REGISTER + OFFSET ADDRESSING

Direct addressing and indirect addressing using a single register are two basic forms of memory access. Another possibility is to use different combinations of direct and indirect references. In the above example we used BX to access different array elements which were placed consecutively in memory like an array. We can also place in BX only the array index and not the exact address and form the exact address when we are going to access the actual memory. This way the same register can be used for accessing different arrays and also the register can be used for index comparison like the following example does.

Example 2.8

001 ; a program to add ten numbers using register + offset addressing
002 [org 0x0100]
003 mov bx, 0 ; initialize array index to zero
004 mov cx, 10 ; load count of numbers in cx
005 mov ax, 0 ; initialize sum to zero
006
007 l1: add ax, [num1+bx] ; add number to ax
008 add bx, 2 ; advance bx to next index
009 sub cx, 1 ; numbers to be added reduced
010 jnz l1 ; if numbers remain add next
011
012 mov [total], ax ; write back sum in memory
013
014 mov ax, 0x4c00 ; terminate program
015 int 0x21
016
017 num1: dw 10, 20, 30, 40, 50, 10, 20, 30, 40, 50
018 total: dw 0

003 This time BX is initialized to zero instead of array base

7 The format of memory access has changed. The array base is added to BX containing array index at the time of memory access.

8 As the array is of words, BX jumps in steps of two, i.e. 0, 2, 4. Higher level languages do appropriate incrementing themselves and we always use sequential array indexes. However in assembly language we always calculate in bytes and therefore we need to take care of the size of one array element which in this case is two.

Inside the debugger we observe that the memory access instruction is shown as “mov ax, [011F+bx]” and the actual memory accessed is the one whose address is the sum of 011F and the value contained in the BX register. This form of access is of the register indirect family and is called base + offset or index + offset depending on whether BX or BP is used or SI or DI is used.