EEM332 Mikroişlemciler: 8086 Adresleme Kipleri

Bu yazı, Başkent Üniversitesi EEM332 Mikroişlemciler ders arşivindeki ilgili notları ve uygulamaları bir araya getiriyor (2005–2015). Doğrudan, dolaylı, taban ve indisli adresleme; segment ve ofsetten fiziksel adresin bulunması. Belgelerin PDF arşiv kopyaları ve varsa kaynak kodları yazının sonunda ZIP olarak indirilebilir.

CPU03_2013

EEM332 Lecture3

Adressing modes in 8086

Addressing Modes:

The CPU can access operands (data) in various ways, called addressing modes. In 80x86 there are 7 addressing modes. The number of addressing modes is determined when the microprocessor is designed & cannot be changed The x86 provides seven distinct addressing

• 1 - Register – 5 - Based relative

• 2 - Immediate – 6 - Indexed relative

• 3 - Direct – 7 - Based indexed relative

• 4 - Register indirect

Register addressing mode involves use of registers to hold the data to be manipulated. Memory is not accessed, so it is relatively fast.

Memory is not accessed, so it is relatively fast.

In immediate addressing mode, as the name implies, when the instruction

is assembled, the operand comes immediately after the opcode. The source operand is a constant.

Note the bracket around the

address

In register indirect addressing mode,

the address of the memory location where the operand resides is held by a register.

Notice that BX is in brackets. The physical address is calculated by shifting DS left one hex position

and adding BX to it

MOV CX, [BX]

Example

CX = destination — register [BX] = source — memory location

1. Generate physical address for memory location

from contents of BX and DS (shifted by 4) — put on address bus

2. Retrieve contents of word in memory — put on data bus

3. Write data into CX

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

C9 551A5

520A DS address bus

MOV CX, [BX]                                           FFFFF

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

551A4 551A5

C9

520A DS

address
bus

520A0 = DS shifted by 4

+3104 = BX

551A4 = physical address FFFFF

memory 8086 00000

3104 BX

data bus

C927 CX

C927

27 551A4

551A4 551A5

C9

520A DS address bus

FFFFF

4. Register indirect addressing mode: Examples

• The address of the memory location where the operand resides is held by a register.

• SI, DI and BX registers are used as the pointers to hold the offset addresses.

• They must be combined with DS to generate the 20-bit physical address

; moves into AL the contents of the memory

location pointed to by DS:BX

; move contents of DS:SI into CL
; move the contents of AH into DS:DI

BX contains 0400h Offset 0400h contains ABCDh SI contains 0500h Offset 0500h contains 1000h DI contains 0600h Offset 0600h contains 0045h

INSTRUCTIONS: RESULTS

1. MOV AX,BX AX: 0400h

2. MOV AX,[BX] AX: ABCDh

3. MOV AX,SI AX: 0500h

4. MOV AX,[SI] AX: 1000h

Direct versus indirect addressing mode

address of the data in memory comes immediately after the instruction operand is a

constant The address is the offset address. The offset address is put in a rectangular bracket Ex: MOV AX,[SI] or SUB DX, [BX] indirect addressing

Direct addressing mode

EX

move contents of DS:2400H into DL

5. Based relative addressing mode:

BX and BP are known as the base registers. In this mode base registers as well as a displacement value are used to calculate the effective address. The default segments used for the calculation of Physical address (PA) are DS for BX,

and SS for BP.

opcode mod r/m displacement

BX

+

BP

effective address

Example: MOV [BX]+1234H, AL                        Displacement

Element n-1

Data Structure +

Element 1 Base Register Element 0

6. Indexed relative addressing mode:

Indexed relative addressing mode works the same as the based relative

addressing mode. Except the registers DI and SI holds the offset address.

