The 8086 Assembly Programming Data Allocation & Addressing Modes

1. The 8086 Assembly ProgrammingData Allocation & Addressing Modes Khaled A. Al-Utaibi alutaibi@uoh.edu.sa

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 AllocationVariables • 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 AllocationArrays • 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 AllocationArrays • 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

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. 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 + 1000H = 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.