1. The 8086 Assembly Programming Data Allocation & Addressing Modes
Khaled A. Al-Utaibi

2. Agenda
Data Allocation
Addressing Modes
Data Addressing Modes

3. Data Allocation
In 8086 assembly, data can be allocated (or defined) using the following directives:
DB (Define Byte) Allocates 1-byte
DW (Define Word) Allocates 2-bytes
DD (Define Double word) Allocates 4-bytes
DQ (Define Quad Word) Allocates 8-bytes
The general format of data allocation is
data-definition-type { init {{, init}} }
Where:
data-definition-type is one of {DB, DW, DD, DQ}, and
init is/are initial value/values
Examples:
DB 0AH
DB 1BH, 20H, 34H
DW 01ACH

4. Data Allocation Variables
A variable is a datum in memory that has been give a name and that may be referred to by name in an instruction.
Variables are created and named and can be initialized by data allocation statements.
The general format for a variable declaration statement is
{variable-name} data-allocation-type {init{{,init}}}
Examples:
V1 DB 12H
V2 DW 0ABCDH

5. Data Allocation Arrays
We can allocate an array by specifying more than one initial (init) value in a data allocation statement.
The general format for an array declaration statement is
{array-name} data-allocation-type {init{{,init}}}
Examples:
A1 DB 12H, 10H, 88H, 44H
A2 DB 14H, 15H, 23H
13H, 17H, 24H

6. Data Allocation Arrays
The DUP (Duplicate) construct facilitates initialization of large array.
The general format of DUP construct is
expr DUP (init)
Where:
expr is a numeric experssion for the number of elements to be allocated and initialized, and
init is the initial value to be given to each element
Examples:
A1 DB 10 DUP(0)
A2 DW 20 DUP(1234H)

7. Addressing Modes
Addressing modes define how to identify the operand(s) of each instruction.
An addressing mode specifies how to calculate the memory address of an operand using information held in –
registers and/or
constants contained within the instruction or elsewhere.
Addressing modes are specified by the instruction set architecture of the CPU.

8. Data Addressing Modes
Data addressing modes are concerned with instructions accessing/manipulating data such as:
Data movement instructions (e.g., MOV, IN, OUT, etc.)
Arithmetic instructions (e.g., ADD, SUB, MUL, DIV, etc.)
Logical instructions (e.g., AND, OR, NOT, CMP, etc.)
There are 7 data addressing modes in the 8086:
(1) Register addressing
(2) Immediate addressing
(3) Direct addressing
(4) Register indirect addressing
(5) Base-plus-index addressing
(6) Register relative addressing
(7) Base relative-plus-index addressing
Figure 1 shows illustrates examples of data addressing modes

9. Figure 1: Data Addressing Modes
MOV [BX+SI], AX
Register AX

10. Register Addressing
Register addressing transfers a copy of a byte (8- bits) or word (16-bits) from one register (the source) to another register (the destination) as shown in Figure 2.
Figure 2: The MOV instruction showing the source,
destination, and direction of data flow.

11. Register Addressing
The 8086 contains the following register names which can be used with register addressing:
8-bits registers: AH, AL, BH, BL, CH, CL, DH, and DL.
16-bits registers: AX, BX, CX, DX, SP, BP, SI, and DI.
Note that mixing an 8-bit register addressing with a 16-bit register addressing is not allowed results in an error when assembled.
Table 1 shows examples of register addressing.

12. Table 1: Examples of Register Addressing.

13. Immediate Addressing
Immediate addressing transfers the source, an immediate byte or word, into the destination register or memory location.
Example: the MOV AX, 3456H (See Figure 3)
This instruction copies a word-sized constant (3456H) into register AX.
Note that AH = 34H and AL = 56H.
Note that immediate data are constant data, whereas the data transferred from a register or memory location are variable data.
Table 2 shows examples of immediate addressing.

