1. 94201.lecture19-22-subroutines
Often need to perform the same program activity many times
High-level language: function, procedure, method
Assembly language: subroutine
Want ability to:
encapsulate program activity in program text
invoke the activity from elsewhere in program
Control flow
pass control to the activity
execute the activity
return control to the invocation point
30 Oct-01
Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

2. Subroutines as Encapsulated Actvities
program
control flow
activity:
done
encapsulated activity
invocation point
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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3. 94201.lecture19-22-subroutines
Subroutines (contd)
If only used once, imbed inline
activity:
done
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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4. What if invoked more than once?
Subroutines (contd.)
What if invoked more than once?
encapsulated activity
program
activity:
done
invocation points
how can the activity know which one to “return” to?
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

5. Subroutine CALL and RET
During invocation: must save invocation point information
During return: must return to saved invocation point
Machine-level implementation mechanisms:
CALL target (invoke target subroutine)
PUSH IP save return address on run-time stack
JMP target pass control to activity
RET (return from subroutine)
POP IP return to pushed address
IP value AFTER fetching CALL instruction!
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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6. --responsibility of subroutine!! old top Next SP old top
activity:
RET
CALL activity
(after fetch
IP = = Next)
PUSH IP
JMP activity
after execution
IP = = activity
Stack
program
CALL activity
Next:
RET
POP IP
after execution
IP = = Next
Stack SP
Next
N.B. only works if return address is on top of stack when RET is executed
--responsibility of subroutine!!
old top
Next SP
old top
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

7. Nested Subroutine Calls
program
CALL
Next1:
activity1:
CALL
Next2:
RET
activity2:
RET
old top
Next1
old top
Next2
Next1
old top
Run-
Time
Stack
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

8. Subroutine Parameters
increase the generality of an activity
allow invocations to behave differently
allows caller (the code that invokes) and callee (the invoked activity) to communicate information
Example: could have a dedicated subroutine that displays the value 245
void Display245( ); nice, but .....
could generalize by adding a parameter
allow a 16-bit value to be displayed to be communicated at invocation
void DisplaySigned( word Value );
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

9. Subroutine Parameters (contd)
could further generalize by adding another parameter
to communicate the base for displaying Value
void Display16( word Value, byte Base);
(Base- 0 : binary, 1 : hex, 2 : decimal)
text of function/subroutine is static – write in terms of parameters
behaviour of each invocation is dynamic – depends on arguments – the actual parameter values passed to specific invocations
parameters can be communicated in various ways:
global variables  
registers
on the stack  POLICY for , 
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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10. 94201.lecture19-22-subroutines
Global Variable
A shared (static) memory variable
to communicate invocation argument:
caller: puts argument value in variable, then calls
callee: reads argument from variable
Example:
MOV WORD PTR [Value], 245
CALL activity
. . .
activity: MOV AX, [Value]
RET
Value: DW
Caller
Callee
Global variable
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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11. 94201.lecture19-22-subroutines
Global Variables
not widely used in practice
problems in large programs – software engineering issues
in :
turns out to be essential
only way to communicate in some cases
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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12. Subroutine Parameter Passing
Via Registers:
could exchange arguments in registers
caller: load registers with values
callee: read values from registers
used in assignments 2 and 3
OK, but ... finite number of registers
what to do if more parameters than registers?
Via the Run-Time Stack
caller: push invocation arguments onto stack
callee: indexes into stack to access arguments
POLICY for
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13. 94201.lecture19-22-subroutines
Subroutines (contd)
Many more subroutine issues to deal with:
register save/restore
parameters
order and type of invocation arguments in stack
indexing into stack – standard entry code
pass by value vs. pass by reference
return-value argument
prototype/documentation
clearing invocation arguments off of stack
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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14. Register Save / Restore
subroutines need to use registers
what to do if registers that contain values that are useful to caller, but subroutine wants to use? One solution:
caller responsible for saving all useful values before calling
callee free to use any register
caller responsible for restoring values saved before call
an alternate solution:
callee (subroutine) must save
and restore all registers used
an alternate alternate solution: hybrid of above
caller responsible for some, callee responsible for some !?
POLICY in this course
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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15. Parameter Passing on Stack
Design decisions
Issues in Passing Parameters on the Stack:
order and type of invocation arguments in stack
Common specs needed for programmers writing:
1. implementation of subroutine
2. code that calls subroutine
Document using C++-like prototype:
return-value and type
name of subroutine
parameters: number, type, name & order
Policy in !
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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16. C++-like Subroutine Prototypes
Example:
void Display16( word Value, byte Base);
Order in stack?
push right-to-left according to prototype
in above example: push Base 1st, then Value
no return-value
subroutine name
2 parameters:
16-bit Value
8-bit Base
Policy in
94.201
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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17. Subroutine Calling Policies
Implications of POLICIES (so far)
caller: push arguments, then CALL
callee:
save all registers to be used
must index into stack to access arguments
must restore saved registers before return
pushes return address
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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18. Parameter Access in Called Subroutine
Want consistent way for callee to deal with stack ( … need another POLICY  )
Callee will use BP to index to arguments
Callee will set-up stack frame:
PUSH BP ; save BP
MOV BP, SP ; BP points to top of stack
Callee now saves (PUSHes) any other registers it uses
When callee is finished:
restore (POP) saved registers (BP too!), then
RET
Policy: standard entry code
Policy: standard exit code
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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19. 94201.lecture19-22-subroutines
Stack Frame
indexing into stack – use BP based or indexed
every subroutine invocation results in a new stack frame on top of the stack SP
saved
registers
BP – constant
Standard
Stack
Frame
old BP
BP + 0 BP
return address
BP + 2
arguments
High level languages save this and more:
BP + 4 or more
stack
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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20. 94201.lecture19-22-subroutines
Stack Frame Example
Recall Previous Example:
void Display16( word Value, byte Base);
call set up:
MOV AL, 0 ; Base = binary
PUSH AX
PUSH [BX + SI] ; Value to display
CALL Display16
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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21. 94201.lecture19-22-subroutines
Stack Frame Example
stack frame after standard entry code:
in body of Display16:
MOV AX, [BP + 4] ; get Value
. . .
CMP BYTE PTR [BP + 6], 0 ; binary ? SP
saved
registers BP
old BP
return address
BP + 2
BP + 4
Value
BP + 6
Base
could WORD PTR be used?
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

