The Intel 8086 - architecture - MN/MX modes

1. 8086 Microprocessor

2. Architecture of 8086
• The architecture of 8086 includes
– Arithmetic Logic Unit (ALU)
– Flags
– General registers
– Instruction byte queue
– Segment registers

3. EU & BIU
• The 8086 CPU logic has been partitioned into
two functional units namely Bus Interface Unit
(BIU) and Execution Unit (EU)
• The major reason for this separation is to
increase the processing speed of the processor
• The BIU has to interact with memory and input
and output devices in fetching the instructions
and data required by the EU
• EU is responsible for executing the instructions
of the programs and to carry out the required
processing

4. EU & BIU

5. BUS INTERFACE UNIT (BU)
The BIU performs all bus operations for EU .
Fetching instructions
Responsible for executing all external bus
cycles.
Read operands and write result.
EXECUTION UNIT (EU)
Execution unit contains the complete
infrastructure required to execute an instruction

6. Architecture Diagram

7. Execution Unit
• The Execution Unit (EU) has
– Control unit
– Instruction decoder
– Arithmetic and Logical Unit (ALU)
– General registers
– Flag register
– Pointers
– Index registers

8. Execution Unit
• Control unit is responsible for the co-ordination
of all other units of the processor
• ALU performs various arithmetic and logical
operations over the data
• The instruction decoder translates the
instructions fetched from the memory into a
series of actions that are carried out by the EU

9. Execution Unit - Registers
• General registers are used for temporary storage
and manipulation of data and instructions
• Accumulator register consists of two 8-bit
registers AL and AH, which can be combined
together and used as a 16-bit register AX
• Accumulator can be used for I/O operations and
string manipulation

10. Execution Unit - Registers
• Base register consists of two 8-bit registers BL
and BH, which can be combined together and
used as a 16-bit register BX
• BX register usually contains a data pointer used
for based, based indexed or register indirect
addressing. Similar to 8085 H-L register.
• Count register consists of two 8-bit registers CL
and CH, which can be combined together and
used as a 16-bit register CX
• Count register can be used as a counter in string
manipulation and shift/rotate instructions

11. Execution Unit - Registers
• Data register consists of two 8-bit registers DL
and DH, which can be combined together and
used as a 16-bit register DX
• Data register can be used to hold 16 bit result in
16 in 16x16 multiplication.

12. Execution Unit - Registers
Pointer Registers
Stack pointer and BP are used to access data in
the stack segment.
SP is used as an offset from the current SS during
execution of instructions that involve the stack
segment in external memory.
BP is used in based addressing mode.

13. Execution Unit - Registers
Index Register
Source index register (SI) and Destination Index
Registers are used in indexed addressing.

14. Execution Unit - Registers

15. Execution Unit - Flags

16. Execution Unit - Flags
• Overflow Flag (OF) - set if the size of the exceeds the
capacity of the destination location.
• Direction Flag (DF) – It is used with string operations.
When set , it causing string instructions to auto –
decrement or to process strings from right to left.
• Interrupt-enable Flag (IF) - setting this bit enables
maskable interrupts . When IF = 0 , all maskable
interrupt are disable.
• Single-step Flag (Trap F) – put 8086 in the single step
mode.

17. Execution Unit - Flags
• Sign Flag (SF) - set if the most significant bit of the
result is one.
• Zero Flag (ZF) - set if the result is zero.
• Auxiliary carry Flag (AF) - set if there was a carry
from or borrow to bits 0-3 in the AL register.
• Parity Flag (PF) - set if parity (the number of "1" bits)
in the low-order byte of the result is even.
• Carry Flag (CF) - set if there was a carry from or
borrow to the most significant bit during last result
calculation

18. Execution Unit - Flags

19. Execution Unit - Pointers
• Stack Pointer (SP) is a 16-bit register pointing to
program stack
• Base Pointer (BP) is a 16-bit register pointing to data
in stack segment. BP register is usually used for based,
based indexed or register indirect addressing.
• Source Index (SI) is a 16-bit register. SI is used for
indexed, based indexed and register indirect
addressing, as well as a source data addresses in string
manipulation instructions.
• Destination Index (DI) is a 16-bit register. DI is used
for indexed, based indexed and register indirect
addressing, as well as a destination data addresses in
string manipulation instructions.

