Chapter 14
Functions
Function
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Smaller, simpler, subcomponent of program
Provides abstraction
• hide low-level details
• give high-level structure to program,
easier to understand overall program flow
• enables separable, independent development
C functions
• zero or multiple arguments passed in • single result returned (optional)
• return value is always a particular type
In other languages, called procedures, subroutines, …
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Example of High-Level Structure
main() {
SetupBoard(); /* place pieces on board */
DetermineSides(); /* choose black/white */
/* Play game */
do {
WhitesTurn();
BlacksTurn();
} while (NoOutcomeYet());
}
Structure of program
is evident, even without knowing implementation.
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Functions in C
Declaration (also called prototype) int Factorial(int n);
Function call — used in expression
a = x + Factorial(f + g);
type of return value
name of function
types of all arguments
1. evaluate arguments
2, execute function
3. use return value in expression
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Function Definition
State type, name, types of arguments
• must match function declaration
• give name to each argument (doesn’t have to match declaration)
int Factorial(int n)
{
int i;
int result = 1;
for (i = 1; i <= n; i++)
result *= i;
return result;
}
gives control back to calling function and returns value
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Why Declaration?
Since function definition also includes
return and argument types, why is declaration needed?
• Use might be seen before definition. Compiler needs to know return and arg types and number of arguments.
• Definition might be in a different file, written by a different programmer.
• include a "header" file with function declarations only • compile separately, link together to make executable
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Example
double ValueInDollars(double amount, double rate);
main() {
declaration
function call (invocation)
...
dollars = ValueInDollars(francs,
DOLLARS_PER_FRANC);
printf("%f francs equals %f dollars.\n",
francs, dollars);
...
}
definition
double ValueInDollars(double amount, double rate)
{
return amount * rate;
}
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Implementing Functions: Overview
Activation record
• information about each function, including arguments and local variables
• stored on run-time stack Calling function
push new activation record
copy values into arguments
call function
get result from stack
Called function
execute code put result in
activation record pop activation record
from stack return
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Run-Time Stack
Recall that local variables are stored
on the run-time stack in an activation record
Frame pointer (R5) points to the beginning of a
region of activation record that stores local variables for the current function
When a new function is called,
its activation record is pushed on the stack;
when it returns,
its activation record is popped off of the stack.
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Run-Time Stack
Memory
Memory
Memory
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R6
R5
Watt
main
R6
R5
R6 R5
main
main
Before call
During call
After call
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Activation Record
int NoName(int a, int b)
{
int w, x, y;
.
. R5 .
return y;
}
locals
args
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y
x
w
dynamic link
return address
return value
a
b
bookkeeping
Name
Type
Offset
Scope
a b w x y
int int int int int
4 5 0 -1 -2
NoName NoName NoName NoName NoName
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Activation Record Bookkeeping
Return value
• space for value returned by function
• allocated even if function does not return a value
Return address
• save pointer to next instruction in calling function
• convenient location to store R7 in case another function (JSR)
is called
Dynamic link
• callers frame pointer
• used to pop this activation record from stack
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Example Function Call
int Volta(int q, int r)
{
int k;
int m;
...
return k;
}
int Watt(int a)
{
int w;
...
...
return w; }
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w = Volta(w,10);
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Calling the Function
w = Volta(w, 10);
; push second arg
AND R0, R0, #0
ADD R0, R0, #10
ADD R6, R6, #-1
STR R0, R6, #0
; push first argument
LDR R0, R5, #0
ADD R6, R6, #-1
STR R0, R6, #0
; call subroutine
JSR Volta
new R6 q R6 r R5 w
25
10
25
Note: Caller needs to know number and type of arguments, doesn't know about local variables.
xFD00
dyn link ret addr ret val a
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Starting the Callee Function
; leave space for return value
ADD R6, R6, #-1
; push return address
ADD R6, R6, #-1
new R6
new R5
m
k
dyn link
xFCFB
x3100
25
10
25
ret addr STR R7, R6, #0 ret val
; push dyn link (callers frame ptr)R6 ADD R6, R6, #-1
q
STR R5, R6, #0
; set new frame pointer
ADD R5, R6, #-1
; allocate space for locals
ADD R6, R6, #-2
r R5 w
xFD00
dyn link ret addr ret val a
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Ending the Callee Function
return k;
; copy k into return value
LDR R0, R5, #0
STR R0, R5, #3
; pop local variables
ADD R6, R5, #1
; pop dynamic link (into R5)
LDR R5, R6, #0
ADD R6, R6, #1
; pop return addr (into R7)
LDR R7, R6, #0
ADD R6, R6, #1
; return control to caller
RET
R6 m R5 k
dyn link
ret addr new R6 ret val
-43
217
xFCFB
x3100
217
25
10
25
new R5
xFD00
q
r
w
dyn link ret addr ret val a
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Resuming the Caller Function
w = Volta(w,10);
JSR Volta
; load return value (top of stack) R6
ret val
217
25
10
217
LDR R0, R6, #0
; perform assignment
STR R0, R5, #0
; pop return value
ADD R6, R6, #1
; pop arguments
ADD R6, R6, #2
q new R6 r
R5 wdyn link ret addr
xFD00
ret val a
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Summary of LC-3 Function Call Implementation
1. Caller pushes arguments (last to first).
2. Caller invokes subroutine (JSR).
3. Callee allocates return value, pushes R7 and R5.
4. Callee allocates space for local variables.
5. Callee executes function code.
6. Callee stores result into return value slot.
7. Callee pops local vars, pops R5, pops R7.
8. Callee returns (JMP R7).
9. Caller loads return value and pops arguments.
10. Caller resumes computation...
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