOVE.L -4(A6), D0 Read f from stack into D0 UNLK A6RTS

i

<ret adr>

h

g

j$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

Memory

A7(SP)

$1234A6

Before subroutine starts

© 2006


CMPUT 229


f = (g + h) - (i + j); return f; }

MK68K assembly:LINK.w A6,#-4 Make room in stack for 1 more itemMOVE.L 8(A6) , D0 Load g from stack into D0ADD.L 12(A6), D0 D0 g+hMOVE.L 16(A6), D1 Load i into D1ADD.L 20(A6), D1 D1 i+j SUB.L D1, D0 D0 D0-D1MOVE.L D0, -4(A6) Write f into stackMOVE.L -4(A6), D0 Read f from stack into D0UNLK A6RTS

$1234

i

<ret adr>

h

g

j

Memory

A7

$8000A6

After the LINK instruction

A6

$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

© 2006


CMPUT 229


f = (g + h) - (i + j); return f; }


f

$1234

i

<ret adr>

h

g

j

Memory

A7

$8000A6

Before UNLK instruction

A6

$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

© 2006


CMPUT 229


f = (g + h) - (i + j); return f; }


f

$1234

i

<ret adr>

h

g

j

Memory

A7

$1234A6

After UNLK instruction

$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

© 2006


CMPUT 229


f = (g + h) - (i + j); return f; }


f

$1234

i

<ret adr>

h

g

j

Memory

A7

$1234A6

After RTS instruction

$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

© 2006


CMPUT 229


f = (g + h) - (i + j); return f; }


f

$1234

i

<ret adr>

h

g

j

Memory

A7

$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

Optimization

© 2006


CMPUT 229


f = (g + h) - (i + j); return f; }


i

<ret adr>

h

g

j

Memory

A7

$8014

$8018

$8010

$800C

$8008

$8004

$8000

$7FFC

Further

Optimization

Documents

Subroutines (Part 1)