Lecture 11

Lecture 11
Data Addressing mode continued..

Review: Addressing Modes
• Operand can be place either in one of the processor register or
in memory.
• There are different ways to get the operands.
• The way in which the operand is taken from register or memory
is named as addressing mode.
THE ADDRESSING MODE:
Specifies a rule for interpreting or modifying the address field of the
instruction before the operand is actually referenced.

Review: Addressing Modes in Microprocessor
8086/8088
Types of Addressing Modes:
(1) Stack-Memory Addressing Modes
(2) Data Addressing Modes
(3) Program-Memory Addressing Modes

2. Data Addressing Modes
1. Immediate Addressing Mode
2. Direct Addressing Mode
3. Indirect Addressing Mode
4. Register Addressing Mode
5. Register Indirect Addressing Mode
6. Implied Addressing Mode
7. Auto increment or Auto decrement Addressing
Mode
8. Relative Addressing Mode
9. Indexed Addressing Mode
10. Base Register Addressing Mode
 An addressing mode means the method by which an
operand can be specified in a register or a memory
location

In this mode the address field of the instruction gives
• the address where the effective address is stored in memory.
• Control unit fetches the instruction from the memory and uses
its address part to access memory again to read the effective
address.

So, what you get it?
• The instruction specifies the indirect address where the
effective address of the operand is placed.
• Or, in other words: The memory address is specified
where the actual address of operand is placed.
MOV A, ([2802h])
Means: Move A ← ([2802])
It moves the data from memory location specified by the location 2802 toA.

Mechanism:
MOV A, ([2802h] ) A ← ([2802])
It moves the data from memory location specified by the
location 2802 to A.
Before After
2807
2806 FF
2805
2804
2803 06
2802 28
2801
2800
A 2807
2806 FF
2805
2804
2803 06
2802 28
2801
2800
A FF
A ← ([2802]) A ← FF
2 bytes
read
processor

4:RegisterAddressingMode
• When the address field specifies a processor register,
the instruction is said to be in register-mode.
• Instruction gets its source data from a register.
• Data resulting from the operation is storedin another register.
Data length depends on register beingused.
– 8-bit registers:AH,AL,BH,BL,CH,CL,DH, DL.
– 16-bit registers:AX,BX,CX,DX,SP,BP,SI,DI.
– 32-bit registers: EAX,EBX,ECX,EDX,ESP,EBP,EDI, ESI.
– 64-bit registers: RAX,RBX,RCX,RDX,RSP,RBP,RDI, RSI,
Advantage:
The address field of the instruction uses fewer bits to select a register than would have been required to specify a
memory address directly.

Simple Example:
• The operand is specified with in one of the processor register.
• Instruction specifies the register in which the operand is
stored.
Move
CX ← AX Here Ax has the operand move to
another register
MOV CX , AX
Add
Here Bx has the operand by default added into AXADD BX AX ← AX+ BX

Some more points:
Both the operands are in registers
Memory is not accessed when this addressing mode is executed
Limited number of registers
Very small address field is needed to address registers.
Shorter instructions

SomemoreExamples:
• – MOVAX,BX ;Copy the 16-bit content of BXto
AX
– MOVAL,BL ;Copy the 8-bit content of BLtoAL
– MOVSI,DI ;Copy DIinto SI
– MOV DS,AX ;CopyAXintoDS
Note
that the instructionmust useregisters of the
samesize.
– Cannot mix between 8-bit and 16-bitregisters.
– Will result in an error whenassembled.
•

Register Addressing Exceptions

5:Register Indirect Addressing Mode
Transfers a byte or word between a register and a memory location
addressed by an index or base
register
The address of the memory location where the operand resides is held
by a register
The registers used for this purpose are SI, DI, and BX
They must be combined with DS in order to generate the 20-bit physical
address.
 Theregisters must be specified using abracket [ ] or ().
In this mode the instruction specifies a register in the CPU
whose contents give the address of the operand in the memory. In other
words, the selected register contains the address of
the operand rather than the operand itself.

