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8086 Hardware Specification

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8086 Hardware Specification Segment 5 8086 Hardware Specification Content: • 8086 Modes of operation. • Pin diagram and pin function of 8086. • 8284A Clock generator operation and pin functions. Prepared By: Mohammed Abdul Kader Lecturer, EEE, IIUC 8086 modes of operation: 8086 can operate in two modes (MN/푀푋) Minimum mode: The 8086 processor works in a single processor environment. All control signals for memory and I/O are generated by the microprocessor. Maximum mode is designed to be used when a coprocessor exists in the system. 8086 works in a multiprocessor environment. Control signals for memory and I/O are generated by an external BUS Controller (e.g. Intel 8288) Pin Functions There are 40 pins in 8086 microprocessor. They are divided as: • Common Pins • Maximum Mode Pins • Minimum Mode Pins 8086 Pins 8086 Pins (Common Pins) AD15-AD0 (Address/Data Bus) and ALE (Address latch enable) The 8086 address/data bus lines compose the multiplexed address/data bus on the 8086. These lines contains address bits when ALE=1 and data bits when ALE=0. These pins enter a high-impedance state when a hold acknowledge occurs. ALE=1 AD15-AD0 act as address pins ALE=0 AD15-AD0 act as data pins A19/S6 – A16/S3 (Address/ Status bus) • Other 4 address lines (addresses are 20 bits) are the A16/S3, A17/S4, A18/S5 and A19/S6 (Multiplexed with status lines) • During the first clock period of bus cycle (read or write cycle), the entire 20- bit address is available on these lines. During all other clock cycles for memory and I/O operations, AD15 – AD0 contain the 16 bit data and S3, S4, S5 and S6 become status lines. S3 and S4 lines are decoded as follows(during memory operation): During I/O operation: • A18/S5 and A19/S6 will stay low during the first clock period. A17/S4 A16/S3 Function • During all other cycles, A18/S5 0 0 Extra Segment indicates the status of the 8086 0 1 Stack Segment interrupt enable flag and A19/S6 1 0 Code or no segment become low, a low A19/S6 pin 1 1 Data Segment indicates that the 8086 is on the bus. 8086 Pins (Common Pins) [Continued] 푹푫 (Read Signal) • If 푅퐷 =0, the data bus receives data from the memory or I/O devices. • This pin floats to its high-impedance state during a hold acknowledge. READY • If READY =0 , the microprocessor enters into wait states and remains idle. • If the READY pin=1 , it has no effect on the operation of the microprocessor. • The READY input is controlled to insert wait states into the timing of the microprocessor. 푻푬푺푻 • The 푻푬푺푻 pin is an input that is tested by the WAIT instruction. • If 푇퐸푆푇 is a logic 0, the wait instruction functions as a NOP and if 푇퐸푆푇 is logic 1, the wait instruction waits for 푇퐸푆푇 to become a logic zero. INTR Interrupt request is used to request a hardware interrupt. If INTR is held high when IF=1 the 8086 enters an interrupt acknowledge cycle (INTA becomes active) after the current instructions has completed execution. NMI The Non-maskable interrupt input is similar to INTR except that the NMI interrupt does not check to see whether the IF flag bit is a logic 1. If NMI is activated, this interrupt input uses interrupt vector 2. 8086 Pins (Common Pins) [Continued] RESET The RESET input causes the microprocessor to reset itself if this pin is held high for a minimum of four clocking period. It causes the 8086 to initialize register DS, SS, ES, IP and flags to all zeros and CS to FFFFH. CLK The CLK (clock) pin provides the basic timing signal to the microprocessor. The clock must have a duty cycle of 33% (high for one third of the clocking period and low for two thirds) to provide proper internal timing for the 8086. Vcc The power supply input provides a +5.0 V, ±10% signal to the microprocessor. GND The ground connection is the return for the power supply. Note that the 8086 microprocessors have two pins leveled GND- both must be connected to the ground for the proper operation. MN/푴푿 The minimum/maximum mode pin selects either minimum mode or maximum mode operation for the microprocessor. If minimum mode is selected, the MN/푀푋 pin must be connected directly to +5.0 V. 푩푯푬/푺ퟕ (Bus High Enable) The 8086 outputs a low on this pin during read, write and interrupt acknowledge cycles in which data are to be transferred in a high order byte (AD15-AD8) of the data bus. The state of S7 is always logic 1. 