2. OPERATION MODES OF 8086
MICROPROCESSORS:
• There are two operation modes of operations in the 8086 microprocessors and they
are as follows:
• Maximum mode and
• Minimum mode
• Maximum mode: The maximum mode is a multiprocessor system.
• Minimum mode: The minimum mode is a single processor system.
3. FEATURES OF 8086
MICROPROCESSORS:
• The Intel 8086 microprocessor is the foremost microprocessor introduced in the
year of 1978 by the Intel Company and it is a 40 pin IC chip.
• It uses 20 line address bus and 16 line data bus.
• The 16 bit lower order address bus lines are multiplexed with complete 16-bit
data lines. The next four higher order address lines are multiplexed with the
status lines.
4. THE INTERNAL ARCHITECTURE OF
8086 MICROPROCESSORS:
• The internal architecture of the 8086 microprocessors is divided into two
independent functional units and they are as follows:
• Bus interface unit
• Execution unit
5. •BUS INTERFACE UNIT:
• Bus interface unit: The bus interface unit is responsible for the transfer of
instructions, information, and address to the execution unit through the system bus.
The bus interface unit exchanges the information with the device outside the
microprocessor and fetches the instructions from the memory. The bus interface
unit consists of three functional parts and they are as below
• 1.Instruction pointer (IP)
• 2.Segment register
• 3.Instruction queue
6. • 1.Instruction pointer: The abbreviation of the instruction pointer is the IP and it
is a 16-bit register that keeps the address of the memory location of the coming
instruction to be executed.
• 2.Segment register: The memory space 1MB of 8086 is segmented into four
blocks and each block is specified by a register with the maximum size of 64 KB.
The four segment registers are as follows:
• Code segment (CS)
• Data segment register (DS)
• Stack segment (SS)
• Extra segment (ES)
• These registers are used to store a 16-bit data and the starting address of the
four memory segments always begin at a memory location with OH as the least
significance and for example, it is like 1234OH.
7. • 3.Instruction queue: The bus interface unit performs its operations in parallel with
the execution unit. The bus interface unit fetches the instruction byte while
execution unit is executing the operation; the pre fetched instruction is saved in a
group of a high-speed register and is known as the instruction queue.
8. •EXECUTION UNIT:
• Execution unit: The execution unit performs four kinds of operations and they are as
follows:
1.Fetch operation
2.Decode operation
3.Execute operation and
4.Write operation
Receiving the instructions from the instruction queue of bus interface unit is the fetch
operation. The decode operation is the decoding of the fetched instruction, the
processing operation on the data is called the execute operation. The storing of the
complete result at the destination is called write operation.
9. • The functional parts of the execution unit are as follows:
1.General purpose registers (GPRS)
2.Pointer and indexed register
3.Arithmetic logic unit (ALU)
4.Flag register
5.Timing and control unit
10. 1.General purpose registers (GPRS): The 8086 microprocessor consists of four general
purpose 16-bit registers and they are AX, BX, CX, and DX. The 16-bit registers can also be
used as the 8bit registers and they are AH, AL, BH, BL, CH, CL, DH and DL.
2.Pointer and indexed registers: The 8086 microprocessor has two pointers and two
indexed registers. They are as follows:
Stag pointer
Base pointer
Source index and
Destination index
11. 3.Arithmetic logic unit (ALU): The arithmetic logic unit is a 16 bit ALU and it
performs the logical and arithmetic operations of 8 bit and as well as the 16 bit.
4.Flag register: The 8086 microprocessor consists of 16-bit flag register among this
16-bit trap we use the 9 flags and the remaining 6 flags are unused. Among the
nine flags, we divided into two categories and they are as follows:
Status flags and
Control flags
12. • The six status flags are as follows:
• Carry flag
• Auxiliary carry
• Zero flag
• Sign flag
• Trap
• Parity flag
13. • The three control flags are as follows:
• Interrupt flag
• Direction flag and
• Overflow
5.Timing and control unit: The control unit of the execution unit directs all the
internal operations and also responsible for the generation of the control signals.
