Datorarkitektur I
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Datorarkitektur I
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Let’s Have a Look into the CPU
I/O
n
I/O
2
I/O
1
THE CONTROL UNIT
System Bus
1. Why is a Control Unit Needed inside the CPU?
Main
Memory
CPU
2. Microoperations and Control Signals
3. The Control Unit - Basic Tasks
System Bus
IR
PC
Control
Unit
Address Bus
ALU
Data Bus
Registers
5. Microprogrammed Control
Control Bus
CPU
4. Hardwired Control
Internal
CPU Bus
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Datorarkitektur I
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Datorarkitektur I
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Instruction Execution
Let’s Have a Look into the CPU (cont’d)
•
•
The CPU executes a sequence of instructions.
The execution of an instruction is organized as an
instruction cycle: it is performed as a succession of
several steps;
The question that has to be answered:
•
How are the elements inside the CPU and the
interface to the external datapath controlled in order
to work properly?
Fetch
instruction
Execute
instruction
To perform this control, that’s
the task of the Control Unit
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•
Each step is executed as a set of several
microoperations.
•
The task performed by any microoperation falls in
one of the following categories:
- Transfer data from one register to another;
- Transfer data from a register to an external
interface (system bus);
- Transfer data from an external interface to a
register;
- Perform an arithmetic or logic operation, using
registers for input and output.
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Datorarkitektur I
Microoperations and Control Signals
Instruction Execution (cont’d)
Control signals
PCin
Status&Cond.
Flags
PC
Control
unit
PCout
MARin
System bus
MAR
IRin
MBRin
Clock
R0in
Yin
Examples:
b) signals for adding content of Y to that of R0 (result in Z):
R0out, Add, Zin
IRout
MBRout
In order to allow the execution of a microoperation, one
or several control signals have to be issued; they allow
the corresponding data transfer and/or computation to
be performed.
a) signals for transferring content of register R0 to R1:
R0out, R1in
IR
MBR
Clear Y
Fö 11- 6
c) signals for reading a memory location; address in R3:
R3out, MARin, Read
R0
Y
R0out
•
Yout
Add
•
XOR
ALU
Rn-1in
Carry-in
Zin
Rn-1
The CPU executes an instruction as a sequence of
control steps. In each control step one or several
microoperations are executed.
One clock pulse triggers the activities
corresponding to one control step ⇒ for each
clock pulse the control unit generates the control
signals corresponding to the microoperations to be
executed in the respective control step.
Z
Rn-1out
Zout
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Microoperations and Control Signals (cont’d)
Microoperations and Control Signals (cont’d)
instruction:
ADD
R1,R3
R1 ← R1 + R3
instruction:
BR
1
2
3
4
5
6
PCout, MARin, Read, Clear Y, Carry-in, Add, Zin
Zout, PCin
MBRout, IRin
R1out, Yin
R3out, Add, Zin
Zout, R1in, End
instruction:
ADD
R1,(R3)
fetch ins.
PC←PC+1
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unconditional branch (with
relative addressing - see lect. 5)
1
2
3
4
5
6
PCout, MARin, Read, Clear Y, Carry-in, Add, Zin
Zout, PCin
MBRout, IRin
PCout, Yin
(displacement-field)IRout, Add, Zin
Zout, PCin, End
R1 ← R1 + [R3]
control steps and control signals:
1
2
3
4
5
6
7
target
control steps and control signals:
fetch ins.
PC←PC+1
fetch ins.
PC←PC+1
control steps and control signals:
PCout, MARin, Read, Clear Y, Carry-in, Add, Zin
Zout, PCin
MBRout, IRin
indirect addressing
R3out, MARin, Read
R1out, Yin
MBRout, Add, Zin
Zout, R1in, End
Comments:
• The first (three) control steps are identical for each
instruction; they perform instruction fetch and
increment the PC. The following steps depend on
the actual instruction (stored in the IR).
• If a control step issues a read, the value will be
available in the MBR after one additional step.
• Several microoperations can be performed in the
same control step if they don’t conflict (for example,
only one of them is allowed to output on the bus)
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Control Unit
Control Unit (cont’d)
The control unit is driven by the processor clock.
The signals to be generated at a certain moment
depend on:
- the actual step to be executed;
- the condition and status flags of the processor;
- the actual instruction executed;
- external signals received on the system bus
(e.g. interrupt signals).
IR
Control
unit
Control signals
on system bus
Signals from
system bus
Clock
•
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Control signals internal to the CPU
Status&Cond.
Flags
The basic task of the control unit:
- for each instruction the control unit causes the
CPU to go through a sequence of control steps;
- in each control step the control unit issues a set
of signals which cause the corresponding
microoperations to be executed.