8086 Pins (Minimum Mode Pins) M/푰푶 • This pin selects memory or I/O. • When the 8086 executes an I/O instruction such as IN or OUT, it outputs a LOW on M/IO . • On the other hand, the 8086 outputs HIGH on the pin when it executes a memory reference instruction such as MOV AX, [SI]. 푾푹 When 푾푹 =0, the processor is performing a write memory or write I/O operation depending on the M/푰푶 signal. 푰푵푻푨 • The interrupt acknowledge signal is a response to the INTR input pin. • The 푰푵푻푨 pin is normally used to gate the interrupt vector number onto the data bus in response to an interrupt request. ALE (Address latch enable) Used to multiplex the AD0-AD15 into A0-A15 (when ALE=1)and D0-D15 (when ALE=0). 8086 Pins (Minimum Mode Pins) [Continued] 푫푻/푹 (Data Transmit/receive) The data transmit/receive signal shows that the microprocessor data bus is transmitting (푫푻/푹 =1) or receiving (푫푻/푹 = ퟎ) data. The signal is used to enable external data bus buffers. DEN (Data bus enable) Data bus enable activates external data bus buffers. (e.g. Data bus buffer of 8255A) HOLD • If the HOLD signal is a logic 1, the microprocessor stops executing software and places its address, data, and control bus at the high impedance state. • If the HOLD pin is logic 0, the microprocessor executes software normally. HALDA (Hold Acknowledgement) Hold acknowledgement indicates that the 8086 has entered the hold state. 8086 Maximum Mode Pins 푺ퟐ, 푺ퟏ, 풂풏풅 푺ퟎ The status bits indicate the function of the current bus cycle. These signals are normally decoded by the 8288 bus controller. Table shows the function of these three status bits in the maximum mode. 푺ퟐ 푺ퟏ 푺ퟎ Function 0 0 0 Interrupt acknowledge 0 0 1 I/O Read 0 1 0 I/O Write 0 1 1 Halt 1 0 0 Opcode Fetch 1 0 1 Memory Read 1 1 0 Memory write 1 1 1 Passive 퐑퐐/퐆퐓퐈 and 퐑퐐/퐆퐓ퟎ The request/grant pins request direct memory accesses (DMA) during maximum mode operation. These lines are bidirectional and are used to both request and grant a DMA operation. [DMA: Direct memory access normally occurs between an I/O device and memory without the use of microprocessor. A DMA read transfers data from memory to the I/O device. A DMA write transfers data from an I/O device to memory] 8086 Maximum Mode Pins (Continued) 푳푶푪푲 The 8086 outputs LOW on the 퐿푂퐶퐾 pin to prevent other bus masters from gaining control of the system bus. QS1 and QS0 The 8086 outputs to QS1 and QS0 pins to provide status to allow external tracking of the internal 8086 instruction queue as follows- QS1 QS0 Function 0 0 No operation 0 1 First byte of opcode from queue 1 0 Empty the queue 1 1 Subsequent byte from queue Clock Generator (8284A) The 8284A provides the following basic functions or signals: • Clock Generation. • RESET synchronization. • READY synchronization and • A TTL level peripheral clock signal. Pin Functions: The 8284A is an 18-pin integrated circuit designed specially for use with the 8086/8088 microprocessor. The following is a list of each pin and its function. The address enable pins are provided to qualify the bus ready signals, RDY1 and RDY2, respectively. Wait states are generated by the READY pin of the 8088/8086 microprocessor, which is controlled by these two pins. Clock Generator (8284A): Pin functions The bus ready inputs are provided from a device located on the system data bus, in conjunction with the , pins, to cause wait state in an 8086/8088- based system. Ready synchronization selection input selects either one or two stages of synchronization for the RDY1 and RDY2 inputs. When it is low, two stages of ready synchronization are provided. When it is high, a stage of ready synchronization is provided. READY is an output pin that connects to the 8086/8088 READY input. This signal is synchronized with the RDY1 and RDY2 inputs. The crystal oscillator pins connect to an external crystal used as the timing source for the clock generator and all its functions. Clock Generator (8284A): Pin functions (Cont.) Frequency/Crystal select input chooses the clocking source for the 8284A. If this pin is high, clock is supplied from external source connected to EFI pin. If this pin is low, clock is supplied from crystal oscillator. External frequency input is an input pin to provide external clock pulse for the 8086/8088 microprocessor. The input external clock should be three times greater than the desired output clock. The clock output pin provides the CLK input signal to the 8086/8088 microprocessor and other components in the system. The CLK input has an output signal that is one-third of the crystal or EFI input frequency, and has a 33% duty cycle, which is required by the 8086/8088.
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