8086 [2]

Ahad

Internal!

External?

8086 vs 8088

U?

8086MIN

MN33

READY22

CLK19

RESET21

INTR18

HLDA30

HOLD31

NMI17

TEST23

AD016

AD115

AD214

AD313

AD412

AD511

AD610

AD79

AD88

AD97

AD106

AD115

AD124

AD133

AD142

AD1539

A16/S338

A17/S437

A18/S536

A19/S635

BHE/S734

DEN26

DT/R27

M/IO28

RD32

WR29

ALE25

INTA24

U?

8088MIN

MN33

READY22

CLK19

RESET21

INTR18

HLDA30

HOLD31

NMI17

TEST23

AD016

AD115

AD214

AD313

AD412

AD511

AD610

AD79

A88

A97

A106

A115

A124

A133

A142

A1539

A16/S338

A17/S437

A18/S536

A19/S635

SSO34

DEN26

DT/R27

IO/M28

RD32

WR29

ALE25

INTA24

16_bit Data Bus

20_bit Address

8_bit Data Bus

20_bit Address

80888086

Only external bus of 8088 is 8bit

Pin configuration:

8086/8088 Busses• Address Bus

– 20 address lines so a 220 byte address space.– Pins A0-A19 provide the address– For 8086, A0-A15 are multiplexed with D0-D15 to form

AD0-AD15– For 8088, A0-A7 are multiplexed with D0-D7 to form

AD0-AD7• Data Bus

– For 8086, 16 bit data bus D0-D15 (multiplexed as AD0-AD15)

– For 8088, 8 bit data bus D0-D7 (multiplexed as AD0-AD7)

Control pins

• Control Bus– For memory access, the following pins are used:RD’, WR’, M/IO’, DT/R’, DEN’, ALE, BHE’

– Other input signals to control 8086 performance:clk ,reset , ready , hold , test’, intr , nmi ,mn’/mx

- The intr and hold are acknowledged through intra and holda respectively.

8086 Pin Assignment

8086 Pin DescriptionVcc (pin 40) : Power supply input

GND (pin 1 and 20) : Ground pin is the return of the power supply

AD0…AD7, A8...A15 , A19/S6, A18/S5, A17/S4, A16/S3 : 20-bit Address Bus

When ALE=1 [Address Latch Enable] AD8-AD15 becomes address bits.

When ALE=0 it becomes data bits.

A19/S6, A18/S5, A17/S4, A16/S3 Address/status bus -

S4 S3 function .0 0 extra segment 0 1 stack segment1 0 code or no segment1 1 data segment

These two status bits could be used to address four separate 1MByte mem banks – by decoding them as A21 and A20

S5 indicates the condition of the IF flag bitS6 always a logic 0

MN/MX’ (33, input) : Indicates Operating mode – min mode or max mode

READY (input, Active High) : Takes µP to wait state0 µP enters into wait states n remains idle.1 no effect on the operation of the µP

CLK (input) : Provides basic timing for the processor CLK must have a duty cycle of 33% (high for 1/3 of the clocking period & low for 2/3)

DEN’ (output) : activates external data bus buffers. It is LOW when processor wants to receive data or processor is giving out data

DT/R’ (output) : Data Transmit/Receive. 1 data from µP to memory0 data is from memory to µP

M/IO’ (output) : selects mem or I/O.It indicates that the mP address bus contains – either a memory address or an I/O port address. 1 µP access I/O Device

0 µP access memory

RD’ (output) : When Low, µP is performing a read operation0 the data bus is receptive to data from the mem or I/O devices connected to the system.

WR’ (output) : When Low, µP is performing a write operationIt is outputting data to a mem or I/O device. During the time WR=0, the data bus contains a valid data for mem or I/O.

• ALE (output) : Address Latch Enable, Active Highto latch address 1 µP is using AD0..AD7, A19/S6, A18/S5, A17/S4, A16/S3 as address bus.This address can be a mem address or an I/O port number.

RESET (input, Active High) : At least 4 clock cycles causes the µP immediately terminate its present activity.

TEST’ (input , Active Low) : it is an input pin – that is tested by the WAIT instruction.

0 the WAIT instr. functions as an NOP.1 the WAIT instr. waits for TEST to

become a logic 0.

HOLD (input , Active High) : it requests a DMA [Direct Memory Access]1 the mP stops executing software and places its address, data, and control bus at the high-impedance state.0 mP executes SW normally.

HLDA (output , Active High) :Hold Acknowledge – indicates that the mP has entered the HOLD state.

INTR (input , Active High) : Interrupt requestIt is used to request a hardware interrupt. 1 when IF = 1, mP enters an interrupt acknowledge cycle (INTA becomes active) after the current instruction has completed execution. [IF - Interrupt Flag -Set by user to disable hardware interrupts temporarily]

INTA’ (output , Active Low) : Interrupt Acknowledge

NMI (input , Active High) : Non-maskable interruptsimilar to INTR – except that the NMI interrupt does not check to see whether the IF flag is 1.

• The AD0-AD15 lines are a 16-bit multiplexed addressed or data bus.

• During the 1st clock cycle, AD0-AD15 are the low order 16-bit address.

• The 8086 has a total of 20 address line, the upper 4 lines are multiplexed with the state signal that is A16/S3, A17/S4, A18/S5, A19/S6.

• For memory and i/o operations, AD15-AD0 contain the 16 bit data and S3,S4,S5,S6 become the status line.

• BHE/S7 is used as best high enable during the 1st clock cycle of an instruction execution.

• BHE can be used in conjunction with AD0 to select the memory

• RD is low when the data is read from memory or I/O location

• TEST is an input pin and is only used by the wait instruction

• 8086 enters a wait state after execution of the wait instruction until a low is seen on the test pin.

• ALE is an address latch enable is an o/p signal provided by the 8086

• - can be used to demultiplexed AD0 to AD15 in to A10 toA15 and D0 to D15.

• M/IO is an 8086 output signal to distinguish a memory access and i/o access.

33. MN/MX

• The 8086 can operate in two modes:1. minimum mode and 2. maximum mode

For minimum mode, a unique processor system with a single 8086

For maximum mode, a multi-processor system with more than one 8086.

Minimum-Mode and Maximum-Mode System

•

Signals common to both minimum and maximum mode

Minimum-Mode and Maximum-Mode System (cont.)

Unique minimum-mode signals

Minimum-Mode and Maximum-Mode System (cont.)

Unique maximum-mode signals