•
Fö 11- 10
Techniques for implementation of the control unit:
1. Hardwired control
2. Microprogrammed control
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Hardwired Control
Hardwired Control (cont’d)
•
In this case, the control unit is a combinatorial
circuit; it gets a set of inputs (from IR, flags, clock,
system bus) and transforms them into a set of
control signals.
IR
Instruction
decoder
I2
I3
In
T1
T2
Zin = T1 + T5 ⋅ (ADDreg + BR) + T6 ⋅ ADDreg_ind + . . .
Generation of signal End:
- step 6 of ADD with register addressing
- step 7 of ADD with register-indirect addressing
- step 6 of BR
- ------------------End = T6 ⋅ (ADDreg + BR) + T7 ⋅ ADDreg_ind + . . .
End
Tn
From system bus
Encoder block
(combinatorial signal
generator)
Status&Cond.
Flags
Clock
Reset Step counter
and decoder
I1
Generation of signal Zin:
- first step of all instructions (fetch instruction)
- step 5 of ADD with register addressing
- step 5 of BR
- step 6 of ADD with register-indirect addressing
- -------------------
Control signals
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Control signals
Control
unit
•
Hardwired control provides highest speed.
•
RISCs are implemented with hardwired control.
•
If the instruction set becomes very complex
(CISCs) implementing hardwired control is very
difficult. In this case microprogrammed control units
are used.
Clock
IR
PC
R0
In order to allow execution of register-to-register
operations in a single clock cycle, RISCs (and other
modern processors) use three-bus CPU structures
(see following slide).
Rn-1
ALU
•
Status&Cond.
Flags
Hardwired Control (cont’d)
MBR
MAR
System bus
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Datorarkitektur I
Microprogrammed Control
•
•
Microprogrammed Control Unit
Status&Cond.
Flags
IR
Address
generator
(AG)
µPC
Control
store
(CS)
Microprogrammed control - basic idea:
• All microroutines corresponding to the machine
instructions are stored in the control store.
• The control unit generates the sequence of control
signals for a certain machine instruction by reading
from the control store the CWs of the microroutine
corresponding to the respective instruction.
gen. addr
INCR
•
Control word (CW): a sequence of Nsig bits, where
Nsig is the total number of control signals; each bit
in a CW corresponds to one control signal.
Each control step during execution of an instruction
defines a certain CW; it represents a combination
of 1s and 0s corresponding to the active and nonactive control signals.
Microroutine: a sequence of CWs corresponding to
the control sequence of a machine instruction. An
individual CW in a microroutine is called a
microinstruction.
Fö 11- 16
Clock
The control unit is implemented just like another
very simple CPU, inside the CPU, executing
microroutines stored in the control store.
Sequencer
Signals from
system bus
IAB: Internal Address Bus
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IAB
Control buffer/
decoder
(CB/D)
end, end-fetch
branch
Control signals
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Control Store Organization
Microroutine Executed for Conditional Branch
Addr-fetch
Addr-interr.
Addr-instr0
Addr-instr1
Addr-instrn
Control store
----------------------------------------end-fetch
Fetch
instruction
----------------------------------------branch addr-fetch
Interrupt
routine
----------------------------------------end
Execute
instr. code 0
----------------------------------------end
Execute
instr. code 1
----------------------------------------end
Execute
instr. code n
A_fetch
+1
+2
+3
A_CB
+1
+2
+3
+4
+5
•
•
•
The control store contains the microprogram
(sometimes called firmware).
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Fö 11- 19
Summary
•
•
•
•
•
•
•
•
The control unit is in charge of coordinating the
activities inside the CPU and the interaction with
the outside. It is doing this by issuing in each clock
cycle the appropriate control signals.
A set of control signals activates the
microoperations which have to be executed in a
given control step.
Control units can be implemented hardwired or
microprogrammed.
A hardwired control unit is a combinatorial circuit
which gets a set of inputs and transforms them into
a set of control signals.
A microprogrammed control unit is implemented
like another CPU inside the CPU. It executes
microprogrammes stored in the control store.
Each instruction of the microprogram practically
represents the set of signals which the control unit
has to issue in the respective control step.
Hardwired controllers are faster then
microprogrammed ones. They are used in all
RISCs.
If the instruction set is complex, hardwired
controllers become too complicated. Therefore
CISCs are implemented with microprogrammed
controllers.
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The microroutines contain, beside CWs, also
branches which have to be interpreted by the
microprogrammed controller.
The sequencer is controlling the right execution
sequence of microinstructions. The sequencer is a
small control unit of the control unit.
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Datorarkitektur I
PCout, MARin, Read, Clear Y, Carry-in, Add, Zin
Zout, PCin
MBRout, IRin
end-fetch
this produces the jump to A_CB
-------------branch to A_CB+2 if N set
end
PCout, Yin
(displacement-field)IRout, Add, Zin
Zout, PCin
end