Memory Organization Intro to Pointers. CIS 15 : Spring 2007

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1 Memory Organization Intro to Pointers CIS 15 : Spring 2007

2 Functionalia Today: Midterm Exam Computation Architecture Memory Addressing Pointers

3 Midterm Average 26/50 Median 29/50, or a 58% (low - normally want a 66%) Any curves will occur at the very end of the course. Final Exam is comprised only of those problems that most people messed up on. I will go through the problems now. Pay attention as this will clue you in what the Final Exam might be comprised of. Go through the problems. Re-grade Policy Follows

4 Regrade Policy If you feel that there was an error in grading your homework, project or exam, then you need to write on a piece of paper a description of the error. STAPLE the paper to your homework, project or exam and leave it with me to be regraded. Know that I mark with a list of expectations for each homework assignment, project and exam problem, knowing where to take off points so if your complaint is that too many points were taken off for one kind of mistake or another in your program, then generally those types of things will not change in a regrade. (Do Not Submit Every Homework for a regrade!)

5 Regrade Evaluation Policy If there is a genuine error in the marking, like I thought something was missing, but it is really there, then you will likely get points restored. HOWEVER, a regrade means that the entire assignment or exam will be remarked, so be aware that your mark can go DOWN as well as up. Regrades take a while to process, so be patient if you need the work to study from, then make a copy of it before you turn it in for a regrade.

6 ENIAC and Memory Archtecture 1940s (U. Penn) - One of the First Programmable Computers (Eckart, ) Programs were combinations of patches wires - faster to execute - reconfigurable BUT static Amazing performance for its time (calculate artillery faster than realtime)

7 ENIAC (1940s) 1940s (U. Penn - Eckert, Burks, and Mauchly) - One of the First Electronically Programmable Computers Programs were combinations of patches wires faster to execute reconfigurable BUT static Amazing performance for its time (calculate artillery faster than realtime)

8 von Neuman Architecture ENIAC et al. proposed (along with von Neuman) the stored-program architecture, popularly called the von Neuman architecture. Program and Data are stored in the same memory storage device. Basis of modern computational devices: PCs, ipods, etc...

Harvard Architecture - Program and Data are stored in different (sometimes different bit-widths) Modern uses: fast DSP chips, and

9 Harvard Architecture von Neuman bottleneck computational power is limited by the memory bus and data throughput (not the CPU or other individual components) What other models of computational architecture is there? Harvard Architecture - Program and Data are stored in different (sometimes different bit-widths) Modern uses: fast DSP chips, and small microcontrollers (PIC 8,12,16...) Neural Networks: Program and Data are contained within a model based upon metaphors of the neurons in the brain and their connectivity

10 Accessing Memory in C and C++ With C and C++, we can manipulate our data (and our programs!) at the memory level through the use of pointers. Pointers are a special variables that hold memory addresses. Memory addresses identify locations in a computer s memory. Address Value A Pointer Holds the memory address 0x and uses it to indirectly access the value 3 0x x Can use Pointers to access (i.e. point) to some piece of data (int, or custom). 0x

11 Accessing Memory in C and C++ Address Value 0x is shown in HEXADECIMAL format (base-16). (indicated by preceding 0x). What is the size of the data (i.e. integers) that is stored in the memory? 0x x x

12 HexEditors View your own files on your computer in the way that the computer sees it using a Hex Editor (there are many types out there on the internet).

13 Windows used to have the Blue Screen Of Death. When it crashed you could see a DUMP of the current physical memory.

14 Use the & operator to get the address of a variable. int x = 1; int y = 2; Output? int z = 3; Values cout << x << endl; cout << y << endl; cout << z << endl; Addresses and Sizes cout << &x << << sizeof(x) << endl; cout << &y << << sizeof(y) << endl; cout << &z << << sizeof(z) << endl;

15 Use the & operator to get the address of a variable. int x = 1; int y = 2; int z = 3; Values cout << x << endl; cout << y << endl; cout << z << endl; Addresses and Sizes cout << &x << << sizeof(x) << endl; cout << &y << << sizeof(y) << endl; cout << &z << << sizeof(z) << endl; xbffffc4c 4 0xbffffc48 4 0xbffffc44 4 Why are these reversed?

16 Use the & operator to get the address of a variable. int x = 1; int y = 2; int z = 3; Values cout << x << endl; cout << y << endl; cout << z << endl; Addresses and Sizes cout << &x << << sizeof(x) << endl; cout << &y << << sizeof(y) << endl; cout << &z << << sizeof(z) << endl; xbffffc4c 4 0xbffffc48 4 0xbffffc44 4 Reversed because variables are stored on a stack.

17 Don t Confuse this use of & with... We now have seen 4 DIFFERENT uses of the & character in C/C++ 1. Passing Function Parameters by Reference void swap(int & x, int & y); 2. As the Logical AND used in conditionals boolean x; if(false && x) { As the bit-wise AND for use in bit arithmetic. int x, y; y = 0x00F0 & x; 4. Gives the memory address of a variable. int x = 0; cout << Address of x: << &x;

18 Array Names are Pointers int numbers[5] = {1, 2, 3, 4, 5}; showvalues(numbers, 5);... void showvalues(int values[], int size) { for(int c = 0; c < size; c++) cout << values[c] << endl; }

19 Pointers Versus Reference Variables Reference Variables are aliases, used in accessing variables from within functions. Pointers are lower-level, but their effect is exactly like Reference Variables. Pointers require some more work and alot of care on the part of the programmer. Why not Reference Variables all the time? Pointers are fundamental in dynamic memory allocation, and C-strings. Reference Variables do not exist in C!

Advanced Programming Concepts. CIS 15 : Spring 2007

Advanced Programming Concepts. CIS 15 : Spring 2007 Advanced Programming Concepts CIS 15 : Spring 2007 Mondays and Thursdays 12:15 pm to 1:30 pm 232 Ingersoll Extension Instructor: Chipp Jansen (not Tennenbaum as listed) E-mail: chipp@sci.brooklyn.cuny.edu