addressing modes

Addressing Modes

Prima Dewi PurnamasariMicroprocessor

Electrical Engineering Department Universitas Indonesia

Microprocessor (c) Prima Dewi Purnamasari 20112

Data addressing modes

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Data Addressing Modes

Register addressing:MOV CX,DX or MOV ECX,EDX

Immediate addressing: MOV AL,22H or MOV EAX,12345678H

Direct addressing:MOV CX,LIST

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Data Addressing Modes (cont’d)

Register Indirect addressing:MOV AX,[BX]

Base-plus-index adressing:

MOV [BX+DI], CL MOV [EAX+EBX],CL

Register relative addressing:

MOV AX,[BX+4] MOV AX,ARRAY[BX]

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Data Addressing Modes (cont’d)

Base relative-plus-index addressing:

MOV AX,ARRAY[BX+DI] MOV AX,[BX+DI+4]

Scaled-index addressing:

MOV EDX,[EAX+4*EBX]

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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

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Example of register-addressed instructions

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Example: MOV BX,CX

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Register Addressing Example 3.1 shows a sequence of assembled

instructions that copy various data between 8-, 16-, and 32-bit registers

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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 3456H into register AX

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Immediate Addressing Example 3.2 shows various immediate

instructions in a short program that places a 0000H into the 16-bit registers AX, BX, CX

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Direct Data Addressing Two basic forms of direct data addressing:

direct addressing, which applies to a MOV between a memory location and accumulator (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

instruction A MOV instruction is 3-byte long instruction

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Direct Data Addressing Displacement Addressing: MOV CL,DATA

almost identical with direct addressing except that the instruction is four bytes wide

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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

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Register Indirect Addressing

Sometimes, indirect addressing requires specifying 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 system (Fig.3.7 & Example 3.6)

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.model small

.datadatas dw 10 dup (?)

.code

.startupmov ax,1234hmov bx,offset datasmov cx,10

again: mov [bx],ax inc ax inc bx inc bx loop again.exitend


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 ESP MOV DL, [EAX+EBX]

Figure 3.8 shows the sample instruction of locating data with this scheme

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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

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Register Relative Addressing In its, the data in a segment of memory are

addressed by adding the displacement to the contents of a base or an 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 subtracted from the register, as in MOV AL,[SI-1]

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Register Relative Addressing (cont’d)

It is possible to address array data with register relative addressing such as one does with base-plus-index addressing See Fig.3.11 and study the example 3.8

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Base Relative-Plus-Index Addressing

This mode often addresses a two-dimensional 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]

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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

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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

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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

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When data are addressed in a structure, use the structure name and the field name to select a field from the structure (Example 3.12)

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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 See Fig.3.14 It is called a far jump because it can jump to any

memory location for the next instruction

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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

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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 relative 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 beginning at memory location TABLE

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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)

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Mock quiz

1. Ada berapa jenis bus? Sebutkan!2. Tuliskan micro operation dari instruksi write!

(apa saja yang terjadi di bus, memori dan mikroprosesor)

3. Buatlah sebuah program lengkap, dengan model small, untuk mengkopikan sebuah nilai 1234H ke register AX, BX, CX dan DX

4. Apakah alamat akhir dari segment jika alamat awalnya 1234H?

5. Sebutkan pasangan segmen register dari ofset berikut ini:

IP, BX, BP, SP

addressing modes

Documents

register ax

mov bx di

addressing mode

mov al

immediate data

mov ecx

mov edx

arraybx di mov ax