Example: MOV             AL, [SI]+2000H
01000
01001

IP 01002

01003

CS 01004

Index Register Element n-1

DS

SS

ES Array of data

FS +

GS

Element 1 AX Displacement BX Element 0

CX

DX

SP

BP

SI

DI

Adresing modes

7. Based Indexed addressing mode:

The combination of the based and indexed addressing modes. One base register and one index register are used.

opcode mod r/m displacement

DI

+

SI

effective address

Examples: Based and Index Addressing Modes

INSTRUCTION MEANING

SUB CX,[DI+4]        Subtract from CX the word located in the data segment at an

offset designated by the contents of DI + 4

MOV AH,[4+DI]        Move to AH the byte located in the data segment at an offset

designated by the contents of 4 + DI

CMP AX,[BP]+4        Compare the contents of AX with the word on the STACK

that is located at SS + the contents of BP + 4

CMP AX,4+[BP]        Compare the contents of AX with the word on the STACK

that is located at SS + 4 + the contents of BP

ADD CX,NUMBERS[SI]   Add to CX the word in memory located in the array named

NUMBERS that is offset from the beginning of the array by the value in SI

Accessing the STACK

• When BP is used in register indirect mode, it is understood that

the segment address is in SS

• Thus BP can be used to access the STACK without changing the

STACK or the Stack Pointer (SP)

• Example:

MOV     BP,SP             ; Set BP to the value in the stack pointer
MOV     AX,[BP]           ; move the top word on the stack to AX
MOV     BX,[BP+2]         ; move the second word from the top to BX
MOV     CX,[BP+4]         ; move the third word from the top to CX

Moves the 1st (top three) values in the stack to AX, BX, and CX

respectively without changing SP

cpu03_08

Addressing Modes:

The CPU can access operands (data) in various ways, called addressing modes. In 80x86 there are 7

addressing modes

1. register

2. immediate

3. direct

4. register indirect

5. based relative

6. indexed relative

7. based indexed relative

The default segment register is DS

Direct addressing mode

address of the data in memory comes immediately after the instruction operand is a

constant The address is the offset address. The offset address is put in a rectangular bracket Ex: MOV AX,[SI] or SUB DX, [BX]

EX

move contents of DS:2400H into DL

• Direct Addressing Mode

• PA = Segment Base : Direct Address

• MOV CX, [1234H]

Instruction (program code)

01000 8B

01001 0E

0000 IP 01002 34

01003 12

0100 CS XX

01004

0200 DS

SS

ES

FS

GS

AX Data

BX

XXXX CX 03234 ED

DX 03235 BE

SP

BP

SI BEED

DI

4. Register indirect addressing mode:

• The address of the memory location where the operand resides is held by a register.

• SI, DI and BX registers are used as the pointers to hold the offset addresses.

• They must be combined with DS to generate the 20-bit physical address

; moves into AL the contents of the memory

location pointed to by DS:BX

; move contents of DS:SI into CL
; move the contents of AH into DS:DI

BX contains 0400h Offset 0400h contains ABCDh SI contains 0500h Offset 0500h contains 1000h DI contains 0600h Offset 0600h contains 0045h

INSTRUCTIONS: RESULTS

1. MOV AX,BX AX: 0400h

2. MOV AX,[BX] AX: ABCDh

3. MOV AX,SI AX: 0500h

4. MOV AX,[SI] AX: 1000h

MOV CX, [BX]

CX = destination — register [BX] = source — memory location

1. Generate physical address for memory location

from contents of BX and DS (shifted by 4) — put on address bus

2. Retrieve contents of word in memory — put on

data bus

3. Write data into CX

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

C9 551A5

520A DS address bus

MOV CX, [BX]                               FFFFF

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

551A4 551A5

C9

520A DS address bus 520A0 = DS shifted by 4

+3104 = BX

551A4 = physical address FFFFF

memory 8086 00000

3104 BX

data bus

C927 CX C927

27 551A4

551A4 551A5

C9

520A DS address bus

FFFFF

5. Based relative addressing mode:

BX and BP are known as the base registers. In this mode base registers as well as a displacement value are used to calculate the effective address. The default segments used for the calculation of Physical address (PA) are DS for BX,

and SS for BP.

opcode mod r/m displacement

BX

+

BP

effective address

Example: MOV [BX]+1234H, AL

01000 88 01001 07 Element n-1 0000 IP 01002 34 Displacement 01003 12

0100 CS XX

01004 0200 DS Data Structure

SS +

ES

FS Element 1 GS Base Register Element 0

ED AX

1000 BX

CX 04234

DX 04235

SP

BP

1234 SI

DI

6. Indexed relative addressing mode:

Indexed relative addressing mode works the same as the based relative

addressing mode. Except the registers DI and SI holds the offset address.