Ali Asghar Manjotho, LecturerCSE-MUET
Simple Example
• The instruction specifies the register in which the
memory address of operand is placed.
• It do not specify the operand itself but its location with in
the memory where operand is placed.
Move
Ax ← [[BH][BL]]MOV Ax , (BX)
It moves the data from memory location specified by BX register pair to
Ax.

Mechanism
2807
2806
2805 A9
2804
2803
2802
2801
2800
BH 28
BL 05
2807
2806
2805 A9
2804
2803
2802
2801
2800
AX A9
MOV Ax , (BX) Ax ← ([BH][BL])
It moves the data from memory location specified by HL register pair to
Ax.
Before After
Ax
AX ← (2805) AX ← A9
BH 28
BL 05

Some more points
EA of operand(s) is/are specified in register.
Physical address is computed using segment register and EA.
Data in the physical address is an operand

Example 1: with physical address consideration
Example:
MOV AX, [BX] ; moves into AX the contents of
the memory location pointed
to by DS:BX, 1000:1234
The physical address is calculated as
1000x10+1234=11234H
The same rules apply when using register SI or DI.
Example:
MOV CL, [SI]
MOV [DI],AH
; move contents of DS:SI into CL
; move contents of AH into DS:DI

Example 2 with physical address consideration
Example 1 : MOV AX, [BX] given that DS = 5004, [BX] = 0020, calculatecontent
of registerAX
Solution:DS = 5004, [BX] = 0020,
Physical Address=50040+0020=50060.
50060 contain 0015H
0015H is moved to memoryAX
0000
0002
.
.
.
‘
0020 15
0021 00
50060
Little Endian

Example 3: with physical address consideration
 Example:
 Assume that DS = 1120, SI = 2498, and AX = 17FE Show the contents
of memory locations after the execution of
MOV [SI], AX ; move contents of AX into DS:SI
 Solution:
The contents of AX are moved into memory locations with logical
address DS:SI and DS:SI + 1;
The physical address starts at DS (shifted left) + SI = 11200+2498
13698. According to the little endian convention,
Low address 13698H contains FE, the low byte,
High address 13699H will contain 17, the high byte.

5: Register Indirect Addressing Mode Exceptions

Question problem
1st instruc
500
Next Instruction
950
450
700
800
900
325
300
Address
200
201
202
Memory
300
399
400
500
600
702
PC 200
AC
800
PC = Program Counter
AC/AX = Accumulator
BX= base register
DX=data register
Find out the effective address of operand
and accumulator & operand value by
addressing modes and also identify their
modes.
Load Ax , ([500])
Load Ax, BX
Load Ax, (DX)
Homework:
Load Ax, (([500]))
Load AX,DX
Load AX, ((DX))
Load AX, (((DX)))
BX 702
DX 400

Example problem
1st instruct
500
Next Instruction
950
450
700
800
900
325
300
Address
200
201
202
Memory
300
399
400
500
600
702
PC 200
AC
800
• Instruction contains the address 500.
• Address at 500 is 800.
• So effective address of operand is 800.
• The data stored at 800 is 300.
Load Ax , ([500])
Effective Address = 800
Operand = 300
AC 300
BX 702
DX 400

Example problem
1st instruc
500
Next Instruction
950
450
700
800
900
325
300
Address
200
201
202
Memory
300
399
400
500
600
702
PC 200
BX 702
DX 400
AX
800
4. Register Addressing Mode
• Register BX contains 702.
• As operand is in register so no any memory
location.
Effective Address = Nil
Operand = 702
AX 702
Load Ax, BX

Example problem
Address
200
201
202
Memory
300
399
400
Load to AC Mode
Address = 500
Next Instruction
950
450
500
800
900
325
300
500
600
702
PC 200
BX 702
DX 400
AX
800
5. Register Indirect Addressing
Mode
• Register DX contains 400.
• So effective address of operand is
400.
• The data stored at 400 is 500.
Effective Address = 400
Operand = 500
AC/AX 500
Load Ax, (DX)

Example problem
Addressing Mode Effective
address
Accumulator Operand
Direct Addressing Mode
Immediate Addressing Mode null
500
500
800
500
Indirect Addressing mode 800 300 300
Register Addressing Mode Null 702 702
Register Indirect Addressing
Mode
400 500 500

Lecture 11

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Lecture 11