Block diagram with functions

1. 8086 Architecture

2. Introduction
• Intel First 16 bit microprocessors
• 40 pin IC
• Made up n-channel depletion mode silicon
gate technology.
• It has two stage pipelining implementation in
instruction execution.
• The CPU logic are divided into Execution
Unit(EU) & Bus Interface Unit (BIU).

3. • BIU provides interface with external bus, and
executes all bus operations.
• EU takes the instruction from object code
queue of BIU and executes it.
• It has two mode of operations.
- Minimum mode : single processor
environment.
- Maximum mode : Multi-processor
environment.

4. Architecture

5. 8086 Pin Diagram

6. Bus Interface Unit
• BIU contains Bus interface logic, Segment
Registers, Memory addressing logic, and a 6-
byte instruction object code queue.
• If the EU executes an instruction, then BIU
- resets the queue
- fetches the instruction from the new address
- passes the instructions to the EU
- begins refilling the queue from new location

7. Execution Unit
• EU contains instruction decoder, ALU,General
purpose registers, pointers and index
registers,flag registers and control circuitry.
• EU responsible for,
– The execution of all instructions
– Providing address to the BIU for fetching
data/instruction
– Manipulating various registers as well as flag
registers.

8. Register Set
• It has fourteen 16 bit registers.
- Data register Group
- Segment Register Group
- Pointer
- Index Register Group
- Program Counter
- Flag register

9. Data registers
• Four 16 bit registers ,
– AX,BX,CX,DX
– AX reg serves as a primary accumlator
– BX reg serves as a base reg while computing the data
memory address.
– CX reg serves to hold count in multi iteration instructions
or repetitions.
– DX reg to used in I/O instructions.

11. Segment Registers
• Code segment Register
• Data segment Register
• Stack segment Register
• Extra segment Register
• It s 16 bit register whose have 16 KB memory
space each.

12. Pointer & Index registers
• Stack pointer
• Base pointer
• Source Index
• Destination Index
• Instruction Index
• Instruction Pointer

13. Flag Register

14. • Carry Flag : its used in some shift & rotate
operations.
• Parity Flag: it is set if result of the word
operation contains an even number of 1s
• Auxiliary carry flag: its used in BCD operations
• Zero Flag: if the result of operation is zero
• Sign flag: set if after ALU operations ,the result
of MSB is 1.

15. • Overflow Flag: set if MSB bit is overflow as
carry or borrow. It has 3 control flags ,
• Direction Flag : used string operations ,set if
instructions to auto decrement or from right
to left.
• Interrupt Enable Flag : set if 8086 to recognize
the external interrupt request.
• Trap Flag : set to put the processor into single
step mode for debugging.