pin description(cont.)

Figure shows the Pin functions of 8086.

The following pin functions are for the minimum mode operation of 8086.

• M/IO – Memory/IO : This is a status line logically equivalent to S2 in maximum mode. When it is low, it indicates the CPU is having an I/O operation, and when it is high, it indicates that the CPU is having a memory operation. This line becomes active high in the previous T4 and remains active till final T4 of the current cycle. It is tristated during local bus “hold acknowledge “.

• INTA – Interrupt Acknowledge : This signal is used as a read strobe for interrupt acknowledge cycles. i.e. when it goes low, the processor has accepted the interrupt.

• ALE – Address Latch Enable : This output signal indicates the availability of the valid address on the address/data lines, and is connected to latch enable input of latches. This signal is active high and is never tristated.

• DT/R – Data Transmit/Receive: This output is used to decide the direction of data flow through the transreceivers (bidirectional buffers). When the processor sends out data, this signal is high and when the processor is receiving data, this signal is low.

• DEN – Data Enable : This signal indicates the availability of valid data over the address/data lines. It is used to enable the transreceivers ( bidirectional buffers ) to separate the data from the multiplexed address/data signal. It is active from the middle of T2 until the middle of T4. This is tristated during ‘ hold acknowledge’ cycle.

• HOLD, HLDA- Acknowledge : When the HOLD line goes high, it indicates to the processor that another master is requesting the bus access. The processor, after receiving the HOLD request, issues the hold acknowledge signal on HLDA pin, in the middle of the next clock cycle after completing the current bus cycle.

• At the same time, the processor floats the local bus and control lines. When the processor detects the HOLD line low, it lowers the HLDA signal. HOLD is an asynchronous input, and is should be externally synchronized. If the DMA request is made while the CPU is performing a memory or I/O cycle, it will release the local bus during T4 provided :

1.The request occurs on or before T2 state of the current cycle.
2.The current cycle is not operating over the lower byte of a word.
3.The current cycle is not the first acknowledge of an interrupt acknowledge sequence.
4. A Lock instruction is not being executed.

The following pin functions are applicable for maximum mode operation of 8086.

• S2, S1, S0 – Status Lines : These are the status lines which reflect the type of operation, being carried out by the processor. These become activity during T4 of the previous cycle and active during T1 and T2 of the current bus cycles.

• LOCK : This output pin indicates that other system bus master will be prevented fromgaining the system bus, while the LOCK signal is low. The LOCK signal is activated by the ‘LOCK’ prefix instruction and remains active until the completion of the next instruction. When the CPU is executing a critical instruction which requires the system bus, the LOCK prefix instruction ensures that other processors connected in the system will not gain the control of the bus.

The 8086, while executing the prefixed instruction, asserts the bus lock signal output, which may be connected to an external bus controller. By prefetching the instruction, there is a considerable speeding up in instruction execution in 8086. This is known as instruction pipelining.

S2	S1	S0	Indication
0 0 0 0 1 1 1 1	0 0 1 1 0 0 1 1	0 1 0 1 0 1 0 1	Interrupt Acknowledge Read I/O port Write I/O port Halt Code Access Read Memory Write Memory Passive

• At the starting the CS:IP is loaded with the required address from which the execution is to be started. Initially, the queue will be empty an the microprocessor starts a fetch operation to bring one byte (the first byte) of instruction code, if the CS:IP address is odd or two bytes at a time, if the CS:IP address is even.

• The first byte is a complete opcode in case of some instruction (one byte opcode instruction) and is a part of opcode, in case of some instructions ( two byte opcode instructions), the remaining part of code lie in second byte.

• The second byte is then decoded in continuation with the first byte to decide the instruction length and the number of subsequent bytes to be treated as instruction data. The queue is updated after every byte is read from the queue but the fetch cycle is initiated by BIU only if at least two bytes of the queue are empty and the EU may be concurrently executing the fetched instructions.

• The next byte after the instruction is completed is again the first opcode byte of the next instruction. A similar procedure is repeated till the complete execution of the program. The fetch operation of the next instruction is overlapped with the execution of the current instruction. As in the architecture, there are two separate units, namely Execution unit and Bus interface unit.

• While the execution unit is busy in executing an instruction, after it is completely decoded, the bus interface unit may be fetching the bytes of the next instruction from memory, depending upon the queue status.

QS1	QS0	Indication
0 0 1 1	0 1 0 1	No Operation First Byte of the opcode from the queue Empty Queue Subsequent Byte from the Queue

• RQ/GT0, RQ/GT1 – Request/Grant : These pins are used by the other local bus master in maximum mode, to force the processor to release the local bus at the end of the processor current bus cycle.

• Each of the pin is bidirectional with RQ/GT0 having higher priority than RQ/GT1. RQ/GT pins have internal pull-up resistors and may be left unconnected. Request/Grant sequence is as follows:

1.A pulse of one clock wide from another bus master requests the bus access to 8086.
2.During T4(current) or T1(next) clock cycle, a pulse one clock wide from 8086 to the requesting master, indicates that the 8086 has allowed the local bus to float and that it will enter the ‘hold acknowledge’ state at next cycle. The CPU bus interface unit is likely to be disconnected from the local bus of the system.
3.A one clock wide pulse from the another master indicates to the 8086 that the hold request is about to end and the 8086 may regain control of the local bus at the next clock cycle. Thus each master to master exchange of the local bus is a sequence of 3 pulses. There must be at least one dead clock cycle after each bus exchange. The request and grant pulses are active low.For the bus request those are received while 8086 is performing memory or I/O cycle, the granting of the bus is governed by the rules as in case of HOLD and HLDA in minimum mode.