20. Execution Unit - Pointers

21. Bus Interface Unit
• The BIU has
– Instruction stream byte queue
– A set of segment registers
– Instruction pointer

22. BIU – Instruction Byte Queue
• 8086 instructions vary from 1 to 6 bytes
• Therefore fetch and execution are taking place
concurrently in order to improve the
performance of the microprocessor
• The BIU feeds the instruction stream to the
execution unit through a 6 byte prefetch queue

23. BIU – Instruction Byte Queue
• Execution and decoding of certain instructions do not
require the use of buses
• While such instructions are executed, the BIU fetches up to
six instruction bytes for the following instructions (the
subsequent instructions)
• The BIU store these prefetched bytes in a first-in-first out
register by name instruction byte queue
• When the EU is ready for its next instruction, it simply
reads the instruction byte(s) for the instruction from the
queue in BIU

24. Segment: Offset Notation
• The total addressable memory size is 1MB
• Most of the processor instructions use 16-bit
pointers the processor can effectively address
only 64 KB of memory
• To access memory outside of 64 KB the CPU
uses special segment registers to specify where
the code, stack and data 64 KB segments are
positioned within 1 MB of memory

25. Segment: Offset Notation
• A simple scheme would be to order the bytes in a serial
fashion and number them from 0 (or 1) to the end of
memory
• The scheme used in the 8086 is called segmentation
• Every address has two parts, a SEGMENT and an
OFFSET (Segmnet:Offset )
• The segment indicates the starting of a 64 kilobyte
portion of memory, in multiples of 16
• The offset indicates the position within the 64k portion
• Absolute address = (segment * 16) + offset

26. Segment Registers
• The memory of 8086 is divided into 4 segments
namely
– Code segment (program memory)
– Data segment (data memory)
– Stack memory (stack segment)
– Extra memory (extra segment)

27. Different Areas in Memory
• Program memory – Program can be located anywhere in
memory
• Data memory – The processor can access data in any one
out of 4 available segments
• Stack memory – A stack is a section of the memory set
aside to store addresses and data while a subprogram
executes
• Extra segment – This segment is also similar to data
memory where additional data may be stored and
maintained

28. Segment Registers
• Code Segment (CS) register is a 16-bit register containing
address of 64 KB segment with processor instructions
• The processor uses CS segment for all accesses to
instructions referenced by instruction pointer (IP) register
• Stack Segment (SS) register is a 16-bit register containing
address of 64KB segment with program stack
• By default, the processor assumes that all data referenced
by the stack pointer (SP) and base pointer (BP) registers is
located in the stack segment

29. Segment Registers
• Data Segment (DS) register is a 16-bit register containing
address of 64KB segment with program data
• By default, the processor assumes that all data referenced
by general registers (AX, BX, CX, DX) and index register
(SI, DI) is located in the data segment
• Extra Segment (ES) register is a 16-bit register containing
address of 64KB segment, usually with program data
• By default, the processor assumes that the DI register
references the ES segment in string manipulation
instructions

30. Segment Registers
If a location 109F0 of Code Segment is to be addressed to fetch
An instruction, the physical address will be calculated as follows
CSR = 010A
IP = F950
Effective Address = 109F0

35. Minimum-Mode and Maximum-
Mode System

36. Minimum-Mode and Maximum-
Mode System (cont.)
Signals common to both minimum and maximum mode

37. Minimum-Mode and Maximum-
Mode System (cont.)
Unique minimum-mode signals

38. Minimum-Mode and Maximum-
Mode System (cont.)
Unique maximum-mode signals

39. PIN DESCRIPTION -8086
Pin 1, 20
Ground
Pin 40
Vcc
Pin 19
CLK
Pin 17
INTR
Pin 18
NMI

40. AD15 –AD0 [ Pin 2- 16 &39]
The signal have dual function as in case of the 8085. They act as bus
during the first part of machine cycle and as data bus in the later part.
A19/S6 –A16/S3 [Pin 35- 38]
Contains address information in the first part and status bits in the
later part. The status bits, when decoded, indicates the type of
operations (eg. Memory access) being performed and the segment
register being used.
S4 S3 SEGMENT REGISTER
0 0 ES
0 1 SS
1 0 CS
1 1 DS
S5 = IF
S6 = 0
(ALWAYS)

41. BHE/S7 [pin 34]
MN/MX [pin 33]
High – Minimum Mode Operation
Low – Maximum Mode Operation
RD [pin 32]
Read or receive data from M or I/O device
TEST [pin 23]
Relate to wait instruction. The instruction puts the 8086 in
idle state which ends only when the TEST input goes low

