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Chapter: Microprocessor and Microcontroller : 8086 Microprocessor

8086-Minimum mode of operation

1 Minimum Mode Interface 2 Maximum Mode Interface

8086-Minimum mode of operation



1 Minimum Mode Interface


ü Address/Data bus: 20 bits vs 8 bits multiplexed

ü Status signals: A16-A19 multiplexed with status signals S3-S6 respectively


a.     S3 and S4 together form a 2 bit binary code that identifies which of the internal segment registers was used to generate the physical address that was output on the address bus during the current bus cycle.

b.     S5 is the logic level of the internal interrupt enable flag, s6 is always logic 0.


Control Signals:


ü Address Latch Enable (ALE) is a pulse to logic 1 that signals external circuitry when a valid address is on the bus. This address can be latched in external circuitry on the 1-to-0 edge of the pulse at ALE.


ü IO/M line: memory or I/O transfer is selected (complement for 8086)

ü DT/R line: direction of data is selected

ü SSO (System Status Output) line: =1 when data is read from memory and =0 when code is read from memory (only for 8088)


ü BHE (Bank High Enable) line : =0 for most significant byte of data for 8086 and also carries S7


ü RD line: =0 when a read cycle is in progress

ü WR line: =0 when a write cyle is in progress

ü DEN line: (Data enable) Enables the external devices to supply data to the processor.

ü Ready line: can be used to insert wait states into the bus cycle so that it is extended by a number of clock periods


Interrupt signals:


ü INTR (Interrupt request) :=1 shows there is a service request, sampled at the final clock cycle of each instruction acquisition cycle.


ü INTA : Processor responds with two pulses going to 0 when it services the interrupt and waits for the interrupt service number after the second pulse.


ü TEST: Processor suspends operation when =1. Resumes operation when=0. Used to syncronize the processor to external events.


ü NMI (Nonmaskable interrupt) : A leading edge transition causes the processor go to the interrupt routine after the current instruction is executed.


ü RESET : =0 Starts the reset sequence.


2 Maximum Mode Interface


ü For multiprocessor environment 8288 Bus Controller is used for bus control

ü WR¯ ,IO/M¯ ,DT/R¯ ,DEN¯ ,ALE, INTA¯ signals are not available

ü Instead

a.     MRDC¯ (memory read command)

b.     MWRT¯ (memory write command)

c.      AMWC¯ (advanced memory write command)

d.     IORC¯ (I/O read command)

e.      IOWC¯ (I/O write command)

f.       AIOWC¯ (Advanced I/O write command)

g.     INTA¯ (interrupt acknowledge)

ü The signals shown above are produced by 8288 depending on the state of S0, S1 and S2.


a.     DEN, DT/R¯ and ALE signals are the same as minimum-mode systems

b.     LOCK¯ : when =0, prevents other processors from using the bus

c.      QS0 and QS1 (queue status signals): informs about the status of the queue

d.     RQ¯ /GT ¯ 0 and RQ¯ /GT ¯ 1 are used instead of HOLD and HLDA lines in a multiprocessor environment as request/grant lines.




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