Example: MOV                    AL, [SI]+2000H
01000
01001

IP 01002

01003

CS

01004 Element n-1 DS Index Register

SS

ES Array of data

FS +

GS

Element 1 AX Displacement BX Element 0

CX

DX

SP

BP

SI

DI

Adresing modes

7. Based Indexed addressing mode:

The combination of the based and indexed addressing modes. One base register and one index register are used.

opcode mod r/m displacement

DI

+

SI

effective address

Examples: Based and Index Addressing Modes

INSTRUCTION MEANING

SUB CX,[DI+4]           Subtract from CX the word located in the data segment at an

offset designated by the contents of DI + 4

MOV AH,[4+DI]           Move to AH the byte located in the data segment at an offset

designated by the contents of 4 + DI

CMP AX,[BP]+4           Compare the contents of AX with the word on the STACK

that is located at SS + the contents of BP + 4

CMP AX,4+[BP]           Compare the contents of AX with the word on the STACK

that is located at SS + 4 + the contents of BP

ADD CX,NUMBERS[SI]      Add to CX the word in memory located in the array named

NUMBERS that is offset from the beginning of the array by the value in SI

Accessing the STACK

• When BP is used in register indirect mode, it is understood that

the segment address is in SS

• Thus BP can be used to access the STACK without changing the

STACK or the Stack Pointer (SP)

• Example:

MOV      BP,SP             ; Set BP to the value in the stack pointer
MOV      AX,[BP]           ; move the top word on the stack to AX
MOV      BX,[BP+2]         ; move the second word from the top to BX
MOV      CX,[BP+4]         ; move the third word from the top to CX

Moves the 1st (top three) values in the stack to AX, BX, and CX

respectively without changing SP

Cp04_06

Addressing Modes:

Immediate operand addressing mode Register operand addressing mode Memory operand addressing mode Direct addressing mode Register indirect addressing mode Based addressing mode Indexed addressing mode Based-indexed addressing mode

The default segment register is DS

• Direct Addressing Mode

• PA = Segment Base : Direct Address

• MOV CX, [1234H]

Instruction (program code)

01000 8B

01001 0E

0000 IP 01002 34

01003 12

0100 CS XX

01004

0200 DS

SS

ES

FS

GS

AX Data

BX

XXXX CX 03234 ED

DX 03235 BE

SP

BP

SI BEED

DI

• Effective address is found in either the BX, BP, SI, DI register

• SUB DX, [BX]

Examples

ASSUMPTION: ASSUME all specified offsets are in the DATA Segment

BX contains 0400h Offset 0400h contains ABCDh SI contains 0500h Offset 0500h contains 1000h DI contains 0600h Offset 0600h contains 0045h

INSTRUCTIONS: RESULTS

1. MOV AX,BX AX: 0400h

2. MOV AX,[BX] AX: ABCDh

3. MOV AX,SI AX: 0500h

4. MOV AX,[SI] AX: 1000h

mov      al,      [bp]     ;al gets 8 bits at SS:BP

mov      ah,      [bx]     ;ah gets 8 bits at DS:BX

mov      ax,      [di]     ;ax gets 16 bits at DS:SI

mov      eax,     [si]     ;eax gets 32 bits at DS:SI

MOV CX, [BX]

