What happens if we use “illegal” offsets?? 
 

First of all, what is an array? 

 

An array is a regular order or arrangement containing a fixed number of contiguously storage elements (that is, there are no gaps between elements) of the same data type (although they may have different values). For example, you could have an array of integers, an array of characters or an array of anything that has a defined data type.

For example, we could store the first 10 prime numbers (1, 2, 3, 5, 7, 11, 13, 17, 19, 23) in RAM by storing them as individual integer variables one for each number. Using the following c declaration:

Since an array is a fixed number of contiguous storage all of the same data type, in RAM (assuming that the first contiguous block was at address 2000) it would look like this (2-bytes per integer): 

 

Another example

An array which contains 5 integer values of type int called example could be represented this way:

example

The blank panels represent an element of the array (the array’s objects). These particular objects or elements, are the type int. They are also in sequential order. By being in sequential order, if the computer knows where the first element is, then it is relatively easy for it to find the second, the third, and so forth. Also since this is an array of type int, we are asking for 4-bytes per element (4*5=20 contiguous bytes of RAM)                

Note: The array did not begin with the number 1. Rather it began with the number 0. Each element is offset from the beginning of an array starting at 0.

Why is the first element labeled 0?

Because it is zero elements away from the beginning of the array. This means that the second element is actually 1 away from the beginning the array. The third element is 2 away from the beginning of the array and so forth.

Get the picture? 

Any array of size N has an offset range of 0 to N-1. In our case the range is from 0 to 4.

Please note: Arrays can have more than one dimension. A one-dimensional array is called a vector or pointer, which contains the address of a location in memory; a two-dimensional array is called a matrix, that is, an array of rows and columns.

What is an address? 

 

It is a location of data, usually in main memory or on a disk. It is an array of storage boxes, each of which is one byte in length. Each box has an address—a unique number assigned to it, which makes it easier to locate. 

 

What is a string? 

 

A string is a sequence of data values, usually bytes, which stand for characters (a

“character string”). A character string differs from a name in that it does not represent anything. For example, a bit string is a sequence of bits. 

 

A character string is usually defined by enclosing the characters in single or double quotes:  

 

WASHINGTON would be a name, but 'WASHINGTON' and "WASHINGTON" would be character strings.  

 

The length of a character string is usually the number of characters in it: 

 

The character string "WASHINGTON" has a length of 10 (the quote marks are not included). 

 

What is an offset? 

 

An offset (subscript) is an index or position in an array, string, or block of memory—usually a non-negative integer.  

 

For example: 0, 1, 2, 3… 

 

It is a value added to a base address to create a second address.  

 

For example, if B represents address 100, then the _expression,

B+5

would indicate the address 105. The 5 in the _expression is the offset.  

 

Let’s take an array and figure out the offset by using the following formula :

ELEMENT ADDRESS = BASE ADDRESS OF THE ARRAY+ (OFFSET # * 4)

*Remember we have to multiply the offset by 4 since we are using integers. Therefore the 4 bytes * 4 elements = 16 continguous bytes of RAM needed!

First, let’s say our base address starts at 100. 

int example[4] ={1,3,5,7}        

Element number/position/index  Addresses                    Reference

            1                                              100 to 103                 example[0]

            2                                              100+4=104 (to 107)    example[1]

            3                                              104+4=108 (to 111)    example[2]

            4                                              108+4=112 (to 115)   example[3]                              

RAM Allocation:

Offset                                       Address: