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 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 8086 - 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.
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