CX = destination — register [BX] = source — memory location

1. Generate physical address for memory location

from contents of BX and DS (shifted by 4) — put on address bus

2. Retrieve contents of word in memory — put on

data bus

3. Write data into CX

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

C9 551A5

520A DS address bus

MOV CX, [BX]                                       FFFFF

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

551A4 551A5

C9

520A DS address bus 520A0 = DS shifted by 4

+3104 = BX

551A4 = physical address FFFFF

memory 8086 00000

3104 BX

data bus

C927 CX C927

27 551A4

551A4 551A5

C9

520A DS address bus

FFFFF

• Based Addressing Mode

• PA = Segment Base : {BX or BP} + {8-bit or 16-bit

displacement}

• Base register : the beginning of a data structure

• See Fig 3.16 (b) in page 74

• Example: MOV [BX]+1234H, AL

01000 88 01001 07

0000 IP 01002 34

01003 12

0100 CS XX

01004

0200 DS

SS

ES Displacement Element n-1

FS

GS

ED AX Data Structure

1000 BX +

CX 04234

DX Element 1 04235 Base Register SP Element 0

BP

1234 SI

DI

Based Addressing

mov    al,      [bp+2]                      ;al gets 8 bits at SS:BP+2

mov    ah,      [bx-4]                      ;ah gets 8 bits at DS:BX-4

opcode mod r/m displacement

BX

+

BP

effective address

• Indexed Addressing Mode

• PA = Segment Base : {SI, DI}+{8-bit or 16-bit

displacement}

• Displacement : the starting address of an array;

Index: selects the specific element in the array

• Example: MOV AL, [SI]+2000H

01000 01001

IP 01002

01003

CS

01004

DS

SS

ES

FS

GS

AX Index Register Element n-1

BX

CX

DX Array of data +

SP

BP Element 1 SI Displacement Element 0

DI

Indexed Indirect Addressing

mov    ax,       [si+1000h]       ;ax gets 16 bits at DS:SI+1000h

mov    eax,      [si+300h]        ;eax gets 32 bits at DS:SI+300h

Mov    [di+100h],      al         ;DS:DI+100h gets 8 bits in al

opcode mod r/m displacement

DI

+

SI

effective address

• Based-Indexed Addressing Mode

• PA= Seg Base: {BX, BP}+{SI,DI}+{8-bit or

16-bit displacement}

• to access complex data structures

• See fig 3.20 in page 80

• Example: MOV AH, [BX][SI]+1234H

opcode : 8A 44 34 12

Based Indexed Indirect Addressing

mov       ax,     [bp+di]             ;ax gets 16 bits at SS:BP+DI

mov       ax,     [di+bp]             ;ax gets 16 bits at DS:BP+DI

mov       eax,    [bx+si+10h]         ;eax gets 32 bits at DS:BX+SI+10h

mov       cx,     [bp+si-7]           ;cx gets 16 bits at SS:BP+SI-7

opcode mod r/m displacement

DI

+

SI

BX

+

BP

effective address

Addressing Mode Examples
mov al,     bl           ;8-bit register addressing
mov di,     bp           ;16-bit register addressing                           Register
mov eax,    eax          ;32-bit register addressing
mov al,     12           ;8-bit immediate, al<-0ch
mov cx,     faceh        ;16-bit immediate, cx<-64,206                         Immediate
mov ebx,    2h           ;32-bit immediate, ebx<-00000002h
mov al,     LIST         ;al<-8 bits stored at label LIST
mov ch,     DATA         ;ch<-8 bits stored at label DATA                      Direct
mov ds,     DATA2        ;ds<-16 bits stored at label DATA2
mov al,     [bp]         ;al<-8 bits stored at SS:BP
mov ah,     [bx]         ;ah<-8 bits stored at DS:BX
mov ax,     [bp]         ;ax<-16 bits stored at SS:BP
mov eax,    [bx]         ;eax<-32 bits stored at DS:BX
mov al,     [bp+2]       ;al<-8 bits stored at SS:(BP+2)
mov ax,     [bx-4]       ;ax<-16 bits stored at DS:(BX-4)
Based
mov al,     LIST[bp]     ;al<-8 bits stored at SS:(BP+LIST)
mov bx,     LIST[bx]     ;bx<-16 bits stored at DS:(BX+LIST)
mov al,     LIST[bp+2] ;al<-8 bits stored at SS:(BP+2+LIST)
mov ax,     LIST[bx-12h] ;ax<-16 bits stored at DS:(BX-

12+LIST)

