8086 [2] Ahad

Internal!

External?

8086 vs _bit Data Bus 20_bit Address 8_bit Data Bus 20_bit Address Only external bus of 8088 is 8bit

Pin configuration:

8086/8088 Busses Address Bus – 20 address lines so a 2 20 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 Description Vcc (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 - S4S3function. 00extra segment 01stack segment 10code or no segment 11data 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 bit S6  always a logic 0

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

READY (input, Active High) : Takes µP to wait state 0  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 memory 0  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 operation 0  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 operation It 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 High to 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 request It 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 interrupt similar 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 1 st 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 1 st 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 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