Addressing Modes

1. Addressing Modes A Course in Microprosessor Electrical Engineering Department University of Indonesia

2. Data Addressing Modes • Fig.3.1 illustrates the MOV instruction and defines the direction of data flow • Fig.3.2 shows all possible data variations of the data-addressing modes using the MOV instruction • Register adressing: MOV CX,DX or MOV ECX,EDX • Immediate addressing: MOV AL,22H or MOV EAX,12345678H • Direct addressing: MOV CX,LIST

3. Data Addressing Modes (cont’d) • Base-plus-index adressing: MOV [BX+DI], CL or MOV [EAX+EBX],CL • Register relative addressing: MOV AX,[BX+4] or MOV AX,ARRAY[BX] • Base relative-plus-index addressing: MOV AX,ARRAY[BX+DI] or MOV AX,[BX+DI+4] • Scaled-index addressing: MOV EDX,[EAX+4*EBX]

4. Register Addressing • It is the most common form and is the easiest to apply • Microprocessor contains 8-bit, 16-bit, 32-bit registers • Never mix an 8-bit register with a 16-bit register, 16-bit register with a 32-bit register, etc., because this results in an error when assembled • Table 3.1 shows many variations of register move instructions • Fig.3.3 shows the operation of the MOV BC,CX instruction

5. Register Addressing • Example 3.1 shows a sequence of assembled instructions that copy various data between 8-, 16-, and 32-bit registers

6. Immediate Addressing • The term immediate implies that the data immediately follow the hexadecimal opcode in the memory • Immediate are constant data • The MOV immediate instruction transfers a copy of the immediate data into a register or a memory location • Fig.3.4 shows the source data (sometimes preceded by #) overwrite the destination data • The instruction copies the 13456H into register AX

7. Register Addressing • Example 3.2 shows various immeidate instructions in a short program that placess a 0000H into the 16-bit registers AX, BX, CX

8. Direct Data Addressing • There two basic forms of direct data addressing: • direct addressing, which applies to a MOV bet-ween a memory location and AL, AX or EAX • displacement addressing, which applies to almost any instruction in the instruction set • Direct Addressing: MOV AL,DATA (Fig.3.5) • Table 3.3 lists the three direct addressed instruc-tion • A MOV instruction is 3-byte long instruction

9. Direct Data Addressing • Displacement Addressing: MOV CL,DATA • almost identical with direct addressing except that the instruction is four bytes wide

10. Register Indirect Addressing • It allows data to be addressed at any memory location through an offset address held in any of the following register: BP, BX, DI, and SI • MOV AX,[BX] Fig.3.6 • Data segment is used by default with register indirect addressing or any other addressing mode that uses BX, DI, or SI to address memory • If register BP addresses memory, the stack segment is used by default

11. Register Indirect Addressing • Sometimes, indirect addressing requires spe-cifying the size of the data with the special assembler directive BYTE PTR, WORD PTR or DWORD PTR • These indicate the size of the memory data addressed by the memory pointer (PTR) • Indirect addressing allows a program to refer to a tabular data located in the memory sys-tem (Fig.3.7 & Example 3.6)

12. Base-Plus-Index Addressing • It indirectly addresses memory data • In 8086 - 80286, this use a base register (BP or BX, holds the beginning location of a memory array) and an index register (DI or SI, ) to indirectly addresses memory • In 80386 and above, this type of addressing allows the combination of any two 32-bit extended registers except #SP • MOV DL, [EAX+EBX] • Figure 3.8 shows the sample instruction of locating data with this scheme

13. Base-Plus-Index Addressing (cont’d) • The major use of this type of addressing is to address elements in a memory array • Fig.3.9 shows the use of BX (base) and DI (index) to access an element in an array of data) • Study Table 3.6 and Example 3.7 as well

14. Register Relative Addressing • In its, the data in a segment of memory are addressed by adding the displacement to the contents of a base ro and index register (BP, BX, DI, or SI) • Fig.3.10 shows the operation of the MOV AX,[BX+ 1000H] instruction • The displacement can be a number added to the register within the [ ], as in MOV AL,[DI+2], or it can be a displacement substracted from the register, as in MOV AL,[SI-1]

15. Register Relative Addressing (cont’d) • It is possible to address array data with regis-ter relative addressing such as one does with base-plus-index addressing • See Fig.3.11 and study the example 3.8

16. Base Relative-Plus-Index Addressing • This mode often addresses a two-dimension-al array of memory data • It is the least-used addressing mode (i.e., too complex for frequent use in a program) • Fig.3.12 shows how the instruction MOV AX, [BX+SI+100H] • Addressing arrays with base relative-plus-index addressing • the displacement addresses the file • the base register addresses a record • the index register addresses an element of a record • Study Example 3.9 and Fig.3.13

17. Scaled-Index Addressing • This type of addressing is unique to the 80386 - Pentium Pro • It uses two 32-bit registers (a base register and an index register) to access the memory • The second register (index) is multiplied by a scaling factor (either 1X, 2X, 4X, or 8X) • MOV AX,[EDI+2*ECX] • See Example 3.10 and Table 3.9

18. Data Structures • A data structure is used to specify how information is stored in a memory array; it can be quite useful with application that use arrays • The start of a structure is identified with the STRUC directive and ended with the ENDS • See Example 3.11 • When data are addressed in a structure, use the structure name and the field name to se-lect a field from the structure (Example 3.12)

19. Program Memory Addressing • Program memory-addressing modes (JMP and CALL) consist of three distinct forms: direct, relative, and indirect • Direct Program Memory Addressing • The instruction store the address with the op-code

20. Program Memory-Addressing Modes • Program memory-addressing modes (JMP and CALL) consist of three distinct forms: direct, relative, and indirect • Direct Program Memory Addressing • The instructions store the address with the op-code • See Fig.3.14 • It is called a far jump because it can jump to any memory location for the next instruction

21. Program Memory Addressing Modes (contd) • Relative Program Memory Addressing • The term relative means “relative to the IP” • See Fig.3.15 • JMP instruction is a one-byte instruction with a one-byte or two-byte displacement that adds to the instruction pointer • Relative JMP and CALL instructions contain either an 8-bit or a 16-bit signed displacement that allows a forward memory reference or a reverse memory reference

22. Program Memory Addressing Modes (contd) • Indirect Program Memory Addressing • Table 3.10 lists some acceptable indirect program jump instructions, which can use any 16-bit register, any relative register, and any re-lative register with a displacement • If a 16-bit register holds the address of a JMP instruction, the jump is near • If a relative register holds the address, the jump is also considered an indirect jump • Fig.3.16 shows a jump table that is stored begin-ning at memory location TABLE

23. Stack Memory-Addressing Modes • Stack holds data temporarily and stores return addresses for procedures • The stack memory is LIFO memory • Use PUSH instruction to place data onto stack • Use POP instruction to remove data from stack • The stack memory is maintained by two registers: SP or ESP, and SS • Study Fig.3.17 • The PUSHA and POPA either push or pop all of the register, except the segment register, on the stack (see example 3.14)