Intel 8086/8088 Microprocessors

Intel 8086 and 8088 Microprocessors are
the basis of all IBM-PC compatible
computers
(8086 introduced in 1978, first IBM-PC released in 1981)


All Intel, AMD and other advanced
microprocessors are based on and are
compatible with the original 8086/8
At Power Up and Reset time, Pentiums,
Athlons etc all look like 8086 processors
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Intel 8086/8088 Microprocessors

Intel 8086 is a 16b microprocessor:


Width of external data bus:






16b data registers, 16b ALU
8086: 16b
8088: 8b
Width of external address bus: 16b+4b=20b
Some techniques to optimise the CPU
performance when it’s executing programs
Segment: Offset memory model
Little-Endian Data Format
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8086/8088 (1)


Original IBM PC used 8088 microprocessor
8088 is similar to the 8086, but it has an
external 8b data bus & only 4B-deep queue




For cost reduction reasons
We can consider 8086 and 8088 together
PC clones often used 8086 for better
performance
8-bit bus reduces performance, but meant
cheaper computers
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8086/8088 (2)




Remember the Fetch-Decode-Execute cycle?
Fetching from EXTERNAL MEMORY is SLOW
The 8086/8 used an instruction queue to
speed up performance
While the processor is decoding and
executing an instruction, its bus interface
can be reading new instructions, since at
that time the bus is not actually in use
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8086/8088 Functional Units
Bus Interface
Unit(BIU)
Fetches Opcodes,
Reads Operands,
Writes Data
Execution Unit
(EU)
8086/8088 MPU
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8086/8088 (3)

8086/8088 consists of two internal units




The execution unit (EU) - executes the
instructions
The bus interface unit (BIU) - fetches
instructions, reads operands and writes results
The 8086 has a 6B prefetch queue
The 8088 has a 4B prefetch queue
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8086/8088 Internal Organisation
EU
BIU
Address Bus 20 bits
AH
AL
BH
BL
CH
CL
DH
DL
SUMMATION
Data Bus
CS
DS
SP
SS
BP
ES
DI
IO
BI
Bus
Control
Internal
Communications
Registers
8088
Bus
Temporary
Registers
Instruction Queue
ALU
EU
Control
1
2
3
4
Flags
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BIU Elements

Instruction Queue: the next instructions or data can be
fetched from memory while the processor is executing
the current instruction


Segment Registers:





The memory interface is slower than the processor execution
time so this speeds up overall performance
CS, DS, SS and ES are 16b registers
Used with the 16b Base registers to generate the 20b address
Allow the 8086/8088 to address 1MB of memory
Changed under program control to point to different segments
as a program executes
Instruction Pointer (IP) contains the Offset Address of
the next instruction, the distance in bytes from the
address given by the current CS register
8086/8088 20-bit Addresses
CS
16-bit Segnment Base Address
0000
IP
16-bit Offset Address
20-bit Physical Address
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Exercise: 20-bit Addressing
1.
2.
CS contains 0A820h,IP contains 0CE24h.
What is the resulting physical address?
CS contains 0B500h, IP contains 0024h.
What is the resulting physical address?
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8086/8 In Circuit (1)



8086/8 microprocessors need support
circuits in a microcomputer system
8086/8 multiplex the address and data
buses on the same pins
This saves pins but at a price:

