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Overview

Overview. Projects – Project 1’s Project 2’s no later than Dec 14th Homework Problem – 14.X Pointer Variables Pass by Value Pass by Reference (or by Pointer) Arrays Evaluations Final Exam Dec 14 th 10:30 – 12:45. Example 14.X. #include <studio.h>

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Overview

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  1. Overview • Projects – Project 1’s Project 2’s no later than Dec 14th • Homework Problem – 14.X • Pointer Variables • Pass by Value • Pass by Reference (or by Pointer) • Arrays • Evaluations • Final Exam Dec 14th 10:30 – 12:45

  2. Example 14.X #include <studio.h> int Multiply(int b, int c); int d = 3; int main() { int a,b,c; int d = 4; a = 1; b = 2; c = d + Multiply (a, b); printf(“%d %d %d %d/n”, a, b, c, d); } int Multiply(int b, int c) { int a; a = b * c; return a; }

  3. Problem 14.X (1) ; #include <studio.h> ; int Multiply (int b, int c); .orig x3000 LD R6, stk ; set run-time stk ptr (R6) LEA R4, global ; set global variable stk Ptr (R4) ADD R6, R6, #-1 ; allocate spot for return value ADD R6, R6, #-1 ; push R7 STR R7, R6, #0 ADD R6, R6, #-1 ; push R5 (callers frame ptr) STR R5, R6, #0 ; int d = 3; AND R0, R0, #0 ; define global value d = 3 ADD R0, R0, #3 STR R0, R4, #0 ; (R4) -> d = 3

  4. Problem 14.X (2) ; int main() ; { ; int a,b,c; ADD R6, R6, #-1 ; set main's frame ptr R5 = (R6) ADD R5, R6, #0 ADD R6, R6, #0 ; (R5) -> a ADD R6, R6, #-1 ; (R5) -1 -> b ADD R6, R6, #-1 ; (R5) -2 -> c ; int d = 4; ADD R6, R6, #-1 ; (R5) -3 -> d = 4 AND R0, R0, #0 ADD R0, R0, #4 STR R0, R5, #-3 ; a = 1; AND R0, R0, #0 ; a = 1 ADD R0, R0, #1 STR R0, R5, #0 ; b = 2; AND R0, R0, #0 ; b = 2 ADD R0, R0, #1 STR R0, R5, #-1 ; c = d + Multiply (a, b); ADD R6, R6, #-1 ; push b onto stk LDR R0, R5, #-1 STR R0, R6, #0 ADD R6, R6, #-1 ; push a onto stk LDR R0, R5, #0 STR R0, R6, #0 JSR mult LDR R0, R6, #0 ; pop Multiply (a, b) value from stk ADD R6, R6, #2 ; pop 2 Multiply arguments ADD R6, R6, #1 LDR R1, R5, #-3 ; add d to Multiply (a, b) ADD R0, R1, R0

  5. Problem 14.X (3) ; printf("%d %d %d %d/n", a, b, c, d); AND R1, R1, #0 ; ASCII Conversion values ADD R1, R1, #15 ADD R1, R1, #15 Add R2, R1, #2 ; load a #32 (blank) in R2 ADD R1, R1, #15 ADD R1, R1, #3 ; load a #48 (ascii 0) in R1 LDR R0, R5, #0 ; display a ADD R0, R0, R1 TRAP x21 ADD R0, R2, #0 TRAP x21 LDR R0, R5, #-1 ; display b ADD R0, R0, R1 TRAP x21 ADD R0, R2, #0 TRAP x21 LDR R0, R5, #-2 ; display c ADD R0, R0, R1 TRAP x21 ADD R0, R2, #0 TRAP x21 LDR R0, R5, #-3 ; display d ADD R0, R0, R1 TRAP x21 AND R0, R0, #0 ; display new line ADD R0, R0, x0A TRAP x21 ; return 0 AND R0, R0, #0 ; return value = zero STR R0, R5, #3 ; (write in return value slot) ADD R6, R5, #4 ; pop main local varables (a,b,c,d) LDR R5, R6, #0 ; pop frame ptr ADD R6, R6, #1 LDR R7, R6, #0 ; pop the return address ADD R6, R6, #1 RET ; }

  6. Problem 14.X (4) ; int Multiply (int b, int c) ; { mult ADD R6, R6, #-1 ; allocate spot for return value ADD R6, R6, #-1 ; push R7 STR R7, R6, #0 ADD R6, R6, #-1 ; push R5 (callers frame ptr) STR R5, R6, #0 ; int a; ADD R6, R6, #-1 ; set Multiply frame ptr R5 = (R6) ADD R5, R6, #0 ADD R6, R6, #0 ; (R5) -> a ; a = b * c; LDR R0, R5, #5 ; load c LDR R1, R5, #4 ; load b loop ADD R1, R1, #-1 ; dec b BRz done ADD R0, R0, #0 BR loop done STR R0, R5, #0 ; store a ; return a; LDR R0, R5, #0 ; load a STR R0, R5, #3 ; (write in return value slot) ADD R6, R5, #1 ; pop multiply local varable LDR R5, R6, #0 ; pop frame ptr ADD R6, R6, #1 LDR R7, R6, #0 ; pop the return address ADD R6, R6, #1 RET ; } stk .FILL xF000 ; top of empty stack global .BLKW x0100 ; beginning of global variables .END