42. READY [Pin 22]
Ready is a signal provided by the memory or I/O devices to the
microprocesssor.When READY = High, the microprocessor
proceeds to process the data in usual manner. When READY =
Low, the micropressor goes into wait state and waits for READY to
become High.
RESET [Pin 21]
Resets the Processor

43. ……….PIN DESCRIPTION
DEN
Data bus Enable. This signal, when low indicates that the
microprocessor address/data bus is to be used as data bus.
HOLD
HLDA

44. Minimum Mode pin Functions
Pin 24 -31
INTA - Pin 24
ALE - Pin 25
Address Latch Enable. Since data and address are
multiplexed on a single bus. ALE is output high to identify
a valid address.
DEN -Pin 26
Data Bus Enable. This signal, when low indicates that the
microprocessor address/data bus is to be used as data bus.

45. DT/R - Pin 27
Data transmission/ Receive
M/ IO – Pin 28
WR – Pin 29
HOLD – Pin 30
HLDA - Pin 31

46. Addressing Modes
• Implied Addressing – The data value/data address is
implicitly associated with the instruction
• Register Addressing – The data is specified by referring
the register or the register pair in which the data is present
• Immediate Addressing – The data itself is provided in the
instruction
• Direct Addressing – The instruction operand specifies the
memory address where data is located

47. Addressing Modes
• Register indirect addressing – The instruction specifies a
register containing an address, where data is located
• Based - 8-bit or 16-bit instruction operand is added to the
contents of a base register (BX or BP), the resulting value
is a pointer to location where data resides
• Indexed - 8-bit or 16-bit instruction operand is added to the
contents of an index register (SI or DI), the resulting value
is a pointer to location where data resides

48. Addressing Modes
• Based Indexed - the contents of a base register (BX or BP)
is added to the contents of an index register (SI or DI), the
resulting value is a pointer to location where data resides
• Based Indexed with displacement - 8-bit or 16-bit
instruction operand is added to the contents of a base
register (BX or BP) and index register (SI or DI), the
resulting value is a pointer to location where data resides

49. Data Transfer Instructions

50. Data Transfer Instructions

51. Arithmetic Instructions

52. Arithmetic Instructions

53. Number Representation

54. Logical Instructions

55. String Instructions

56. Program Transfer Instructions

57. Program Transfer Instructions

58. Processor Control Instructions

59. Assembler Directives
• Assembler directives give instruction to the assembler
where as other instructions discussed in the above section
give instruction to the 8086 microprocessor
• Assembler directives are specific for a particular assembler
• However all the popular assemblers like the Intel 8086
macro assembler, the turbo assembler and the IBM macro
assembler use common assembler directives

60. Important Directives
• The ASSUME directive tell the assembler the name of the logical
segment it should use for a specified segment
• The DB directive is used to declare a byte-type variable or to set aside
one or more storage locations of type byte in memory (Define Byte)
• The DD directive is used to declare a variable of type doubleword or to
reserve memory locations which can be accessed as type doubleword
(Define Doubleword)
• The DQ directive is used to tell the assembler to declare a variable 4
words in length or to reverse 4 words of storage in memory (Define
Quadword)

61. Important Directives
• The ENDS directive is used with the name of a
segment to indicate the end of that logical
segment
• The EQU is used to give a name to some value
or symbol

62. Assembly Language Program
• Writing assembly language programs for 8086 is slightly
different from that of writing assembly language programs
for 8085
• In addition to the instructions that are meant for solving the
problem, some additional instructions are required to
complete the programs
• The purpose of these additional programs is to initialize
various parts of the system, such as segment registers, flags
and programmable port devices
• Some of the instructions are to handle the stack of the 8086
based system

63. Assembly Language Program
• Another purpose of these additional instructions is to
handle the programmable peripheral devices such as ports,
timers and controllers
• The programmable peripheral interfaces should be
assigned suitable control words to make them to function
in the way as we expect
• The best way to approach the initialization task is to make
a checklist of all the registers, programmable devices and
flags in the system we are working on

64. Assembly Language Program
• An 8086 assembly language program has five
columns namely
– Address
– Data or code
– Labels
– Mmnemonics
– Operands
– Comments

65. Assembly Language Program
• The address column is used for the address or
the offset of a code byte or a data byte
• The actual code bytes or data bytes are put in
the data or code column
• A label is a name which represents an address
referred to in a jump or call instruction
• Labels are put in the labels column

66. Assembly Language Program
• The operands column contains the registers,
memory locations or data acted upon by the
instructions
• A comments column gives space to describe the
function of the instruction for future reference