22. Call by Value / Call by Address
Or “pass by value vs. pass by reference”
Pass by value:
argument is a copy of the value of interest
in high-level language like C++, default way to pass simple variables e.g. int
example:
int MyValue;
MyValue = 245;
Display16( MyValue, 0 );
2. A copy of the data value of the variable MyValue is pushed onto stack
1. Immediate value 0 is pushed onto stack
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23. 94201.lecture19-22-subroutines
Call by Value Example
Stack frame after entry code in Display16: SP
saved registers
old BP BP
return address
BP + 2
245
BP + 4
BP + 6
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24. 94201.lecture19-22-subroutines
Call by Value (contd)
NOTE: in subroutine, arguments passed in the stack can be treated like local variables: value can be read & modified
but variable is local – exists ONLY during subroutine execution
no persistent changes! (any modifications cannot be seen by caller)
in Previous Display16 Example:
can change the received copy of MyValue
MOV [BP + 4], AX
changes local variable
but does not change original variable !
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25. 94201.lecture19-22-subroutines
Call by Reference
sometimes need access to caller’s variables, e.g.
1. purpose of subroutine is to modify caller’s variables
2. passing large composite structures on stack can require too much time/space
Pass/Call by reference:
argument is the address of a memory variable
need prototype syntax to differentiate value vs. reference parameters
default will be pass by value: type name e.g. int Value
For pass by reference use type & name e.g. int & Value
pass reference to integer variable
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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26. Call by Reference Example
void SortArray ( int & SortMe[ ], int Size );
Array declaration
X: DW DW
. . .
SizeOfX: DW
Call to sort X
PUSH WORD PTR [ SizeOfX ]
MOV AX, X
PUSH AX
CALL SortArray
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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27. Pass by Reference Example (contd)
after entry code in SortArray: SP
saved registers BP
old BP
BP + 2
return address
BP + 4
address of X
BP + 6
copy of SizeOfX
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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28. Call by Reference Example (contd)
In SortArray after stack frame init code:
. . .
MOV BX, [ BP + 4 ] ; get array address
MOV SI, 0 ; array index = 0
MOV AX, [ BX + SI ] ; get array element
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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29. 94201.lecture19-22-subroutines
Return Values
functions can return values two ways:
1. return value in variable passed by reference
2. return value in some other state variable
global variable?  (NO!)
stack ? Maybe
caller could allocate an extra word in stack before call
eg. SUB SP, 2
CALL . . .
callee could return value there
register (high-level languages do this!)
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30. 94201.lecture19-22-subroutines
Return Values
POLICY in this course (and )
return 8-bit value in AL (AH is undefined)
return 16-bit value in AX
return 32-bit value in DX:AX
as with 32-bit values for DIV instruction
Implications of Return-Value Policy 
do not save/restore register(s) used for return-value (the purpose of the subroutine is to return a value in the register(s))
if 8-bit value (returned in AL) – subroutine is not responsible for persistence of AH value
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

