Addressing modes

•Addressing modes is the way of representing the location of an operand •Three types•Data-Addressing Modes

8086 through 80286: register, immediate, direct, register indirect, base-plus-index, register relative,and base relative-plus-index

80386 and above: scaled-indexMOV( Move data) instruction –moves byte, word or double word

•Program Memory-Addressing ModesCALL and JUMP instructionsprogram relative, direct, and indirect

•Stack Memory-Addressing ModesPUSH and POP instructions

Data Addressing Mode

MOV (move data) instruction: transfer bytes/words/ doublewords of data between registers, or between register and memory

– MOVS: memory-to-memory transfer for strings

• opcode (operation code): MOV

• operands: AX and BX

8086-P4 Data-addressing Modes

1. Register Addressing• Transfer a copy of a byte or word or double word from the source

register or memory location to the destination register or memory location.

• Eg. MOV CX,DX MOV ECX,EDX• Most common form• Instructions must use registers of same size• None of the MOV instruction effect the flag bits.• Segment to segment register move is not allowed• Changing CS register with a mov instruction is not allowed.• While using mov instruction the source register content do not

change but destination changes.• The content of destination changes for all instruction except the

CMP and TEST instructions

Examples of register addressing

Operation of MOV BX,CX instruction

A sequence of assembled instructions

2. Immediate Addressing• Transfer the source-immediate byte or word or double word of data in to

the destination register.• Eg. MOV CX,22H MOV ECX,1233456H• Immediate implies the data immediately follow the hexadecimal opcode in

the memory• In some assemblers # symbol precedes immediate data• The letter H is appended with hexa decimal data

Examples of immediate addressing

Programming

• Assemblers

• Intel ASM

• Microsoft MSAM – MACRO assembler

• Borland TASM – Turbo assembler

Simple Program

Program Explanation• .MODEL Tiny - direct the assembler to assemble the program in to

a single code segment.• .code- start of the code segment• .startup - indicates the starting instruction in the program• .exit- causes the program to exit to dos• END – indicates the end of the program.• Tiny program always assembles as a command (.com) program.• The program places 0000H into the 16 bit registers AX, BX.CX

(immediate addressing), then copy the contents of AX to SI , DI and BP (register addressing).

• Each statement have 4 parts1) label – used to store a symbolic name for memory location2) opcode – holds the instruction3) operand – contains information used by opcode (can have beween 0 and 3 operands)4) comment – comment about the instruction, alwaya begin with a semicolon

• Assembler genetaes a .lst file which includes machine code, address and the program

3. Direct Data Addressing• Moves a byte or word or double word between a memory location and

register.• Does not support memory to memory transfer except for MOVS instruction• Eg mov cx, list• Two basic forms

1) Direct addressing – applies to a mov between memory location and AL,AX.EAX2) Displacement addressing – applies almost any instruction.

• The sddress is formed by adding the displacement to the DS or an alternate segment address.

• Direct addressing• 3 byte long instruction• Eg MOV AL,DATA• DTA is a symbolic name , actual location is 1234H• The effective address is formed by adding 1234 and

segment address.

• Displacement addressing• 4 byte long instruction• In 386 and above it can be upto 7 bytes long.

Program

Program explanation

• MODEL SMALL – allows one data segment and one code segment

• .DATA –informs data segment begins

• Assembles as .exe file

• .startup – loads data segment register with segment address

4. Register Indirect Addressing

Transfer a byte , word or double word between register and memory location addressed by index or base register

The index and base registers are BX,BP,SI,DI

Eg MOV AX,[BX] MOV AL,[ECX]

[] – denote indirect addressing

Operation of register indirect addressing

Examples

• Data segment is used by default with register indirect addressing

• If BP register address memory stack segment is used

• Special assembler directive –BYTE PTR, WORD PTR, DWORD PTR – indicates the size of memory data address by memory pointer ptr.

• Eg. Mov BYTE PTR[DI],10H• Used only with instructions that address memory

location through pointer or index register with immediate data.

An array (TABLE) containing 50 bytesthat are indirectly addressed through register BX

5. Base-Plus-Index Addressing

Transfer a byte , word or double word between register and memory location addressed by base register(BP or BX) plus index register (DI or SI).

Eg. Mov [BX+DI],CLThe base register often holds the beggining

location of the memory array while the index register holds the relative position of the element in the array

Locating data with base-plus-index addressing

Examples

Locating Array data using base plus index addressing

Example program

6. Register Relative Addressing

Moves a byte , word or double word between register and memory location addressed by an index or base register plus a displacement

• BX, DI, or SI addresses the DS segment and• BP addresses the SS segment• Displacement can be added or subtracted• Displacement can be an offset address.• MOV AX, [BX+1000H]

Operation

Examples

Addressing array data with register relative

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

Program addressing array data using program relative addressing

7. Base Relative-Plus-Index Addressing

Transfer a byte , word or double word between register and memory location addressed by a base register(BP or BX) and an index register (DI or SI) plus a displacement.

Eg. MOV AX, [BX+SI+100H]

Operation

Examples

Addressing Arrays with base relative plus indexed

8. Scaled-Index Addressing

• Uses two 32 bit registers to access memory the second register is multiplied by a scale factor

• The scaling factor can be 1X,2X,4X or 8X.

• Eg. MOV AX,[EDI+2*ECX]

Data structures• Used to specify how information is stored in the memory array.• STRUC assembly directive and ENDS statement is used to define

a structure.• Eg defining a structure

INFO STRUCNAMES DB 32 DUP (?)STREET DB 32 DUP (?)CITY DB 16 DUP (?)INFO ENDSNAME1 INFO <‘bob smith’ , ‘Cheruthuruthy’, thrissur’>The literals are surrounded with apostrophes and the entire field is

surrounded with <> symbol.Use a structure name and field name to select a field from the

structure.Eg. Name1.names

Program Memory-Addressing Modes

Used with JMP and CALL instructions

Three distinct forms are

• Direct Program Memory Addressing

• Relative Program Memory Addressing

• Indirect Program Memory Addressing

Direct Program Memory Addressing

• The instruction for direct program memory addressing store the address with the opcode

• Used by high level languages, BASIC – GOTO and GOSUB

• Eg . The address is stored following the opcode in the memory

• Inter segment jump means jump to any memory location with in the memory system.

• Direct jump is often called far jump• CALL instruction uses direct program addressing

Relative Program Memory Addressing

• Relative means relative to instruction pointer• Eg.

• A one byte (+127 and -128) displacement is used in short jump and 2 byte (+/- 32k) displacement is used for near jumps and calls

• These jumps are considered to be intra segment jumps• Means any where with in the current code segment.• Allows forward memory reference or reverse memory reference• 32 bit (+/- 2G)displacement is also allowed.

Indirect Program Memory AddressingUse any 16 bit register, any relative registerIf a 16 bit register is used then the jump is nearEg. JMP BXIf a relative register holds the address it is called indirect-indirect or double indirect jumpEg . Mov BX,4 JMP TABLE[BX]

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– Call instruction uses stack to hold return address– RET instruction remove the return address from the stack

• The stack memory is maintained by two registers: SP or ESP, and SS

• Always operates on word of data (double word with 386 and above)• When pushed –high order 8 bits placed in sp-1and low order 8 bits

placed in sp-2 location, sp is then decremented by 2.• Stack memory address is calculated by adding sp and (ss*10)• When poped the low order bits are removed form sp and high order

from sp+1 then sp is incremented by 2

Example Push and pop instructions

Example Stack program