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ELEC 2220 Computer Systems

Chapter 4. Modular Programming

Soo-Young Lee

Department of Electrical and Computer Engineering

Auburn University

ELEC2220 Auburn University

4. Modular Programming: Chapter Objectives

Subroutine
Subroutine-related instructions
Parameter Passing

4-1. Subroutines

A large task may be partitioned into blocks, and some of the blocks can be implemented as subroutines (modules), especially those executed repeatedly.

Example

Finding the sum of all elements in array[N]

Issues: task partitioning, parameter passing

Main

Why modular programming?
easily understand
easy debugging
localized changes
share modules

EA of array, N

Sum

Sum up elements in array[N]

Subroutine

4-1. Subroutines

Task Partitioning

Define the computation to be carried out by each subroutine.

Parameter Passing

Define the input parameters (constants, variables, etc.) to be passed to the subroutine.
Define the results from the subroutine.
How are they passed between the calling procedure and the subroutine?

4-1. Subroutines

Example

N equ 10

org $800

array ds.b 10

total ds.b 1

org $8000

…

ldx #array

ldab #N

jsr sum

staa total

….

rts

sum
ldaa #0
next:
adda 1,x+
dbne b,next
rts

4-2. Subroutine Instructions

BSR (branch to subroutine)

Save the return address (address of the instruction immediately following the BSR) and initiate execution of the subroutine.
Saving return address

SP  SP – 2
M[SP]:M[SP+1]  return address

Initiate execution of the subroutine

PC  PC + distance ; relative addressing

JSR (jump to subroutine)

The same as BSR except for the addressing mode
Initiate execution of the subroutine

PC  address of the subroutine : all other addressing modes

RTS (return from subroutine)

Retrieve the return address from the stack

PC  M[SP]:M[SP+1]
SP  SP + 2

Never jump to or out of a subroutine using branch instructions !
Don’t forget to initialize SP

Subroutine Sequence

…

Jsr subr

next instr ; addr_of_next

….

subr

first instr ; addr_of_first

….

rts

….

1. Save PC

Addr_of_next

SP

Stack

3. Return

PC  addr_of_next

2. Initiate execution of subr

PC  addr_of_first

4-3. Parameter Passing

4-3-1. Registers

Data to be processed are passed in registers to the subroutine.
Results are returned in registers to the calling procedure.
The number of parameters is limited.

4-3-2. Memory

The global shared memory variables are used for parameter passing.
Subroutine: variable-name specific

4-3-3. Stack

Parameters are passed to and from the subroutine via the stack.

Data are pushed onto the stack, which are popped by the subroutine.
Results are pushed by the subroutine, which are popped by the calling procedure.

Careful with the return address on the stack

4-3. Parameter Passing

4-3-4. Address Passing

Addresses of parameters, instead of their values, are passed.
Subroutine: variable-name independent
Particularly useful when the size of data to be passed is large.
Example: finding the maximum in an array

Find_max( aptr, N)

Int *aptr, N;

{

Int temp_max, i;

temp_max = *aptr;

for(i=1; i<N; i++)

if( *(aptr+i) > temp_max )

temp_max = *(aptr+i);

return(temp_max);

}

int array[100], max;

N=100;

max = find_max(array, N);

….

4-5. Macro

A subroutine may be too costly for a small task due to the call/return overhead.
A small task (code) can be defined as a macro which may be considered as a user-defined instruction.
An assembler copies the code wherever it encounters the macro.