Demultiplexing logic is needed to build up
separate address and data buses to interface
with RAMs and ROMs
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MAXIMUM
MODE
GND
1
Vcc
AD14
AD15
AD13
A16,S3
AD12
A17,S4
AD11
A18,S5
AD10
A19,S6
AD9
/BHE,S7
AD8
MN,/MX
AD7
/RD
AD6
/RQ,/GT0
HOLD
/RQ,/GT1
HLDA
AD4
/LOCK
/WR
AD3
/S2
IO/M
AD2
/S1
DT/R
AD1
/S0
/DEN
AD0
QS0
ALE
NMI
QS1
/INTA
8086
AD5
INTR
/TEST
CLK
READY
GND
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MINIMUM
MODE
20
21
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RESET
12
MAXIMUM
MODE
GND
1
40
MAXIMUM
MODE
MINIMUM
MODE
GND
Vcc
1
40
Vcc
AD14
AD15
A14
A15
AD13
A16,S3
A13
A16,S3
AD12
A17,S4
A12
A17,S4
AD11
A18,S5
A11
A18,S5
AD10
A19,S6
A10
A19,S6
AD9
/BHE,S7
A9
high
AD8
MN,/MX
A8
MN,/MX
AD7
/RD
AD7
/RQ,/GT0
HOLD
AD6
/RQ,/GT1
HLDA
AD5
AD4
/LOCK
/WR
AD3
/S2
AD2
MINIMUM
MODE
/SS0
/RD
/RQ,/GT0
HOLD
/RQ,/GT1
HLDA
AD4
/LOCK
/WR
IO/M
AD3
/S2
IO/M
/S1
DT/R
AD2
/S1
DT/R
AD1
/S0
/DEN
AD1
/S0
/DEN
AD0
QS0
ALE
AD0
QS0
ALE
NMI
QS1
/INTA
NMI
QS1
/INTA
AD6
8086
AD5
8088
INTR
/TEST
INTR
/TEST
CLK
READY
CLK
READY
RESET
GND
GND
20
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20
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RESET
13
8086/8 In Circuit (2)


In Maximum Mode the 8086/8 needs at
least the following: 8288 Bus Controller,
8284A Clock Generator, 74HC373s and
74HC245s
With the aid of these devices the 8086
begins to look like the ideal
microprocessor we looked at earlier
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i8086 Circuit - Maximum Mode
CLK
Vcc
8284A
Clock
Generator
S0#
S1#
S2#
CLK
READY
RESET
8288
Bus
Controller
DEN
DT/R#
ALE
RDY
8086
CPU
MRDC#
MWTC#
AMWC#
IORC#
IOWC#
AIOWC#
INTA#
MN/MX#
LE
OE#
BHE#
AD15:AD0
A19:A16
ADDR/DATA
74LS373
x3
A19:A0,
BHE#
INTR
DIR
EN#
ADDR/Data
74LS245
74LS245
x2
x2
D15:D0
8086/8 Maximum Mode

In maximum mode, the 8288 uses a set of
status signals (S0, S1, S2) to rebuild the
normal bus control signals of the
microprocessor



MRDC#, MWTC#, IORC#, IOWC# etc
Equivalent to MEMR# etc
Look at some special signals briefly
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RESET# Signal





The Active low RESET# signal puts the 8086/8
into a defined state
Clears the flags register, segment registers etc.
Sets the effective program address to 0FFFF0h
(CS=0F000h, IP=0FFF0h)
8086/8 Programs always start at 0FFFF0H after
Reset has been asserted and removed
Continues into latest generation CPUs
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BHE# Signal (8086 Only)



The 8086 processor can address memory a
byte at a time
Its data bus is 16b wide
It uses the BHE# signal and A0 (sometimes
called BLE#) to address bytes using its 16b
bus
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Use of BHE#/A0(BLE#)
Byte-Wide addressing
(8088)
ODD Addresses (8086)
EVEN Addresses (8086)
FFFFF
FFFFF
FFFFE
FFFFE
FFFFD
FFFFC
FFFFD
FFFFC
FFFFB
FFFF9
FFFFA
FFFF8
A19..A1
A19..A1
00002
00005
00004
00001
00003
00002
00000
00001
00000
D15:D8
D7:D0
BHE#
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A0/BLE#
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Use of BHE#/BLE#
BHE#
A0/BLE#
0
0
Whole word (16-bits)
0
1
High byte to/from odd
address
1
0
Low byte to/from even
address
1
1
No selection
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ALE and Address/data Bus
Multiplexing



8086/8 Multiplexes the Address and Data
signals onto the same set of pins
Need off-chip logic to separate the signals
Transparent latches designed just for
address demultiplexing
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ALE and 74HC373 Transparent Latch
Clock
Address/
Data
Bus
Address
Time
Data Time
ALE
Output of
74HC373
Microcomputer AddressBus
74HC373 or equivalent
Address/
Data Bus
In0:In7
ALE
Q0:Q7
System Address Bus
LE
OE#
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TriState Control signal,
OE#, shown connected to
GND for simplicity
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Use of ALE (Address Latch Enable)



