8086 MICROPROCESSOR ARCHITECTURE & SEGMENTATION Visit for more Learning Resources

Multiplexed address & data pin Multiplexed address & status pins Select Minimum/ maximum mode Multplexed address and data bus Interrupt Pins

S4 S3 Segment Register ES 1 SS CS or none DS A19/S6, A18/S5, A17/S4, A16/S3 S4 S3 Segment Register ES 1 SS CS or none DS

BHE/S7 BHE A0 Word/Byte Access Whole word from even address 1 Whole word from even address 1 Upper byte from/ to odd address Lower byte from/ to odd address None

QS1, QS0 QS1 QS0 Status No operation 1 1st byte of op-code queue No operation 1 1st byte of op-code queue Empty queue Subsequent byte of op-code queue

S0, S1, S2 S2 S1 S0 Status Interrupt Acknowledge 1 I/O read I/O write Interrupt Acknowledge 1 I/O read I/O write Halt Op-code Fetch Memory Read Memory Write Passive

Flags Carry flag Overflow Parity flag Direction Auxiliary flag Interrupt enable Zero Trap Sign 6 are status flags 3 are control flag

BIU registers (20 bit adder) EU registers ES Extra Segment CS 8086 Programmer’s Model ES Extra Segment BIU registers (20 bit adder) CS Code Segment SS Stack Segment DS Data Segment IP Instruction Pointer EU registers AX AH AL Accumulator BX BH BL Base Register CX CH CL Count Register DX DH DL Data Register SP Stack Pointer BP Base Pointer SI Source Index Register DI Destination Index Register FLAGS

8086/88 internal registers 16 bits (2 bytes each) AX, BX, CX and DX are two bytes wide and each byte can be accessed separately These registers are used as memory pointers. Flags will be discussed later Segment registers are used as base address for a segment in the 1 M byte of memory

Memory Address Generation Intel Memory Address Generation The BIU has a dedicated adder for determining physical memory addresses Offset Value (16 bits) Segment Register (16 bits) 0 0 0 0 Adder Physical Address (20 Bits)

Byte end of code segment 3FFFF Next instruction byte to be fetch Physical address 30123 0123 Start of code segment 30000 CS: 30000 + IP: 0123 = Physical address 30123 Physical Address generation for code segment

Byte Top of Stack end of code segment 6FFFF SP = FFE0 6FFE0 H Physical address 0123 Start of stack segment 60000 H SS: 60000 + SP: FFE0 = Physical address 6FFE0 Physical Address generation for stack segment

Fig a) Non-pipelined execution of 3 instructions F D E F D E F D E I1 I1 I1 I2 I2 I2 I3 I3 I3 Clock cycle 1 2 3 4 5 6 7 8 9 Fig b) Pipelined execution of 3 instructions F I1 I2 I3 I4 I5 F - Fetch I4 I1 I2 I3 D - Decode D E I1 I2 I3 E - Execute Clock cycle 1 2 3 4 5

Operating modes of 8086 Microprocessor 8086 can be operated in two modes- Minimum Mode Maximum Mode In minimum mode all control signals are generated by 8086 itself, so it is used in single processor system In miximum mode all control signals are generated by bus controller 8288 and not by the processor 8086, so it is used in multiprocessor system. The pin 33 MN/MX is used to set minimum mode or maximum mode of 8086 and also the function of pin no 24 to 31 will change as per the selected mode.

8284 Pin Diagram:

8284 Block Diagram: x1 AEN1 CSYNC F / C Clock OSC Clock Logic x2 EF1 PCLK RDY1 AEN1 Ready Logic READY RDY2 AEN2 Reset Logic RESET RES

8284 Connected to 8086 Mp 8284 X1 Ready X2 8086 Microprocessor AEN1 CLK 8284 F/C Reset RDY1 RDY2 RES R + 5 V RESET KEY C

8288 BUS CONTROLLER 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 Vcc S0 S2 MCE/PDEN DEN CEN INTA IORC AIOWC IOWC IOB CLK S1 DT/R ALE AEN MRDC AMWC MWTC GND the 8288 is a 20-pin chip specially designed to provide all the control signals when the 8088/86 is in maximum mode. The input and output signals are described below.

MRDC MWTC AMWC IORC AIOWC INTA S0 S1 S2 Bus Command Logic AEN CEN CLK IOB DT/ R DEN ALE MCE / PDEN Control Signals Logic

Input signals

Pin Diagram of 74LS245:

Pin Diagram of 74LS245:

Function of 74LS245:

8086 System Minimum mode (74LS373) (74LS245)

Processor Timing Diagram of 8086 (Minimum Mode) for Memory or I/O Read CLOCK __ DT/R ALE AD15 - AD0 A15 - A0 D0 – D15 (from memory) A19/S6 - A16/S3 A19 - A16 S6 - S3 __ IO/M if I/O ACCESS this is HIGH, if MEMORY ACCESS this is LOW ____ RD ______ DEN

Processor Timing Diagram of 8086 (Minimum Mode) for Memory or I/O Write CLOCK __ DT/R ALE AD15 - AD0 A15 - A0 D15 - D0 (from memory) A19/S6 - A16/S3 A19 - A16 S6 - S3 __ IO/M if I/O ACCESS this is HIGH, if MEMORY ACCESS this is LOW ____ WR ______ DEN

8086 System Maximum Mode

Processor Timing Diagram of 8086(Maximum Mode) for Memory or I/O Read CLOCK __ DT/R ALE D0-D15 AD15- AD0 A15 - A0 S2-S0 S2-S0 Active S2-S0 Inacive A19/S6 - A16/S3 A19 - A16 S6 - S3 MDRC/IORC ______ DEN

Processor Timing Diagram of 8086(Maximum Mode) for Memory or I/O Write CLOCK __ DT/R ALE D0-D15 AD15- AD0 A15 - A0 S2-S0 S2-S0 Active S2-S0 Inacive A19/S6 - A16/S3 A19 - A16 S6 - S3 MWTC &IOWC AMWC/AIOWC ______ DEN For more detail contact us