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8086
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The 8086 is Intel’s first 16-bit
microprocessor
The 8086 can run at different clock speeds
 Standard 8086 – 5 MHz
 8086-1 –10 MHz
 8086-2 – 8 MHz
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The 8086 has a 20-bit address
 Can address upto 1 megabyte of memory
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The 8086 uses segmented memory
Prefetches upto 6 instruction bytes from
memory
 To speed up instruction execution
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Larger memory size can be accessed
The internal registers are 16 bit wide. They can address
at most 64 Kbytes at most. The segment+offset approach
produces a 20-bit address to access 1 Mbyte.
Multi-tasking
Each program occupies a particular code segment in
memory. Multiple programs can be stored in different
segments. They can be executed on the same processor.
To switch from one to another, only the content of the
segment register needs to be changed.
 The
memory of 8086-based
microcomputer is organized as bytes.
 The 8086 always access a 16-bit word to
or from memory.
 8086 can be configured as a small
uniprocessor system or as a
multiprocessor system
• Using MN/MX pin
• HIGH –uniprocessor LOW-multiprocessor
 The
8086 family consist of two type of 16
bit microprocessor – the 8086 and 8088.
 The 8086 has a 16-bit external data path
to memory and I/O but the 8088 has a 8bit external data path.
 In most other respects, the processors are
identical.
 8086
is internally divided into two
separate functional units
• Bus Interface Unit (BIU)
 Fetch Instruction
 Read/write data from/to memory or I/O ports
• Execution Unit (EU)
 Execute already fetched instruction
 Segments
can be
contiguous,
partially overlapped,
fully overlapped or
disjoint
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Consider the most commonly used instruction,
MOV destination, source
which attempts to transfer data “from source to destination”.
source is either
• the data itself
• the name of a register, or
• specifying the address where the data is stored
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destination is either
• the name of a register, or
• specifying the address where the data is to be stored
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Addressing modes refer to how the locations of operands
are specified.
 Instructions
using this mode specify the
operand as a signed 8-bit displacement
relative to PC.
 JNC START
 If carry = 0 , then PC is loaded with
current PC contents plus 8-bit signed
value of START;
otherwise, the next instruction is
executed;
 Instructions
using this mode have no
operands.
 CLC clears the carry flag to zero
 40
pins
 16-bit bi-directional data bus.
 20-bit output address bus
 Other pins are for control and status
signals, and communication with
peripherals
 Two operation modes: minimum mode &
maximum mode
 Pins 24 through 31 and pin 34 perform
different functions in minimum mode and
Minimum mode: 8086 generates all
control signals by itself. There are 9 pins
transmitting control signals:
 All are output signals except for HOLD.
 INTA, ALE, DEN, DT/R , M / IO, WR, HLDA,
HOLD, SS0
 Maximum mode: 8088 doesn’t generate
control signals itself. Supporting chips are
used to generate them. The 9 controlsignal pins are saved for other purposes
and the power of the processor will be
maximally utilized.
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 Vcc
and GND (input) provide the power
supply.
 Vcc = 5 volts and GND = 0
 CLK (input) receives the clock signal,
which provides a reference for
synchronization.
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AD0 – AD19 (bidirectional) transmit part of
the address or 16-bit data.
A16/S3 – A19/S6 (output)
carry the most significant 4
bits of the address or 4
status bits.
S3 and S4 indicate the
segment register being
used.
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S5 indicates the Interrupt enable flag.
Low in the S6 indicates 8086 is on the bus, during
hold acknowledge period the 8086 tri-states the
pin and allows another bus master to take control
of the system bus
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Remember that address pins are output !
A latch is like a “buffer” or “bridge” between the CPU
address pins and the system address bus.
The “bridge” allows one-way traffic, which is controlled
by a latch enable signal.
ALE pin from 8088 is used as the latch enable.
 1) When the latch enable is low, the input pins are set at high-
impedance state (don’t care)
 2) When the enable line becomes high, the output is made equal
to the input
 3) If the enable line goes back low again, the output retains its
current value.
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Signal on a tri-state pin may be in one of the three
states:
 1) low (0);
 2) high (1);
 3) open (high impedance)
The “high-impedance” state is equivalent to
disconnecting the pin from both the high and low
voltage sources.
If a pin is in the high-impedance state, it will not
affect the system bus to which it is connected. Thus
the bus can transmit other signals without being
affected.
BHE (output): bus high enable, can be
used to select memory banks
 RD (output):low when reading data from
memory or an I/O location.
 TEST (input): used with WAIT instruction.
 INTR (input): maskable interrupt input,
must be held at high level until
recognized to generate an interrupt.
 NMI (input): nonmaskable interrupt input
activated by a leading edge
 RESET (input): system reset signal
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INTA (tri-state): interrupt acknowledge.
 ALE (output):address latch enable,
indicating that the address is available on
the bus.
 DEN (tri-state): data bus enable,
indicating that the processor is ready to
send or receive data.
 DT/R (tri-state): data transmit/receive,
indicating whether the processor is
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IO/M (tri-state): accessing memory or I/O
?
 WR (tri-state): is the CPU outputting data ?
 HLDA (output):hold acknowledge
 HOLD (input): hold request from external
circuitry
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LOCK (output): lock peripherals off the
system
 RQ/GT1, RQ/GT0 (bi-directional):
request/grant for the direct memory
access (DMA) operation.
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