ALE is used with an external latch
(74HC373) to demultiplex the address and
data lines
74HC373 is transparent when its LE input
(connected to ALE) is high
When ALE goes low, the ‘373 holds the last
data until ALE goes high again
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8288 Bus Controller and Bus
Transceivers
8288 Bus Controller also
generates Direction and
Enable signals for BiDirectional Transeivers
8288
Bus Controller
DEN#
DT/R#
Supports Buffering the
System Data Bus
CPU [D15:D8]
74HC245
Buffered [D15:D8]
EN#
DIR
CPU [D7:D0]
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74HC245
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Buffered [D7:D0]
To Memory and I/O
Systems
EN#
DIR
DIR
24
8086 Read Cycle
T1
T2
T3
T4
CLK
/S0, /S1, /S2
A16..A19, /BHE
001 or 101
Address
Status
S3..S6
ALE
AD0..AD15
Address
A0..A19
float
Valid Data
float
Valid Address
DT/R
DEN
/MRDC or /IORC
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8086 Write Cycle
T1
T2
T3
T4
CLK
/S0, /S1, /S2
A16..A19, /BHE
010 or 110
Address
Status
Address
Valid Data
S3..S6
ALE
AD0..AD15
A0..A19
Valid Address
DT/R
DEN
/MWTC or /IOWC
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8086 Read Cycle
T1
(1 Wait State)
T2
T3
Tw
T4
CLK
/S0, /S1, /S2
A16..A19, /BHE
001 or 101
Address
Status
S3..S6
ALE
8284 RDY
READY
AD0..AD15
Address
A0..A19
float
Valid Data
float
Valid Address
DT/R
DEN
/MRDC or /IORC
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8086/8088 Summary






First Generation (introduced June 1978)
One of the first 16b processors on the
market
16b internal registers
16/8b external data bus
20b address bus (1MB addressable)
Used in 1st generation IBM PCs (1981)
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80186/80188





Evolution of 8086/8088 80186/80188
Increased instruction set
On-chip system components (Clock
generator, DMA, Interrupt, Timers…)
Unsuccessful in PCs
Popular in embedded systems…
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2nd Generation Processor 286







P2 (286) = 2nd Generation Processor
Introduced in 1981
CPU behind IBM AT
Throughput of original IBM AT (6MHz) was about
500% of IBM PC (4.77MHz)
Level of integration: 134k transistors (vs 29k in
8086)
Still a 16b processor…
Available in higher clock frequencies: 25MHz
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2nd Generation Processors 286

Fully backwards compatible to 8086
80286 runs 8086 software without modification

Improved instruction execution
Average instruction takes 4.5 cycles vs. 12 cycles (8086)


Improved instruction set
Real mode and Protected Mode
Multitasking-support. What happens in one area of memory doesn’t affect
other programs. Protected mode supported by Windows 3.0.



16MB addressable physical memory
On-chip MMU (1GB virtual memory)
Non-multiplexed address-bus and data-bus
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Improving Computer Performance




We’ve seen how 16b computer
technology based on the 8086 and
80286 processors developed
These computers are not powerful
enough for today’s applications
How do you improve the performance
of your computer?
Let’s start with the CPU
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CPU Performance (1)




MOST OBVIOUS: Processor Clock Frequency
Increased frequency – increased execution
rate
State of the Art: >4GHz (03/2005)
Memory and I/O access times can be
performance bottleneck – unless you take
some special measures
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CPU Performance (2)

ALU register width



A processor is an n-bit processor, where N represents
the precision of the ALU – N can be 4, 8, 16, 32, or 64
The wider the registers – the more processing per
clock
Data bus width


The wider the data bus the faster we can transfer data
Since the memory and I/O device access times are
finite, the more bits transferred per cycle the better
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CPU Performance (3)





Address bus width
Increased address width doesn’t provide a
‘speed’ increase as such
CPU can directly address more memory
PCs use big programs, which would not fit in a
smaller address space
Overcoming small address space takes time

Impacts on overall system performance
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3rd Generation Processor 386



P3 (386) = 3rd Generation Processor
Introduced: 10/1985
Full 32b processor
(32b registers. 32b internal and external databus. 32b address bus)

275k transistors. CMOS. 132-pin PGA package.
(Supply current Icc=400mA. Roughly the same as 8086 !)


