1. Subroutines … passing data
Mechanisms:
pass by value
pass by result
pass by value result/ value return/ copy restore
pass by reference
pass by name
pass by lazy evaluation …
Techniques:
in registers
in memory locations
in a parameter block
on stack
in the code stream

2. Passing data in registers
simple programs with few parameters
complex programs, e.g. operating systems, with an established standard or ‘parameter convention’
usually, the low numbered registers, e.g. D0, D1, … etc, are used

3. Passing data in registers
e.g. variable delay routine …
move.w #5,D0 ; init delay count
jsr v_delay ; and call delay
stop #$2700
* Insert comments for subroutine here
v_delay move.l D0,-(SP) ; save context
d_loop nop
dbra D0,d_loop ; kill some time
move.l (SP)+,D0 ; restore context
rts
What registers should be saved?
Who decides?
Why not save all registers all the time?

4. e.g. find median v1
lea data,A0 clr.l D0 move.b data_l,D0 jsr find_median move.b D1,median stop #$2700 * Preconditions: * A0 – pointer to list * D0 – list count (byte) * Postconditions: * D1 – median (byte) find_median: movem.l D0/A0,-(sp) lsr.b #1,D0 scs flag add.l D0,A0 move.b (A0),D1
tst.b flag bne done add.b -(A0),D1 lsr.b #1,D1 done movem.l (sp)+,D0/A0 rts median ds.b 1 flag ds.b 1 data dc.b 1,3,5,7,8 data_l dc.b 5 data dc.b 1,3,5,7,7,8 data_l dc.b 6
Median is the middle value if odd
number of values or the average
of the 2 middle values

5. e.g. find median v2
lea data,A0 clr.l D0 move.b data_l,D0 jsr f_median move.b D1,median stop #$2700 * Same pre/post conditions * for all 3 subroutines * D1 is still output!! f_median: movem.l D0/A0,-(sp) btst #0,D0 bne odd_r jsr even bra done odd_r jsr odd done movem.l (sp)+,D0/A0 rts
even lsr.b #1,D0 add.l D0,A0 move.b (A0),D1 add.b -(A0),D1 lsr.b #1,D1 rts odd lsr.b #1,D0 add.l D0,A0 move.b (A0),D1 rts data dc.b 1,3,5,7,7,8 data_l dc.b 6 median ds.b 1

6. e.g. find median v3
lea data,A0 clr.l D0 move.b data_l,D0 jsr f_median move.b D1,median stop #$2700 * Same pre/post conditions * for all 3 subroutines * D1 is still output!! f_median: movem.l D0/A0,-(sp) lsr.b #1,D0 bcs odd_r jsr even bra done odd_r jsr odd done movem.l (sp)+,D0/A0 rts
even add.l D0,A0 move.b (A0),D1 add.b -(A0),D1 lsr.b #1,D1 rts odd add.l D0,A0 data dc.b 1,3,5,7,7,8 data_l dc.b 6 median ds.b 1

7. Passing data in memory
OK for simple programs e.g. test programs, one-time use
severely limits how a subroutine is called
unusual in commercial code
makes the procedure non-reentrant
only one instance possible

8. Passing data in memory e.g. variable delay routine …
move.w #5,delay_cnt ; init delay count
jsr v_delay ; and call delay
stop #$2700
* Comments for subroutine here
v_delay move.l D0,-(SP) ; save context
move.w delay_cnt,D0 ; init count
d_loop nop
dbra D0,d_loop ; kill some time
move.l (SP)+,D0 ; restore context
rts
delay_cnt ds.w 1

9. Passing data in parameter blocks
used for large amounts of data stored in arrays or data structures
pass the address of the data structure
for complex data, i.e. a table containing data and pointers
pass the address of a parameter block
typically, the address is passed in an address register
usually, the low numbered registers, e.g. A0, A1, … etc, are used

10. Passing data in parameter blocks
e.g. add increment to every element in an array
… move.w #5,arr_inc ; init increment lea array,A0 ; init table pointer jsr inc_lst ; and do variable incr …
stop #$2700
inc_lst movem.l ; save context move.w ; get increment move.w ; and length .
inc_lp ; increment element dbra ,inc_lp ; and loop til done movem.l ; restore context rts
array equ *
arr_inc ds.w 1
arr_l dc.w 100
arr_d ds.w 100

11. Mechanisms for Passing Data
pass by value
used in C, Java, Pascal, Ada (in param)
copy of argument passed to S/R
infamous swap example
void swap (int x, int y) { int t = x;
x = y;
y = t; }
int main(void)
{ int a = 5;
int b = 7;
swap( a, b );
printf("a= %d; b= %d\n", a, b);

12. Mechanisms for Passing Data
pass by reference
used in Fortran, Pascal (var param), Delphi, C++, C (simulates pass by reference with pointers)
address of argument passed to S/R
"reference" is not the same as Java "reference"; Java is passing references to objects by value
void swap ( int *px, int *py)
{ int t = *px;
*px = *py;
*py = t; }
int main(void)
{ int a = 5;
int b = 7;
swap( &a, &b );
printf("a= %d; b= %d\n", a, b);

13. Mechanisms for Passing Data
pass by sharing / pass by object
used in Python where all data are objects with an identity (essentially an address), a type and a value.
immutable objects behave like pass by value; mutable objects behave like pass by reference
pass by value result / value return / copy restore
used in Ada (in/out param), Fortran
address of argument passed to S/R
S/R makes a copy of argument
when S/R is finished, copies the argument copy back to original parameter

14. Reading, Expectations Reading: Expectations:
M68000 Assembly Language [pdf, 92p; N. Znotinas] review operation of JSR (BSR) and RTS instructions covered in presentation
Expectations:
you can explain the operation of JSR/BSR, RTS
you can read/write code that uses the above instructions
you can read/write code that passes data to a subroutine via registers, via memory locations and using parameter blocks