14. Figure 3: The operation of the MOV AX,3456H instruction
Figure 3: The operation of the MOV AX,3456H instruction. This instruction
copies the immediate data (3456H) into AX.

15. Table 2: Examples of Immediate Addressing.

16. Direct Addressing
Direct addressing moves a byte or word between a memory location and a register.
The 8086 instruction set does not support a memory-to-memory transfer, except with the MOVS instruction.
Example(1): The MOV CX, LIST
This instruction copies the word-sized contents of memory location LIST into register CX.
LIST is a memory label (variable) defined in the program.
Example(2): MOV AL, [1234H] (See Figure 4)
This instruction copies the byte-sized content of memory location DS:1234
If DS = 1000H, then the memory byte at location 11234H is copied into AL.
Table 3 shows different examples of this type of addressing modes.

17. Figure 4: The operation of the MOV AL,[1234H] instruction when DS = 1000H.

18. Table 3: Examples of Direct Addressing.

19. Register Indirect Addressing
Register indirect addressing transfers a byte or word between a register and a memory location addressed by an index or base register.
The index and base registers are BP, BX, DI, and S1.
Example: The MOV AX, [BX] (See Figure 5)
This instruction copies the word-sized data from the data segment offset address indexed by BX into register.
If DS = 0100H, this instruction addresses a word stored at memory bytes 2000H and 2001H, and transfers it into register AX.
Note that the contents of 2000H are moved into AL and the contents of 2001H are moved into AH.
The [ ] symbols denote indirect addressing in assembly language.
Table 4 shows examples of this type of addressing modes

20. Figure 5: The operation of the MOV AX,[BX] instruction when BX = 1000H and
DS = 0100H.

21. Table 4: Examples of Register Indirect Addressing.

22. Base-Plus-Index Addressing
Base-plus-index addressing transfers a byte or word between a register and the memory location addressed by a base register (BP or BX) plus an index register (DI or SI).
Example: The MOV DX, [BX+DI] (See Figure 6)
This instruction transfers a copy of the word-sized contents of the data segment memory location addressed by BX plus DI into register DX.
If BX = 1000H, DI = 0010H, and DS = 0100H, then this translates into memory address 02010H.
This instruction transfers a copy of the word from location 02010H into the DX register.
Table 5 shows examples of this type of addressing modes

23. Figure 6: An example showing how the base-plus-index addressing mode functions for the MOV DX,[BX_DI ] instruction. Notice that memory address 02010H is accessed because DS = 0100H, BX = 100H, and DI = 0010H.

24. Table 5: Examples of Base plus Index Addressing.

25. Register Relative Addressing
Register relative addressing moves a byte or word between a register and a memory location addressed by an index or base register plus a displacement.
Example(1): MOV AX,[BX+1000H] (See Figure 7)
The instruction loads AX from the data segment address formed by BX plus 1000H.
If BX = 0100H and DS = 0200H, then the address generated = DS x 10H + BX H = 03100H
Example(2): MOV AX, ARRAY[DI] (See Figure 8)
The second instruction loads AX from the data segment memory location in ARRAY plus the contents of DI.
Table 6 shows examples of this type of addressing modes

26. AX BX
Figure 7: The operation Memory of the MOV AX, [BX+1000H] instruction, when BX = 0100H and DS = 0200H.

27. DI
Figure 8: Register relative addressing used to address an element of ARRAY. The displacement addresses the start of ARRAY, and DI accesses an element.

28. Table 6: Examples of Register Relative Addressing.

29. Base Relative Plus Index Addressing
Base relative-plus-index addressing transfers a byte or word between a register and the memory location addressed by a base and an index register plus a displacement.
Example(1): MOV AX, ARRAY[BX+DI]
This instruction uses an address formed by adding ARRAY, BX, and DI.
Example(2): MOV AX, [BX+DI+4]
This instruction uses an address formed by adding BX, DI, and 4.
Table 7 shows examples of this type of addressing modes

30. Table 7: Examples of Base Relative Plus Index Addressing.