Clock speeds: 16-33MHz
P3 processors were far ahead of their time:
It took 10 years before 32b operating systems became mainstream!

First 386 PCs early 1987
(COMPAQ)
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3rd Generation Processor 386

Modes of operation:


Real. Protected. Virtual Real.
Protected mode of 386 is fully compatible
with 286
Protected mode=native mode of operation. Chips are designed for
advanced operating systems such as Windows NT

New virtual real mode
Processor can run with hardware memory protection while simulating
the 8086’s real-mode operation. Multiple copies of e.g. DOS can run
simultaneously, each in a protected area of memory. If a program in
one memory area crashes, the rest of the system is protected.
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Intel 32-bit Architecture:IA-32
Address
Addressing Unit
(AU)
Bus Unit (BU)
Prefetch Queue
Data
Execution Unit (EU)
ALU
Control
Unit (CU)
Instruction Unit (IU)
Registers
The 80386 includes a Bus Interface Unit for reading and providing data and instructions,
witha Prefetch Queue, an IU for controlling the EU with its registers, as well as an AU for
generating memory and I/O addresses
80386 Features








32b general and offset registers
16B prefetch queue
Memory management unit with segmentation unit and
paging unit
32b address and data bus
4GB physical address space
64TB virtual address space
i387 numerical coprocessor
Implementation of real, protected and virtual 8086 modes
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80386 Operating Modes


Protected Mode for Multitasking support
Real Mode (native 8086 mode)


Processor powers up in Real Mode
System Management Mode


Power management or system security
Processor switches to separate address space, while
saving the entire context of the currently running
program or task
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80386 Register Set
Instruction Pointer
31
16 15
EIP
EFLAG
IP
General-Purpose Registers
16 15
31
EFLAG Register
16 15
31
0
8 7
EAX
AH
AL
EBX
BH
BL
ECX
CH
CL
EDX
DH
DL
ESI
SI
EDI
DI
EBP
BP
ESP
SP
FLAG
Segment Registers
15
0
0
CS
SS
DS
ES
FS
GS
E0
80386 Prefetch Queue
Execution Unit
16-byte deep
Instruction Queue
Fetching from
on-chip Queue
is fast
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Bus Interface Unit
32-bit Data
Bus
Reading from off-chip
Memory is slow
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80386 Prefetch Queue

1.
2.
80386 Prefetch queue is 16B deep
The instruction fetch can read from the
prefetch queue faster than from memory
The prefetcher can do some work while
the execution unit is doing other tasks in
parallel
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Coprocessor: i387


The hardware implementation of floating
point processing in the i387 means floating
point operations run at much higher speed.
The i386 can execute all mathematical
expressions using software emulation of the
i387.
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80386: Classic CISC Processor






CISC = Complex Instruction Set Computer
Complex instructions
...but code-size efficient
Micro-encoding of the machine instructions
Extensive addressing capabilities for
memory operations
Few, but very useful CPU registers
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80386 Execution Sequence
Coprocessor
Microcode
ROM
Microcode
Queue
Control Unit
Register
Register
Register
Execution Unit
Decoding Unit
Prefetch Queue
Bus Interface
CISC Processor
Register
ALU
In a microprogrammed CISC the processor fetches the instructions via the bus interface into a
prefetch queue, which transfers them to a decoding unit. The decoding unit breaks the machine
instruction into many elementary micro-instructions and apples them to a microcode queue. The
micro-instructions are transferred from the microcode queue to the control and execution unit which
drives the ALU and the registers
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80386 Complex Instructions





CISC drawback: Most instructions are so
complicated, they have to be broken into a
sequence of micro-steps
These steps are called Micro-Code
Stored in a ROM in the processor core
Micro-code ROM: Access-time and size...
They require extra ROM and decode logic
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RISC: Less is More



RISC = Reduced Instruction Set Computer
20/80 Rule: 20% of the instructions take up
80% of the time
Sometimes executing a sequence of simple
instructions runs quicker than a single
complex machine instruction that has the
same effect
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RISC Ideas (1)

