A description of memory allocation and its types. According, to the M.Sc( 2nd sem) Compiler design syllabus.

1. Presentation
On
Memory Allocation

2. Memory Allocation
Memory allocation is an action of assigning the physical
or the virtual memory address space to a process (its
instructions and data). The two fundamental methods of
memory allocation are static and dynamic memory
allocation.
We have two types of memory allocation or we can say
two methods of binding, static and dynamic binding.

3. 1. Static Memory Allocation
In static memory allocation, the size of the data required by
the process must be known before the execution of the
process initiates.
If the data sizes are not known before the execution of the
process, then they have to be guessed.
If the data size guessed is larger than the required, then it
leads to wastage of memory.
If the guessed size is smaller, then it leads to inappropriate
execution of the process.

4. As all the memory allocation operation required for the
process is done before the execution of the process has
started. So, it leads to faster execution of a process.
Static memory allocation provides more efficiency when
compared by the dynamic memory allocation.

5. 2. Dynamic Memory Allocation
Dynamic Memory Allocation is a process in which we
allocate a block of memory during the run-time of a
program.
The actual size, of the data required, is known at the run
time so, it allocates the exact memory space to the
program thereby, reducing the memory wastage.
Dynamic memory allocation provides flexibility to the
execution of the program.

6. Advantages of Static Memory Allocation
Static memory allocation provides an efficient way of
assigning the memory to a process.
All the memory assigning operations are done before the
execution starts. So, there are no overheads of memory
allocation operations at the time of execution of the program.
Static memory allocation provides faster execution.

7. Advantages Dynamic Memory Allocation
Dynamic memory allocation provides a flexible way of
assigning the memory to a process.
Dynamic memory allocation reduces the
memory wastage as it assigns memory to a process during
the execution of that program. So, it is aware of the exact
memory size required by the program.

8. Disadvantages of static memory allocation
In static memory allocation, the system is unaware of the
memory requirement of the program. So, it has to guess
the memory required for the program.
Static memory allocation leads to memory wastage. As it
estimates the size of memory required by the program. So,
if the estimated size is larger, it will lead to
memory wastage else if the estimated size is smaller, then
the program will execute inappropriately.

9. Disadvantages of Dynamic Memory allocation
Dynamic memory allocation method has an overhead of
assigning the memory to a process during the time of its
execution.
Sometimes the memory allocation actions are repeated
several times during the execution of the program which
leads to more overheads.
The overheads of memory allocation at the time of its
execution slowdowns the execution to some extent.

10. Dynamic memory allocation is possible by 4 functions of
stdlib.h header file.
1.malloc()
2.calloc()
3.realloc()
4.free()
malloc() function
The malloc() function allocates single block of requested
memory.
It doesn't initialize memory at execution time, so it has garbage
value initially.
It returns NULL if memory is not sufficient

11. The syntax of malloc() function is given below
ptr=(cast-type*)malloc(byte-size)

12. calloc() function
The calloc() function allocates multiple block of requested
memory.
It initially initialize all bytes to zero.
It returns NULL if memory is not sufficient.
The syntax of calloc() function is given below:
ptr=(cast-type*)calloc(number, byte-size)

13. realloc() function
If memory is not sufficient for malloc() or calloc(), you can
reallocate the memory by realloc() function. In short, it
changes the memory size.
The syntax of realloc() function.
ptr=realloc(ptr, new-size)

14. free() function
The memory occupied by malloc() or calloc() functions must
be released by calling free() function. Otherwise, it will
consume memory until program exit.
The syntax of free() function.
free(ptr)

15. Array
Arrays are defined as the collection of similar type of data
items stored at contiguous memory locations.
Memory Allocation of the array
All the data elements of an array are stored at
contiguous locations in the main memory. The name of
the array represents the base address or the address of
first element in the main memory. Each element of the
array is represented by a proper indexing.

16. The indexing of the array can be defined in three
ways.
0 (zero - based indexing) : The first element of the array will
be arr[0].
1 (one - based indexing) : The first element of the array will
be arr[1].
n (n - based indexing) : The first element of the array can
reside at any random index number.

17. In the following image, we have shown the memory allocation
of an array arr of size 5. The array follows 0-based indexing
approach. The base address of the array is 100th byte. This will
be the address of arr[0]. Here, the size of int is 4 bytes
therefore each element will take 4 bytes in the memory.