More Addressing Mode Examples

mov al,     [si]         ;al<-8 bits stored at DS:SI
mov ah,     [di]         ;ah<-8 bits stored at DS:DI
mov ax,     [si]         ;ax<-16 bits stored at DS:SI
mov eax,    [di]         ;eax<-32 bits stored at DS:DI
mov ax,     es:[di]      ;ax<-16 bits stored at ES:DI         Indexed
mov al,     [si+2]       ;al<-8 bits stored at DS:(SI+2)
mov ax,     [di-4]       ;ax<-16 bits stored at DS:(DI-4)
mov al,     LIST[si]     ;al<-8 bits stored at DS:(SI+LIST)
mov bx,     LIST[di]     ;bx<-16 bits stored at DS:(DI+LIST)
mov al,     LIST[si+2] ;al<-8 bits stored at DS:(SI+2+LIST)
mov ax,     LIST[di-12h] ;ax<-16 bits stored at DS:(DI-18+LIST)
mov al,     [bp+di]      ;al<-8 bits from SS:(BP+DI)
mov ah,     ds:[bp+si] ;ah<-8 bits from DS:(BP+SI)
mov ax,     [bx+si]      ;ax<-16 bits from DS:(BX+SI)
Based
mov eax,    es:[bx+di] ;eax<-32 bits from ES:(BX+DI)            Indexed
mov al,     LIST[bp+di] ;al<-8 bits from SS:(BP+DI+LIST)
mov ax,     LIST[bx+si] ;ax<-16 bits from DS:(BX+SI+LIST)
mov al,     LIST[bp+di-10h]     ;al<-8 bits from SS:(BP+DI-16+LIST)
mov ax,     LIST[bx+si+1AFH]    ;ax<-16 bits from DS:(BX+SI+431+LIST)

Examples: Based and Index Addressing Modes

INSTRUCTION MEANING

SUB CX,[DI+4]            Subtract from CX the word located in the data segment at an

offset designated by the contents of DI + 4

MOV AH,[4+DI]            Move to AH the byte located in the data segment at an offset

designated by the contents of 4 + DI

CMP AX,[BP]+4            Compare the contents of AX with the word on the STACK

that is located at SS + the contents of BP + 4

CMP AX,4+[BP]            Compare the contents of AX with the word on the STACK

that is located at SS + 4 + the contents of BP

ADD CX,NUMBERS[SI]       Add to CX the word in memory located in the array named

NUMBERS that is offset from the beginning of the array by the value in SI

Accessing the STACK

• When BP is used in register indirect mode, it is understood that

the segment address is in SS

• Thus BP can be used to access the STACK without changing the

STACK or the Stack Pointer (SP)

• Example:

MOV     BP,SP            ; Set BP to the value in the stack pointer
MOV     AX,[BP]          ; move the top word on the stack to AX
MOV     BX,[BP+2]        ; move the second word from the top to BX
MOV     CX,[BP+4]        ; move the third word from the top to CX

Moves the 1st (top three) values in the stack to AX, BX, and CX

respectively without changing SP

Cp04

• Direct Addressing Mode

• PA = Segment Base : Direct Address

• MOV CX, [1234H]

Instruction (program code)

01000 8B

01001 0E

0000 IP 01002 34

01003 12

0100 CS XX

01004

0200 DS

SS

ES

FS

GS

AX Data

BX

XXXX CX 03234 ED

DX 03235 BE

SP

BP

SI BEED

DI

• Effective address is found in either the BX, BP, SI, DI register

• SUB DX, [BX]

Examples

ASSUMPTION: ASSUME all specified offsets are in the DATA Segment

BX contains 0400h Offset 0400h contains ABCDh SI contains 0500h Offset 0500h contains 1000h DI contains 0600h Offset 0600h contains 0045h

INSTRUCTIONS: RESULTS

1. MOV AX,BX AX: 0400h

2. MOV AX,[BX] AX: ABCDh

3. MOV AX,SI AX: 0500h

4. MOV AX,[SI] AX: 1000h

Register Indirect

Addressing
mov   al,   [bp]   ;al gets 8 bits at SS:BP

mov   ah,   [bx]   ;ah gets 8 bits at DS:BX

mov   ax,   [di]   ;ax gets 16 bits at DS:SI

mov   eax, [si]    ;eax gets 32 bits at DS:SI

• Register Indirect Addressing Mode

• PA = Segment Base : Indirect Address {BX,BP,SI,DI}

• example : MOV AX, [SI]