Reduce the instruction set to simplify the
decoding



Smaller Instruction Set -> Simpler Logic ->
Smaller Logic -> Faster Execution
Eliminate microcode – hardwire all
instruction execution
Pipeline instruction decoding and executing
– do more operations in parallel
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RISC Ideas (2)

Load/Store Architecture – only the load and
store instructions can access memory


All other instructions work with the processor
internal registers
This is necessary for single-cycle execution – the
execution unit can’t wait for data to be
read/written
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RISC Ideas (3)



Increase number of internal register due to
Load/Store Architecture
Also registers are more general purpose and less
associated with specific functions
Compiler designed along with the RISC processor
design. Compiler has to be aware of the
processor architecture to produce code that can
be executed efficiently
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Instruction Pipelining - Operations
Can Be Carried Out in Parallel





Read the instruction from memory or the
prefetch queue (instruction fetch phase)
Decode the instruction (decode phase)
Where necessary, fetch the operands
(operand fetch phase)
Execute the instruction (execute phase)
Write back the result (write-back phase)
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Instruction Fetch
Decode
Operand Fetch
Execution
Write-back
Pipelined Execution
Instruction
k
Instruction
k-1
Instruction
k-2
Instruction
k-3
Instruction
k-4
Result k-4
Cycle n+1
Instruction
k+1
Instruction
k
Instruction
k-1
Instruction
k-2
Instruction
k-3
Result k-3
Cycle n+2
Instruction
k+2
Instruction
k+1
Instruction
k
Instruction
k-1
Instruction
k-2
Result k-2
Cycle n+3
Instruction
k+3
Instruction
k+2
Instruction
k+1
Instruction
k
Instruction
k-1
Result k-1
Cycle n+4
Instruction
k+4
Instruction
k+3
Instruction
k+2
Instruction
k+1
Instruction
k
Result k
Cycle n
Superscalar Architecture


The processor may have more than one
pipeline (Pentium…)
Where possible each pipeline works
independently


Not always possible
May achieve average completed execution
of more more than one instruction per clock
cycle
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Pipeline Challenges

More logic per pipeline stage – same
resource can’t be used twice


E.g. can’t re-use ALU for computing implied
addresses
Synchronisation Problems


Delayed Jump/Branch
Data and Register dependency, e.g.
ADD reg1, reg2, reg7
AND reg6, reg1, reg3
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Getting the Benefits of Pipelining




Simplified Instruction decoding
 Simpler, faster logic
On-chip cache memories
 Local memory on-chip to avoid memory
access bottlenecks
Floating Point pipeline for FP coprocessor
Speculative Execution to get around pipeline
flushes
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Software Implications of RISCs

Optimising Compiler must know how
pipeline works
(Compiler must be aware of pipeline delays, and
insert NOPs if need be)
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Lower code density in RISC because
instructions are less efficient
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PowerPC code takes up to 30% more code
to do the same tasks as an x86 CPU
more memory accesses, potential
performance impact...
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80486: IA-32 with RISC elements
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Introduced 04/91
Greatly improved 80386 CPU
Hard-wired implementation of frequently used instructions
(as in RISCs). On average 2 clock cycles/instruction.
5 stage instruction pipeline
Internal L1 Cache Memory (8kB) + cache controller
On-chip Floating Point coprocessor (FPU)
Longer Prefetch Queue (32-bytes as opposed to 16 on the
80386)
Higher frequency operation: up to 120MHz
>1.2M transistors, 0.8mm CMOS. 168-pin PGA.
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D31-D0
Control and
Status Signals
Segmentation
Unit
Paging
Unit
Decoding
Unit
Bus Interface
A31-A0
Cache
(8K
bytes)
Prefetcher
(32-byte
queue)
80486 Block Diagram
Control
Unit
Register
and ALU
Floating
Point Unit
i486 CPU
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Cycle n
Cycle n+1
Cycle n+2
Write-back
ADD eax,
mem32
Decode ADD,
fetch mem32
Decode ADD
(continued)
Add eax and
mem32
Cycle n+3
Write result
into eax
Cycle n+4
06/03/2005
Execution
Decode 2
Decode 1
(memory access)
Instruction Fetch
80486 Pipeline
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