31. Returning Values - Example
void ChkAbsValue ( int & X, int Y );
returns (X == | Y |) in first argument
any potential problems?
Possible solution:
boolean ChkAbsValue( int & X, int Y );
Boolean return value: byte or word
0  false
non-zero  true
Do we still need to pass X by reference?
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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32. Stack Cleanup after Subroutine Call
Must remove arguments from stack
caller or callee ?
POLICY: caller !
Could POP values – but not good idea - subroutine might have modified values passed !
Typical stack cleanup method used by caller
ADD SP, #ofBytes
Policy for stack cleanup in
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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33. Stack Cleanup after Subroutine Call
After RET:
ADD SP, 4 ; clear arguments from stack
saved registers
old values from CALL
old BP
return address SP
arg1
arg2
rest of stack
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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34. Summary of Subroutine Call Mechanism
Call set up:
push arguments onto stack
CALL subroutine
Subroutine entry code:
set up stack frame , BP access into stack
save registers used 
Subroutine body:
perform application code
if return-value needed – calculate and put in appropriate register(s)
Subroutine exit code:
clean up stack – restore registers used
RET
Caller clean up:
remove arguments from stack
save/use return-value
See details of Policies for these mechanisms
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35. 94201.lecture19-22-subroutines
Database Example
Suppose that a database keeps records consisting of:
birth date (day, month year)
name
student number
Record data structures are implemented as:
Day: DB ; range:
Month: DB ; range:
Year: DB ; range:
Name: DB ‘ $’ ; max 20 chars
StudentNum: DW ; unsigned double
DW ; word – range?
The size of each record is: 3 (Date) + 21 (Name) + 4 (StudentNum) = 28 bytes
little endian words!
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36. Database Example (contd)
In each record:
offset to day = 0
offset to month = 1
offset to year = 2
offset to name = 3
offset to student number = 24
Suppose database is implemented as an array of 28 byte records
Limitations? Y2K? Name size? Student Number range?
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

37. Database Example (contd)
Program to manage database might include subroutine:
void ReportDB ( db & MyDB, // ref to database
unsigned int DBSize // number of records );
// purpose: writes contents of db to text file
Assume following subroutine exists:
boolean WriteText ( char & Text[ ] );
// write ‘$’ terminated text string to text file
// returns true iff write was successful
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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38. Database Example (contd)
Requirements:
records are to be reported as:
name student number birth date
sort by: name, student number, birth date
Other support subroutines needed?
depends on approach to solution
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39. Database Example (contd)
Will use approach similar to assignment 3:
RecordText:
DB ‘ ’
NameText:
DB ‘ ’
StudentNumText:
DB ‘123456’
BirthDateText:
DB ‘ dd/mm/yy’
DB ‘$’ ; terminator 
fill in RecordText for a record
pass RecordText address to WriteText
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

40. Database Example (contd)
Could have subroutines to handle the conversion of database info into text:
void ReportName( char & dbName[ ], char & NameTxt[ ] );
where: dbName is ptr to $-term. name text in db record
NameTxt is pointer to display buffer
void ReportNum( unsigned longint Num, char & NumTxt[ ] );
where: Num is 32-bit student number (push most signif word first)
NumTxt is pointer to display buffer
void ReportDate( byte & Date[ ], char & DateTxt[ ] );
where: Date is ptr to 3-byte array [DAY, MONTH, YEAR]
DateTxt is pointer to display buffer
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

41. ; set up to access database records:
ReportDB:
PUSH BP
MOV BP, SP
PUSH ; save regs
; set up to access database records:
MOV BX, [ BP + 4 ] ; BX=start of db aray
MOV SI, 0 ; SI = offset to rec
; process records one at a time
; for ( CX = DBSize; CX > 0; CX – – )
; { clear display buffer;
; put record text into display buffer;
; output text to file then advance to next rec; }
MOV CX, [ BP + 6 ] ; CX = DBSize
Standard entry code
Algorithm description
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42. 94201.lecture19-22-subroutines
Note that documentation uses a combination of C++-style pseudo-code and line-by-line comments!
DoRec: ; process a record
CMP CX, 0 ; loop test: done?
JE DoneReport
; loop: clear display buffer
; for ( DI = RecordText; [DI] != ‘$’; DI++ ) {[DI] = ‘ ’; }
MOV DI, RecordText
CheckChar:
CMP BYTE PTR [DI], ‘$’ ; loop test: DI ? ‘$’
JE DoText ; yes – exit loop
MOV BYTE PTR [DI], ‘ ’ ; blank the char
ADD DI, 1 ; loop inc: DI ++
JMP CheckChar
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
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43. Database Example (contd)
<continuing with Reportdb subroutine>
DoText:
; put name in display buffer – use ReportName
MOV AX, NameText
PUSH AX
MOV AX, BX ; get start address
ADD AX, SI ; add offset to this record
ADD AX, 3 ; add offset to name
CALL ReportName
ADD SP, 4
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Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001
94201.lecture19-22-subroutines

