The document discusses the execution unit and addressing modes of the 8086 microprocessor. It describes the various registers in the execution unit, including the flag register which contains status and control flags. It also details the different types of registers - pointer, index, and their uses. Finally, it explains the seven addressing modes of 8086 including register, immediate, direct, register indirect, based, indexed, and based indexed addressing modes along with examples.

1. Execution Unit &
Addressing Modes of
8086
Dr.Ajay Singh Raghuvanshi
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2. VARIOUS REGISTERS IN EU
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3. FLAG REGISTER
 It is a 16-bit status register within the EU of
8086.
 There are nine flags in 8086.
 Six of these are status flags and three are
control flags.
 Status Flags are used to indicate some
condition produced by an instruction
 Control Flags are used to control certain
operation of the processor
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4. 8086 FLAG REGISTER
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5. STATUS FLAGS
 CF- Carry Flag It is set if carry out of MSB.
 PF- Parity Flag It is set if result has even parity.
 AF- Auxiliary Flag Used for BCD.
 ZF- Zero Flag It is set if result is 0.
 SF- Sign Flag It is set if result is negative.
 OF- Overflow Flag It is set if signed result is
out of the range.
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6. CONTROL FLAGS
 TF-Single step Trap Flag It is to be set if
single step mode (debugging) is needed.
 IF- Interrupt Flag It is to be set, for
allowing or prohibiting interruption
request.
 DF- Direction Flag If set, string
instruction automatically decrements the
address (string data transfers proceed
from high address to the low address.
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7. POINTER AND INDEX
REGISTERS
 There are two pointer and two index
registers in the EU of 8086.
 These registers are used to store the
offset addresses of memory locations
relative to the segment registers.
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8. POINTER REGISTER
Two pointer registers are :
 Stack pointer – The value in the SP
always represents the offset of the next
stack location that can be accessed.
 Base pointer – It also represents an offset
relative to SS register but is employed in
the based addressing mode of 8086.
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9. INDEX REGISTER
Two index registers are:
 Source index (SI)- It is used to store an
offset address for source operand.
 Destination index (DI)- It is used for
storage of an offset address for the
destination operand.
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10. ADDRESSING MODES OF
8086
 The addressing modes available in Intel 8086
are:
1. Register Addressing
2. Immediate Addressing
3. Direct Addressing
4. Register Indirect Addressing
5. Based Relative Addressing
6. Indexed Relative Addressing
7. Based Indexed Relative Addressing
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11. The MOV instruction
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MOV AX , BX
SourceDestination

12. Register Addressing Mode
 With the Register Addressing mode the operand to
be accessed is specified as residing in an internal
register of the 8086.
 Examples
 Both the source and the destination operands
have been specified as the contents of internal
registers of the 8086
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Assembly Language Size Operation
MOV AL, BL 8-bits Copies BL into AL
MOV CH, CL 8-bits Copies CL into CH
MOV AX, CX 16-bits Copies CX into AX
MOV SP, BP 16-bits Copies BP into SP
MOV BX, ES 16-bits Copies ES into BX
MOV ECX, EBX 32-bits Copies EBX into ECX
MOV ESP, EDX 32-bits Copies EDX into ESP
MOV ES, DS — Not allowed (segment-to-segment)
MOV BL, DX — Not allowed (mixed sizes)

13. Immediate Addressing Mode
 If a source operand is part of the instruction
instead of the contents of a register or
memory location, it represents what is called
the immediate operand and is accessed
using immediate addressing mode. Typically
immediate operand represents constant data
Immediate operands can be either a byte or
word of data
 Example: MOV AL , 15H (2 bytes) MOV CX,
4AC2H (3bytes).
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14. Direct Addressing Mode
 Direct addressing differs from immediate
addressing, that the locations following the
instruction op-code hold an “Offset” which is a 16-
bit offset of the storage location of the operand from
the current value in the data segment (DS) register.
offset is combined with the contents of DS in the
BIU to produce the physical address of the operand.
 Example: MOV CX , [1000]
 This stands for move the contents of the memory
location, which is offset by 1000H from the current
value in DS into internal register CX
CX [DS*10+1000]H
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15. Register Indirect Addressing Mode
 Register indirect addressing is similar to direct addressing, that an
effective address is combined with the contents of DS to obtain a
physical address. However it differs in a way that the offset is
specified. Here offset resides in either a pointer register or an index
register within the 8086.The pointer register can be either a base
register BX or a base pointer register BP and the index register can
be source index register SI or the destination index register DI
 Example MOV AX , [SI]
 This instruction moves the contents of the memory location offset by
the value of offset in SI from the current value in DS to the AX
register.
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AX [DS*10+[SI]]H

16. Based Addressing Mode
 In the based addressing mode, the physical address of
the operand is obtained by adding a direct or indirect
displacement of the contents of either base register BX
or base pointer register BP and the current value in DS
and SS respectively.
 Example : MOV [BX + SI] , AL
 This instruction uses base register BX and direct
displacement SI to derive the EA of the destination
operand. The based addressing mode is implemented by
specifying the base register in the brackets followed by a
period and direct displacement .The source operand is
located in the byte accumulator AL
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17. Indexed Addressing Mode
 Indexed addressing mode works identically to
the based addressing but it uses the contents
of the index registers instead of BX or BP, in
the generation of the physical address.
 Example MOV AL , [SI+10]
 The source operand has been specified using
direct index addressing. The notation this
time is such, which is a direct displacement,
prefixes the selected index register, SI.
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18. Based Indexed Addressing Mode
 Combining the based addressing mode and the
indexed addressing mode together results in a
new, more powerful mode known as based
indexed addressing.
 Example: MOV AH , [BX] . BETA [SI]
 Here the source operand is accessed using the
based indexed addressing mode. The effective
address of the source operand is obtained as
offset = [BX]+BETA+[SI]
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