18. Accessing Elements of an array
To access any random element of an array we need the
following information:
1.Base Address of the array.
2.Size of an element in bytes.
3.Which type of indexing, array follows.
Address of any element of a 1D array can be calculated by
using the following formula:
Byte address of element A[i] = base address + size * ( i -
first index)

19. Example :
In an array, A[10 ..... +2 ], Base address (BA) = 999, size of
an element = 2 bytes,
find the location of A[-1].
L(A[-1]) = 999 + [(-1) - (-10)] x 2
= 999 + 18
= 1017

20. 2D Array
The 2D array is organized as matrices which can be represented
as the collection of rows and columns.
How to declare 2D Array
The syntax of declaring two dimensional array is very much
similar to that of a one dimensional array, given as follows.
int arr[max_rows][max_columns];

21. How do we access data in a 2D array
The elements of 2D arrays can be random accessed. Similar to
one dimensional arrays, we can access the individual cells in a
2D array by using the indices of the cells. There are two
indices attached to a particular cell, one is its row number
while the other is its column number.
However, we can store the value stored in any particular cell
of a 2D array to some variable x by using the following syntax.
int x = a[i][j];

22. Mapping 2D array to 1D array
A 3 X 3 two dimensional array is shown in the following image.
However, this array needs to be mapped to a one dimensional
array in order to store it into the memory.

23. There are two main techniques of storing 2D array elements
into memory
1. Row Major ordering
In row major ordering, all the rows of the 2D array are stored
into the memory contiguously. Considering the array shown in
the above image, its memory allocation according to row major
order is shown as follows

24. first, the 1st row of the array is stored into the memory
completely, then the 2nd row of the array is stored into the
memory completely and so on till the last row

25. 2. Column Major ordering
According to the column major ordering, all the columns of
the 2D array are stored into the memory contiguously. The
memory allocation of the array which is shown in in the
above image is given as follows.

26. first, the 1st column of the array is stored into the memory
completely, then the 2nd row of the array is stored into the
memory completely and so on till the last column of the
array.

27. By Row Major Order
If array is declared by a[m][n] where m is the number of
rows while n is the number of columns, then address of an
element a[i][j] of the array stored in row major order is
calculated as,
Address(a[i][j]) = B. A. + (i * n + j) * size
where, B. A. is the base address or the address of the first
element of the array a[0][0] .
Calculating the Address of the random element of
a 2D array

28. By Column major order
If array is declared by a[m][n] where m is the number of
rows while n is the number of columns, then address of an
element a[i][j] of the array stored in row major order is
calculated as,
Address(a[i][j]) = ((j*m)+i)*Size + BA
where BA is the base address of the array.

29. STRUCTURE ALLOCATION
HOW STRUCTURE MEMBERS ARE STORED IN MEMORY?
Always, contiguous memory locations are used to store
structure members in memory. Consider below example to
understand how memory is allocated for structures.

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#include <stdio.h>
#include <string.h>
struct student
{
int id1;
int id2;
char a;
char b;
float percentage;
};
int main()
{
int i;
struct student record1 = {1, 2, 'A', 'B', 90.5};
printf("size of structure in bytes : %dn",
sizeof(record1));
printf("nAddress of id1 = %u", &record1.id1 );
printf("nAddress of id2 = %u", &record1.id2 );
printf("nAddress of a = %u", &record1.a );
printf("nAddress of b = %u", &record1.b );
printf("nAddress of percentage = %u",&record1.percentage);
return 0;
}

31. Output
size of structure in bytes : 16
Address of id1 = 675376768
Address of id2 = 675376772
Address of a = 675376776
Address of b = 675376777
Address of percentage = 675376780
There are 5 members declared for structure in above program.
In 32 bit compiler, 4 bytes of memory is occupied by int
datatype. 1 byte of memory is occupied by char datatype and 4
bytes of memory is occupied by float datatype.
Please refer below table to know from where to where memory
is allocated for each datatype in contiguous (adjacent) location
in memory.

33. The pictorial representation of above structure memory
allocation is given below. This diagram will help you to
understand the memory allocation concept in C very easily

34. Common and Equivalence allocation
Lengthy argument lists in subroutines and user-defined
functions can occur as modularised programs grow
ever larger, requiring more and more information to be
passed between program units.
The COMMONblock, a piece of shared memory in the
computer, is another method for passing information
between program units.
Data stored in a COMMONblock is not passed
between program units via argument lists , but through
the COMMONstatement near the beginning of each
program unit.

35. There are two types of COMMON block:
blank and named.
A program may contain only one blank COMMONblock
but any number of named COMMONblocks.
Every COMMONblock must be declared in every program
unit in which the information stored therein is needed.
In addition, the unique blank COMMONblock must be
declared in the main program

36. Every subroutine or user-defined function that uses data stored in
the COMMON block , blank or named, must have a similar statement to
those above. The variable names do not need to match between
program units but it is vital that their types and the order in which they
appear in the list are identical.
Consider the following program fragment:
Example
PROGRAM MAIN
INTEGER A
REAL F,R,X,Y
COMMON R,A,F
A = -14
R = 99.9
F = 0.2
CALL SUB(X,Y)
… END
SUBROUTINE SUB(P,Q)
INTEGER I
REAL A,B,P,Q
COMMON A,I,B
… END

37. Main Program Common Memory
Storage
Subroutine
R 99.9 A
A -14 I
F 0.2 B
In this example, a blank COMMONblock holds three values:
a REALnumber, an INTEGERnumber, and another REALnumber.
Memory is shared in the COMMONblock in the following way: