Monday, April 25, 2016

Description and Example of Array

** In assembly language, an array is just a sequence of memory bytes or word.

For Example : To define a three-byte array called B_ARRAY, whose initial values are 10h, 20h, and 30h, we can write,

B_ARRAY                  DB                                   10H,20H,30H

The name B_ARRAY  is associated with the first of these bytes, B_ARRAY+1 with the second, and B_ARRAY+2 with the third. If the assembler assigns the offset address 0200h to B_ARRAY, then memory would look like this : 

Symbol                         Address                           Contents

B_ARRAY                            200h                                 10h

B_ARRAY+1                        201h                                 20h 

B_ARRAY +2                       202h                                 30h 

In the same way, an array of words may be defined.

Example : 

W_ARRAY                       DW                                 1000,40,29887,329

sets up an array of four words, with initial value 1000, 40,29887, and 329.
The initial word is associated with the name W_ARRAY, the next one with W_ARRAY+2, the next with B_ARRAY+4, and so on. If the array starts at 0300h, it will look like this : 

Symbol                          Address                           Contents

W_ARRAY                            0300h                                 1000d

W_ARRAY+2                        0302h                                 40d

W_ARRAY +4                       0304                                  29887d

W_ARRAY +6                      0306h                                 329d 

High and low Bytes of a Word

Sometimes we need to refer to the high and low bytes of a word variable.  Suppose we define 

WORD1                        DW                           123H

The low byte of WORLD1 contains 34h, and the high byte contains 12h. The low byte has symbolic address WORD1, and the high byte has symbolic address WORD1+1.

Character Strings

An array of ASCII codes can be initialized with a string of characters.

Example : 

LETTERS                                       DB                           'ABC'

is equivalent to

LETTERS                 DB                    41H,42H,43H

Inside a String the assembler differentiates between upper and lower case. Thus, the string "abc" is translated into three bytes with values 61h,62h, and 63h

It is possible to combine characters and number in one definition;

Example :

MSG     DB                        'HELLO', 0AH, 0DH, 'S'

is equivalent to  

MSG DB                            48H, 45H, 4CH, 4CH, 4FH, 0AH, 0DH, 24H