8086
Ahad
[2]
Internal!
External?
8086 vs 8088
Only external bus of 8088 is 8bit
U?
33
22
19
21
18
U?
MN
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
AD8
AD9
AD10
AD11
AD12
AD13
AD14
AD15
A16/S3
A17/S4
A18/S5
A19/S6
READY
CLK
RESET
INTR
BHE/S7
30
31
17
23
DEN
DT/R
M/IO
HLDA
HO LD
NMI
TEST
8086MIN
RD
WR
ALE
INTA
8086
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
39
38
37
36
35
33
22
19
21
18
16_bit Data Bus
MN
AD0
AD1
AD2
AD3
AD4
AD5
AD6
AD7
A8
A9
A10
A11
A12
A13
A14
A15
A16/S3
A17/S4
A18/S5
A19/S6
READY
CLK
RESET
INTR
20_bit Address
34
SSO
26
27
28
32
29
25
24
30
31
17
23
DEN
DT/R
IO /M
HLDA
HO LD
NMI
TEST
8088MIN
RD
WR
ALE
INTA
8088
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
39
38
37
36
35
34
26
27
28
32
29
25
24
8_bit Data Bus
20_bit Address
Pin configuration:
8086/8088 Busses
• Address Bus
– 20 address lines so a 220 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 AD0AD15)
– 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 Assignment
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 S4
S3
function
.
0
0
extra segment
0
1
stack segment
1
0
code or no segment
1
1
data 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 1st 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
1st 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.
Minimum-Mode and MaximumMode System
Signals common to both minimum and maximum mode
Minimum-Mode and MaximumMode System (cont.)
Unique minimum-mode signals
Minimum-Mode and MaximumMode System (cont.)
Unique maximum-mode signals