01000 8B

01001 04

0000 IP 01002 XX

01003 XX

0100 CS XX

01004

0200 DS

SS

ES

FS

GS

XXXX AX

BX

CX 03234 ED

DX 03235 BE

SP

BP

1234 SI BEED

DI

MOV CX, [BX]

CX = destination — register [BX] = source — memory location

1. Generate physical address for memory location

from contents of BX and DS (shifted by 4) — put on address bus

2. Retrieve contents of word in memory — put on

data bus

3. Write data into CX

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

C9 551A5

520A DS address bus

MOV CX, [BX]                            FFFFF

memory 8086 00000

3104 BX

data bus

02C5 CX

27 551A4

551A4 551A5

C9

520A DS address bus 520A0 = DS shifted by 4

+3104 = BX

551A4 = physical address FFFFF

memory 8086 00000

3104 BX

data bus

C927 CX C927

27 551A4

551A4 551A5

C9

520A DS address bus

FFFFF

• Offsets of data is found by adding some value to the base

register

MOV AX, [BX-2]

• Based Addressing Mode

• PA = Segment Base : {BX or BP} + {8-bit or 16-bit displacement}

• Base register : the beginning of a data structure

• See Fig 3.16 (b) in page 74

• Example: MOV [BX]+1234H, AL

01000 88 01001 07

0000 IP 01002 34

01003 12

0100 CS XX

01004

0200 DS

SS

ES

FS

Displacement Element n-1

GS

ED AX

1000 BX Data Structure +

CX 04234

DX 04235 Element 1 SP Base Register Element 0

BP

1234 SI

DI

• Indexed Addressing Mode

• PA = Segment Base : {SI, DI}+{8-bit or 16-bit displacement}

• Displacement : the starting address of an array; Index: selects the

specific element in the array

• Example: MOV AL, [SI]+2000H

01000 01001

IP 01002

01003

CS

01004

DS

SS

ES

FS

GS

AX Index Register Element n-1

BX

CX

DX Array of data +

SP

BP Element 1 SI Displacement Element 0

DI

• Based-Indexed Addressing Mode

• PA= Seg Base: {BX, BP}+{SI,DI}+{8-bit or 16-bit

displacement}

• to access complex data structures

• See fig 3.20 in page 80

• Example: MOV AH, [BX][SI]+1234H

opcode : 8A 44 34 12

Based Indirect Addressing

mov   al,       [bp+2]             ;al gets 8 bits at SS:BP+2

mov   ah,       [bx-4]             ;ah gets 8 bits at DS:BX-4

opcode mod r/m displacement

BX

+

BP

effective address

Indexed Indirect Addressing

mov   ax,       [si+1000h]       ;ax gets 16 bits at DS:SI+1000h

mov   eax,      [si+300h]        ;eax gets 32 bits at DS:SI+300h

Mov   [di+100h],      al         ;DS:DI+100h gets 8 bits in al

opcode mod r/m displacement

DI

+

SI

effective address

Based Indexed Indirect Addressing

mov    ax,    [bp+di]             ;ax gets 16 bits at SS:BP+DI

mov    ax,    [di+bp]             ;ax gets 16 bits at DS:BP+DI

mov    eax,   [bx+si+10h]         ;eax gets 32 bits at DS:BX+SI+10h

mov    cx,    [bp+si-7]           ;cx gets 16 bits at SS:BP+SI-7

opcode mod r/m displacement

DI

+

SI

BX

+

BP

effective address

Addressing Mode Examples
mov al,    bl           ;8-bit register addressing
mov di,    bp           ;16-bit register addressing             Register
mov eax,   eax          ;32-bit register addressing
mov al,    12           ;8-bit immediate, al<-0ch
mov cx,    faceh        ;16-bit immediate, cx<-64,206           Immediate
mov ebx,   2h           ;32-bit immediate, ebx<-00000002h
