8086 Microprocessor Pin Configuration

8086 was the first 16-bit microprocessor available in 40-pin DIP (Dual Inline Package) chip. Let us now discuss in detail the pin configuration of a 8086 Microprocessor.

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Introduction of 8086 Microprocessor

• Intel 8086 was the first 16-bit HMOS microprocessor.
• All the limitation of 8-bit microprocessor is overcome by 16-bit microprocessor.
• It is a more powerful processor in term of advanced architecture, more processing capability, large memory addressing capability and more powerful instruction set.
• It is available in 40 pin DIP chip (40 pin IC).
• It uses a +5V DC supply for its operation.
• The 8086 uses 20-line address bus. It has a 16-bit data bus.
• The 20 line of the address bus operate in multiplexed mode. The 16-low order address bus lines have been multiplexed with data and 4 high-order address bus lines have been multiplexed with status signals.

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8086 Pin Diagram

8086 was the first 16-bit microprocessor available in 40-pin DIP (Dual Inline Package) chip. Let us now discuss in detail the pin configuration of a 8086 Microprocessor.

A16/S3, A17/S4, A18/S5, A19/S6 :  High order address bus. These are multiplexed with status signals. These lines are high for memory related operation and low for I/O related operation. These are multiplexed with status signal S3 to S4 respectively. The combination of S3 ,S4 determine that which part of memory is to be accessed. Where S6 is always low logic and reserved for future use. S5 for interrupt status

A17/S4	A16/S3	FUNCTION
0	0	Extra segment access
0	1	Stack segment access
1	0	Code segment access
1	1	Data segment access

•BHE/S7 : BHE signal is enable when data bit are 16 bit it transfer of data over higher order data bus. If it is 1 then data is less than 8 bit.

S2, S1, S0 : Status pins. All are active low pin. These are used by the 8288 bus controller for generating all the memory and I/O operation access control signals. Any change in S2, S1, S0 indicates the beginning of a bus cycle

•QS1,QS0 : Queue Status. These signals indicate the status of the internal 8086 instruction queue according to the table shown below

Power supply and frequency signals

It uses 5V DC supply at V_CC pin 40, and uses ground at V_SS pin 1 and 20 for its operation.

Clock signal

Clock signal is provided through Pin-19. It provides timing to the processor for operations. Its frequency is different for different versions, i.e. 5MHz, 8MHz and 10MHz.

Address/data bus

AD0-AD15. These are 16 address/data bus. AD0-AD7 carries low order byte data and AD8AD15 carries higher order byte data. During the first clock cycle, it carries 16-bit address and after that it carries 16-bit data.

Address/status bus

A16-A19/S3-S6. These are the 4 address/status buses. During the first clock cycle, it carries 4-bit address and later it carries status signals.

S7/BHE

BHE stands for Bus High Enable. It is available at pin 34 and used to indicate the transfer of data using data bus D8-D15. This signal is low during the first clock cycle, thereafter it is active.

Read($\overline{RD}$)

It is available at pin 32 and is used to read signal for Read operation.

Ready

It is available at pin 22. It is an acknowledgement signal from I/O devices that data is transferred. It is an active high signal. When it is high, it indicates that the device is ready to transfer data. When it is low, it indicates wait state.

RESET

It is available at pin 21 and is used to restart the execution. It causes the processor to immediately terminate its present activity. This signal is active high for the first 4 clock cycles to RESET the microprocessor.

INTR

It is available at pin 18. It is an interrupt request signal, which is sampled during the last clock cycle of each instruction to determine if the processor considered this as an interrupt or not.

NMI

It stands for non-maskable interrupt and is available at pin 17. It is an edge triggered input, which causes an interrupt request to the microprocessor.

$\overline{TEST}$

This signal is like wait state and is available at pin 23. When this signal is high, then the processor has to wait for IDLE state, else the execution continues.

MN/MX

It stands for Minimum/Maximum and is available at pin 33. It indicates what mode the processor is to operate in; when it is high, it works in the minimum mode and vice-versa.

INTA

It is an interrupt acknowledgement signal and id available at pin 24. When the microprocessor receives this signal, it acknowledges the interrupt.

ALE

It stands for address enable latch and is available at pin 25. A positive pulse is generated each time the processor begins any operation. This signal indicates the availability of a valid address on the address/data lines.

DEN

It stands for Data Enable and is available at pin 26. It is used to enable Transreceiver 8286. The transreceiver is a device used to separate data from the address/data bus.

DT/R

It stands for Data Transmit/Receive signal and is available at pin 27. It decides the direction of data flow through the transreceiver. When it is high, data is transmitted out and vice-versa.

M/IO

This signal is used to distinguish between memory and I/O operations. When it is high, it indicates I/O operation and when it is low indicates the memory operation. It is available at pin 28.

WR

It stands for write signal and is available at pin 29. It is used to write the data into the memory or the output device depending on the status of M/IO signal.

HLDA

It stands for Hold Acknowledgement signal and is available at pin 30. This signal acknowledges the HOLD signal.

HOLD

This signal indicates to the processor that external devices are requesting to access the address/data buses. It is available at pin 31.

LOCK

When this signal is active, it indicates to the other processors not to ask the CPU to leave the system bus. It is activated using the LOCK prefix on any instruction and is available at pin 29.

RQ/GT₁ and RQ/GT₀

These are the Request/Grant signals used by the other processors requesting the CPU to release the system bus. When the signal is received by CPU, then it sends acknowledgment. RQ/GT₀ has a higher priority than RQ/GT₁.

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