  7. Problem 14.X (5)Stack EFF0 EFF1 EFF2 EFF3 mult R5 a local variable EFF4 (R5) EFF5 (R7) EFF6 multiply return value EFF7 a {b} EFF8 b {c} pass values EFF9 d EFFA c EFFB b EFFC main R5 a local variables EFFD (R5) EFFE (R7) EFFF main return value F000 init R6

  8. Problem 14.X (6) Memory Map & Assignments Program: x3000 + Functions: main() and mult(int A, int B) Run-time-stack: xF000 – Global-Variable-Stack: After program + Function context (top to bottom): Pass variables to called function Local Variables (Context-Ptr points to first local variable) Preserved Context-Ptr and PC Return value Register Assignments: R0: Trap pass values R4: Gobal-Stack Ptr R5: Context Page Ptr R6: Run-time-stack Ptr R7: Preserved PC for calling function

  9. Problem 14.X (7) Compilation Main() Global Variable Table: Name Type Location from (R4) Initial Value d int 0 3 Main() Local Variable Table: Name Type Location from (R5) Initial Value a int 0 - b int -1 - c int -2 - d int -3 4 Mult() Local Variable Table: Name Type Location Initial Value a int 0 -

  10. Problem 14.X (8) Execution Run-time-Global-Variable-Stack: x3061 R4 -> d X3062 Run-time-stack: xEFF0 xEFF1 xEFF2 xEFF3 {mult}R5 -> a [addr:(R5)-0] mult local variable xEFF4 (R5) preserved for return to main xEFF5 (R7) preserved for return to main xEFF6 mult return value begin mult context page xEFF7 a {becomes mult b} pass parameter to mult xEFF8 b {becomes mult c} “ “ xEFF9 d [addr:(R5)-3] main local variable xEFFA c [addr:(R5)-2] “ “ xEFFB b [addr:(R5)-1] “ “ xEFFC {main}R5 -> a [addr:(R5)-0] “ “ xEFFD (R5) preserved for return to system xEFFE (R7) preserved for return to system xEFFF main return value begin main context page xF000 init R6 -> Output: 1 2 6 4

  11. Pointers in C • Declaration int *p; /* p is a pointer to an int */ • A pointer in C is always a pointer to a particular data type:int*, double*, char*, etc. • Operators *p-- returns the value pointed to by p &z-- returns the address of variable z

  12. Declaring Pointer variables int *ptr; ptr is a pointer variable that points to an int variable char *cp; cp is a pointer variable that points to a character variable double *dp; dp is a pointer variable that points to a double variable * is referred to as the indirection operator, or the dereference operator

  13. Address Operator & int object; int *ptr; object = 4; ptr = &objectpointer variable ptr points to variable object

  14. Example Define local variables: int object; int *ptr; Now, let’s assign values to them: object = 4; ptr = &object *ptr = *ptr + 1;

  15. Example: Parameter Passing by Value #include <stdio.h> void Swap(int firstVal, int secondVal); int main() { int valueA = 3; int valueB = 4; Swap(valueA, valueB); } void Swap(int firstVal, int secondVal) { int tempVal; /* Holds firstVal when swapping */ tempVal = firstVal; firstVal = secondVal; secondVal = tempVal; } Snapshot before return from subroutine

  16. Example: Parameter Passing by Reference #include <stdio.h> void NewSwap(int *firstVal, int *secondVal); int main() { int valueA = 3; int valueB = 4; NewSwap(&valueA, &valueB); } void NewSwap(int *firstVal, int *secondVal) { int tempVal; /* Holds firstVal when swapping */ tempVal = *firstVal; *firstVal = *secondVal; *secondVal = tempVal; } Snapshots during the exchange

  17. Scanf( ) function • Recall reading from the keyboard in C: scanf(“%d”, &input); • Why do we use &input ?

  18. Pointer Example #include <stdio.h> int IntDivide(int x, int y, int *quoPtr, int *remPtr); int main() { int dividend; /* The number to be divided */ int divisor; /* The number to divide by */ int quotient; /* Integer result of division */ int remainder; /* Integer remainder of division */ int error; /* Did something go wrong? */ printf("Input dividend: "); scanf("%d", &dividend); printf("Input divisor: "); scanf("%d", &divisor); error = IntDivide(dividend,divisor,&quotient,&remainder); if (!error) /* !error indicates no error */ printf("Answer: %d remainder %d\n", quotient, remainder); else printf("IntDivide failed.\n"); } int IntDivide(int x, int y, int *quoPtr, int *remPtr) { if (y != 0) { *quoPtr = x / y; /* Modify *quoPtr */ *remPtr = x % y; /* Modify *remPtr */ return 0; } else return -1; }

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