mov al,    LIST         ;al<-8 bits stored at label LIST
mov ch,    DATA         ;ch<-8 bits stored at label DATA        Direct
mov ds,    DATA2        ;ds<-16 bits stored at label DATA2
mov al,    [bp]         ;al<-8 bits stored at SS:BP
mov ah,    [bx]         ;ah<-8 bits stored at DS:BX
mov ax,    [bp]         ;ax<-16 bits stored at SS:BP
mov eax,   [bx]         ;eax<-32 bits stored at DS:BX
mov al,    [bp+2]       ;al<-8 bits stored at SS:(BP+2)
mov ax,    [bx-4]       ;ax<-16 bits stored at DS:(BX-4)
Based
mov al,    LIST[bp]     ;al<-8 bits stored at SS:(BP+LIST)
mov bx,    LIST[bx]     ;bx<-16 bits stored at DS:(BX+LIST)
mov al,    LIST[bp+2] ;al<-8 bits stored at SS:(BP+2+LIST)
mov ax,    LIST[bx-12h] ;ax<-16 bits stored at DS:(BX-

12+LIST)

More Addressing Mode Examples

mov al,     [si]         ;al<-8 bits stored at DS:SI
mov ah,     [di]         ;ah<-8 bits stored at DS:DI
mov ax,     [si]         ;ax<-16 bits stored at DS:SI
mov eax,    [di]         ;eax<-32 bits stored at DS:DI
mov ax,     es:[di]      ;ax<-16 bits stored at ES:DI         Indexed
mov al,     [si+2]       ;al<-8 bits stored at DS:(SI+2)
mov ax,     [di-4]       ;ax<-16 bits stored at DS:(DI-4)
mov al,     LIST[si]     ;al<-8 bits stored at DS:(SI+LIST)
mov bx,     LIST[di]     ;bx<-16 bits stored at DS:(DI+LIST)
mov al,     LIST[si+2] ;al<-8 bits stored at DS:(SI+2+LIST)
mov ax,     LIST[di-12h] ;ax<-16 bits stored at DS:(DI-18+LIST)
mov al,     [bp+di]      ;al<-8 bits from SS:(BP+DI)
mov ah,     ds:[bp+si] ;ah<-8 bits from DS:(BP+SI)
mov ax,     [bx+si]      ;ax<-16 bits from DS:(BX+SI)          Based
mov eax,    es:[bx+di] ;eax<-32 bits from ES:(BX+DI)           Indexed
mov al,     LIST[bp+di] ;al<-8 bits from SS:(BP+DI+LIST)
mov ax,     LIST[bx+si] ;ax<-16 bits from DS:(BX+SI+LIST)
mov al,     LIST[bp+di-10h]     ;al<-8 bits from SS:(BP+DI-16+LIST)
mov ax,     LIST[bx+si+1AFH]    ;ax<-16 bits from DS:(BX+SI+431+LIST)

Examples: Based and Index Addressing Modes

INSTRUCTION MEANING

SUB CX,[DI+4]        Subtract from CX the word located in the data segment at an

offset designated by the contents of DI + 4

MOV AH,[4+DI]        Move to AH the byte located in the data segment at an offset

designated by the contents of 4 + DI

CMP AX,[BP]+4        Compare the contents of AX with the word on the STACK

that is located at SS + the contents of BP + 4

CMP AX,4+[BP]        Compare the contents of AX with the word on the STACK

that is located at SS + 4 + the contents of BP

ADD CX,NUMBERS[SI]   Add to CX the word in memory located in the array named

NUMBERS that is offset from the beginning of the array by the value in SI

Accessing the STACK

• When BP is used in register indirect mode, it is understood that

the segment address is in SS

• Thus BP can be used to access the STACK without changing the

STACK or the Stack Pointer (SP)

• Example:

MOV     BP,SP            ; Set BP to the value in the stack pointer
MOV     AX,[BP]          ; move the top word on the stack to AX
MOV     BX,[BP+2]        ; move the second word from the top to BX
MOV     CX,[BP+4]        ; move the third word from the top to CX

Moves the 1st (top three) values in the stack to AX, BX, and CX

respectively without changing SP