44. Aside: Pushing Address in a Structure
Above: explicit calculation of an address that is automatically generated due to addressing mode
Some processors support LEA “Load Effective Address” instruction: calculates address specified by addressing mode
loads destination with address, does not go to memory at address 
replace:
MOV AX, BX ; get start address
ADD AX, SI ; add offset to this record
ADD AX, 3 ; add offset to name
with:
LEA AX, [BX + SI + 3 ]
LEA not supported in p86 yet
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45. 94201.lecture19-22-subroutines
Other Issues in ReportNum?
how to convert 32-bit value for output?
order of generating digits?
problems with DIV?
<Returning to Reportdb example:>
; put student number name into
; display buffer – use ReportNum
; recall:
; void ReportNum( unsigned longint Num, char & NumTxt[ ] );
; Num is 32-bit student number (push most signif word first)
; NumTxt is pointer to display buffer
MOV AX, StudentNumText
PUSH AX
PUSH [ BX + SI + 26 ] ; most signif word
PUSH [ BX + SI + 24 ] ; least signif word
CALL ReportNum
ADD SP, 6
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46. 94201.lecture19-22-subroutines
;put birth date in display buffer -use ReportDate; recall
; void ReportDate( byte & Date[ ], char & DateTxt[] );
; Date is ptr to 3-byte array [ DAY, MONTH, YEAR ]
; DateTxt is pointer to display buffer
MOV AX, BirthDateText
PUSH AX
MOV AX, BX ; get start address
ADD AX, SI ; add offset to this record
; offset to [DATE, MONTH, YEAR ] = 0
CALL ReportDate
ADD SP, 4
Issues in ReportDate?
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47. 94201.lecture19-22-subroutines
; output record text to file
; use: boolean WriteText ( char & Text[ ] );
MOV AX, RecordText
PUSH AX
CALL WriteText
ADD SP, 2
; check for OK write to file:
CMP AL, 0
JNE WriteOK
; do file write error handling here
. . .
WriteOK:
; advance to next record:
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48. 94201.lecture19-22-subroutines
WriteOK:
; advance to next record:
ADD SI, 2
; loop decrement:
SUB CX, 1
JMP DoRec
DoneReport:
; exit code: restore saved registers (including BP)
POP
RET
<end of Reportdb subroutine!>
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49. 94201.lecture19-22-subroutines
Further Example
;void ReportName( char & dbName[ ], char & NameTxt[ ]);
; where: dbName is pointer to $-terminated
; name text in database record
; NameTxt is pointer to display buffer
; for interest – suppose that text is to be right justified in 20-
; char display buffer
ReportName:
; standard entry code:
PUSH BP
MOV BP, SP
; save registers used
PUSH
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50. 94201.lecture19-22-subroutines
Example (contd)
; set up for access: BX = address of name text
MOV BX, [BP + 4 ]
; loop to find end of name
; for ( SI = 0; [BX + SI] != ‘$’; SI ++ ) { empty body }
MOV SI, 0 ; char index = 0
CharCheck:
CMP BYTE PTR [BX + SI ], ‘$’
JE FoundEnd
ADD SI, 1
JMP CharCheck
FoundEnd:
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51. 94201.lecture19-22-subroutines
Example (contd)
FoundEnd:
; at this point, SI = number of char’s in name
Want:
(name text) $ BX
+ SI
(display buffer) DI
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52. 94201.lecture19-22-subroutines
MOV DI, [ BP + 6 ] ; DI = address of dest buffer
ADD DI, 19 ; DI = address of last char
; adjust SI to point to char, not ‘$’
SUB SI, 1 ; SI = offset to last char in name
; Now copy the name into the buffer:
; for ( ; SI > 0; SI – – ) { [DI] = [BX + SI] }
CheckCopyDone:
CMP SI, 0
JLE CopyDone
MOV AL, [BX + SI]
MOV [DI], AL
SUB SI, 1
JMP CheckCopyDone
CopyDone:
; standard exit code –
; restore registers, etc
POP . . .
RET
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53. 94201.lecture19-22-subroutines
Example (contd)
Could accomplish same thing without SI!
; loop to find end of name
; for ( BX = [BP + 4]; [BX] != ‘$’; BX ++ )
; { empty body }
; loop to copy name backwards
; for (DI = [BP + 6] + 19 ; BX > [BP + 4] ; BX – –)
; { [DI] = [BX – 1] }
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54. 94201.lecture19-22-subroutines
Example (contd) -1
[BP+4]
(name text) $ BX
(display buffer) DI
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