Memory Usage in Programs

Transcription

1 Memory Usage in Programs ECE2036 Lecture 4 ECE2036 Memory Usage in Programs Spring / 11

2 Memory Usage in Programs The way that memory is assigned to your program is fundamental to understanding how the compiler does it s job when compiling your program This in turn leads to designing better programs and better skills at debugging your programs Memory is allocated in one of four different logical memory regions: 1 Instruction Memory This area of memory is where the assembly language instructions are stored This is usually in the in the lowest address region 2 Global Data Memory This area is where any variable that is defined at global scope is allocated It is usually right after the instruction memory area More on this later 3 Heap Memory This area is where dynamic memory is allocated and stored It is usually right after the Global Data Memory region The use of the heap will be covered later in the class, when we have more experience with C/C++ programming in general 4 Stack Memory This area is memory for local varaibles are stored It is usually the higheset memory region assigned to your program The stack grows and shrinks as the program executes This will be discussed in detail later ECE2036 Memory Usage in Programs Spring / 11

3 Typical Memory Layout Address Instruction Memory 11,066 LDI R0,1 LDM R1,Length Data Memory ,320 Heap Memory (Covered Later in Class) 14,680,768 Stack Memory 3,221,222,448 ECE2036 Memory Usage in Programs Spring / 11

4 Declaring Variables in C/C++ 1 All variables in a C or C++ program must be declared before then can be assigned a value or used in an expression 2 Variable declaractions consist of a type declaration followed by a variable name followed by a semicolon Variable names must contain only alphabetic characters (upper or lower case), the underscore character, and numeric dibits, but they cannot start with a numeric 3 The type declaration can be one of the built in types pre defined by the C/C++ language, or a user defined type that we will discuss later 4 An array of variables can be declared by appending the array length in square brackets after the variable name 5 All variables in C/C++ have a scope, which specifies when the variable is created (assigned a memory location), when it can be accessed (read or written) and when it is no longer available (removed from memory) This is discussed in detail later ECE2036 Memory Usage in Programs Spring / 11

5 Example Variable Declarations i n t i ; / / Signed i n t e g e r ( with p o s i t i v e and negative value unsigned j = 5; / / Unsigned i n t e g e r, with only p o s i t i v e values unsigned a [ 1 0 ] ; / / Array of 10 unsigned i n t e g e r s / / F l o a t i n g p o i n t v a r i a b l e s have f r a c t i o n a l p a r t s / / Such as 6125 double d ; / / 64 b i t f l o a t i n g p o i n t value f l o a t f = 14582; / / 32 b i t f l o a t i n g p o i n t value char ch = A ; / / Character v a r i a b l e, i n i t i a l value of A char cha [ 5 ] ; / / Array of 5 character v a r i a b l e s char cha1 [ ] = " Array o f Characters " ; s t r i n g s t r = " This i s a t e s t " ; / / S t r i n g v a r i a b l e with i n i t i a l value / / Array v a r i a b l e s can also be e x p l i c i t l y i n i t i a l i z e d unsigned ua [ 1 0 ] = { 100, 200, 300, 400, 500, 600, 700, 800, 900, 10 / / The array l e n g t h value can be omitted / / i f an i n i t i a l i z e r i s s p e c i f i e d i n t i a [ ] = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100 } ; ECE2036 Memory Usage in Programs Spring / 11

6 Global Variables Global Variables are declared outside of any subroutine in C/C++ The scope of a global variable is the entire lifetime of the program The variable is assigned a memory location and initialized before the first line of code in the program If a global variable is not given an initializer, it is give the value of binary zero Global Variables can be read or written by any part of the C/C++ program The use of global variables is generally discouraged However there are many cases where a global variable is the best solution in a particular program design ECE2036 Memory Usage in Programs Spring / 11

7 Global Variables in Memory / / Sample v a r i a b l e s f o r memory l a y o u t ( next s l i d e ) i n t i 1 = 10; char a = A ; i n t j = 1000; i n t k = 0; char aa [ 1 0 ] = { a, b, c, d, e, f, g, h, i, j } ; double d = ; f l o a t e = 25601; char t e s t [ ] = " t h i s i s a t e s t " ; i n t k1 = 1; ECE2036 Memory Usage in Programs Spring / 11

8 Global Variables in Memory Address Contents Declaration int i1 = 10; char a = A ; int j = 1000; int k = 0; char aa[10] = { double d = 102; float e = char t[] = "th int k1 = 1; ECE2036 Memory Usage in Programs Spring / 11

9 Local Variables 1 Local Variables are declared within a subroutine (function) implementation 2 Local variables are assigned a memory location on the Stack when the subroutine is called, and the memory is released when the subroutine exits 3 Local variables can have the same name as global variables! In this case, a reference to the variable within the subroutine refers to the local variable, not the global variable 4 Best programming practice dictates declaring variables at exactly the point they are needed in the subroutine, rather than declaring all of them in a group at the beginning of the subroutine 5 The scope of a local variable is within the subroutine only Subroutine A cannot refer to a local variable defined in subroutine B This would not compile ECE2036 Memory Usage in Programs Spring / 11

10 A Common Mistake A common mistake in declaring and using variables is declaring a new variable, when you really mean using a previoiusly declared value / / I l l u s t r a t e a common mistake / / George F Riley, Georgia Tech, CS1372 i n t Average ( i n t d [ ], i n t L ) { / / Compute average of array d of l e n g t h L i n t i = 0; / / Array index i n t Sum = 0; / / Sum of array values while ( i < L ) { Sum = Sum + d [ i ] ; / / The mistake i s below The code show makes a new v a r i a b l e i / / and does not increment the i declared above / / Thus the while loop never e x i t s i n t i = i + 1; / / Wrong! / / Corr ect code would be : / / i = i + 1; } return Sum / L ; } ECE2036 Memory Usage in Programs Spring / 11

11 Simple C++ Program Snippet / / Memory A l l o c a t i o n Code Snippet / / George F Riley, Georgia Tech, Summer 2008 i n t g1 ; / / Global variable, in global data i n t g2 = 10; / / Another global variable, also in global data i n t Sub1 ( / / I i n s t r u c t i o n s f o r Sub1 i n i n s t r u c t i o n memory i n t i, / / Parameter i i s i s a l l o c a t e d on the stack i n t j ) / / Parameter j i s also a l l o c a t d on the stack { i n t k = i j ; / / Local v a r i a b l e k i s a l l o c a t e d on the stack g1 = k ; / / Global variables are always v i s i b l e, even in subroutines / / m1 = 5; / / Sub1 cannot r e f e r to m1 defined in main / / g3 = 7; / / Sub1 cannot r e f e r to g3, i t i s not declared yet i = 7; / / Sub1 CAN change the value of subroutine parameters } / / When Sub1 exits, a l l stack variables are removed i n t g3 = 20; / / Another global, declared outside any subroutine i n t main ( ) / / I n s t r u c t i o n s f o r main i n i n s t r u c t i o n memory, a f t e r Sub1 { i n t m1 = 10; / / Local v a r i a b l e i n main, on the stack g3 = m1; / / Global v i s i b l e i n main as w e l l / / k = 5; / / Main cannot r e f e r to l o c a l v a r i a b l e k i n Sub1 Sub1 (m1, g3 ) ; / / Call Sub1, passing the VALUE of m1 and g3 } / / When main exits, a l l stack variables are removed ECE2036 Memory Usage in Programs Spring / 11