8086 Internal Block diagram (Intel Corp.)
eNotes
By
Prof. S. Jagannathan, HOD – Department of Electronics and Communication Engineering, R.V. College of Engineering, Bangalore
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore
Session - III ADVANCED MICROPROCESSORS
Contents • • • Block Diagram of 8086 segment registers 8086 flag register format
8086 Internal Block diagram (Intel Corp.)
The block diagram of 8086 is as shown. This can be subdivided into two parts, namely the Bus Interface Unit and Execution Unit. The Bus Interface Unit consists of segment registers, adder to generate 20 bit address and instruction prefetch queue. Once this address is sent out of BIU, the instruction and data bytes are fetched from memory and they fill a First In First Out 6 byte queue.
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore
Execution Unit: The execution unit consists of scratch pad registers such as 16-bit AX, BX, CX and DX and pointers like SP (Stack Pointer), BP (Base Pointer) and finally index registers such as source index and destination index registers. The 16-bit scratch pad registers can be split into two 8-bit registers. For example, AX can be split into AH and AL registers. The segment registers and their default offsets are given below. Segment Register CS DS SS ES Default Offset IP (Instruction Pointer) SI, DI SP, BP DI
The Arithmetic and Logic Unit adjacent to these registers perform all the operations. The results of these operations can affect the condition flags. Different registers and their operations are listed below: Register AX AL AH BX CX CL DX Operations Word multiply, Word divide, word I/O Byte Multiply, Byte Divide, Byte I/O, translate, Decimal Arithmetic Byte Multiply, Byte Divide Translate String Operations, Loops Variable Shift and Rotate Word Multiply, word Divide, Indirect I/O
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore
8086/8088 MPU IP CS DS SS ES AX BX CX DX SP BP SI DI SR Instruction Pointer Code Segment Register Data Segment Register Stack Segment Register Extra Segment Register AH BE CE DH AL BL CL DL
MEMORY 00000016
Code Segment (64Kb)
Data Segment (64Kb)
Stack Segment (64Kb)
Extra Segment (64Kb)
Stack Pointer Register Break Pointer Register Source Index Register Destination Index Register Status Register
FFFFF16
Generation of 20-bit Physical Address:
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore
LOGICAL ADDRESS
SEGMENT REGISTER
0000
ADDER
20 BIT PHYSICAL MEMORY ADDRESS
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore
8086 flag register format
BIT 15 U 14 U 13 U 12 U 11 0F 10
DF
9 IF
8
TF
7
SF
6
ZF
5 U
4
AF
3 U
2
PF
1 U
0
CF
U= UNDEFINED
(a) (b) (c) (d) (e) (f) (g) (h) (i)
(a) (b) (c) (d) (e) (f) (g) (h) (i)
: CARRY FLAG – SET BY CARRY OUT OF MSB : PARITY FLAG – SET IF RESULT HAS EVEN PARITY : AUXILIARY CARRY FLAG FOR BCD : ZERO FLAG – SET IF RESULT = 0 : SIGN FLAG = MSB OF RESULT : SINGLE STEP TRAP FLAG : INTERRUPT ENABLE FLAG : STRING DIRECTION FLAG : OVERFLOW FLAG
There are three internal buses, namely A bus, B bus and C bus, which interconnect the various blocks inside 8086. The execution of instruction in 8086 is as follows: The microprocessor unit (MPU) sends out a 20-bit physical address to the memory and fetches the first instruction of a program from the memory. Subsequent addresses are sent
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore
out and the queue is filled upto 6 bytes. The instructions are decoded and further data (if necessary) are fetched from memory. After the execution of the instruction, the results may go back to memory or to the output peripheral devices as the case may be.
Prof. S. Jagannathan,HOD – Dept of E & C Engg,R.V. C. E, Bangalore