MicroProgrammed Explained .

GROUP MEMBERS :
•Muhammad Umar (R1F15Bscs0015)

• Hardwired
• Microprogrammed
Instruction code
Combinational
Logic Circuits
Memory
Sequence
Counter
.
.
Control
signals
Control
signals
Next Address
Generator
(sequencer)
CAR Control
Memory
CDR
Decodin
g
Circuit
Memory
.
.
CAR: Control Address Register
CDR: Control Data RegisterInstruction code
Thursday, October 26, 2017
3

Control Unit :
The control unit (CU) is a component of a
computer's central processing unit (CPU)
that directs the operation of the processor.
It tells the computer's memory,
arithmetic/logic unit and input and output
devices on how to respond to a program's
instructions.
4

Hardwire :
Hardwire control is a control
mechanism that generates control signals
by using an appropriate Finite State
Machine (FSM). Actually we sort it on
circuit.
The disadvantage is that wrong wiring
waste waste the whole circuit and time .
5

Microprogrammed :
Microprogrammed control is a control
mechanism that generates control signals
by reading a memory called a Control
Storage (CS) that certain control signals.
It’s like a chips or bio’s and digital clock
watch (Masjid) is a chiped
microprogrammed.
Flexibility (change time etc )…
6

HARDWIRED CONTROL
UNIT
MICROPROGRAMMED
CONTROL UNIT
1) Speed is fast.
2) More costlier.
3) Occurrence of error is more.
4) Control functions implemented in
hardware
5) Not flexible to accommodate new
system specification or new
instruction redesign is required.
6) Difficult to handle complex instruction
sets.
7) Complicated design process.
8) Complex decoding and sequencing
logic.
9) More chip area.
10) Application:
Mostly RISC microprocessor.
1) Speed is slow.
2) Cheaper.
3) Occurrence of error is less.
4) Control functions implemented in
software
5) More flexible to accommodate new
system specification or new
instructions
6) Easier to handle complex instruction
sets
7) Orderly, systematic and simple
design process.
8) Easier decoding and sequencing
logic
9) Less chip area.
10) Application:
Mainframes some microprocessors
7

Control Signals.
Control Variables.
Control Word.
Control Memory.
Microinstructions.
Microprogram.
8

• Control Signals :
Group of bits used to select paths in Multiplexer,decoder
and Arithematic logic units.
• Control Variables :
Multiple Micro-operations
i.e a=a+b
• Binary variables specify micro-operations
• Control Word :
String of 1’s and 0’s represented control variables.
• Control Memory :
Memory certain control words
9

• Micro-instructions:
Instructions that makes microprogrammed
are called micro-instruction.
• In micro instruction write program and stored in chip
o Control words stored in control memory.
o Specify control signals for execution of micro operation.
• Micro-program:
Sequence of micro-instruction
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• Are the funcional, or
atomic, operations of a
processor.
• A single micro-operation
generally involves a
transfer between
registers, transfer
between registers and
external bus, or a
simple ALU operation.
11

 Read-only memory (ROM)
 Content of word in ROM at given address specifies
microinstruction
 Each computer instruction initiates series of
microinstructions (micro program) in control memory
 These microinstructions generate micro operations to
• Fetch instruction from main memory
• Evaluate effective address
• Execute operation specified by instruction
• Return control to fetch phase for next instruction
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Micro-program are stored in ROM. That
memory is called Control Memory.
(Like A Library)
Address
Control
memory
(ROM)
Control word
(microinstruction)
13

 Control memory:
• Contains microprograms (set of microinstructions)
• Microinstruction contains
 Bits initiate microoperations
 Bits determine address of next microinstruction
Control
word
Next Address
Generator
(sequencer)
CAR
Control
Memory
(ROM)
CDR
External
input
14

 Control address register (CAR) :
• Specifies address of next microinstruction
 Next address generator (microprogram
sequencer)
• Determines address sequence for control memory
 Microprogram sequencer functions
• Increment CAR by one
• Transfer external address into CAR
• Load initial address into CAR to start control
operations
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 Control data register (CDR)- or pipeline
register:
• Holds microinstruction read from control memory
• Allows execution of microoperations specified by
control word simultaneously with generation of next
microinstruction
 Control unit can operate without CDR
Control
word
Next Address
Generator
(sequencer)
CAR
Control
Memory
(ROM)
External
input
16

Routine :
• Every block of code is “Routine.”
• Group of microinstructions stored in control
memory.
Each computer instruction has its own
microprogram routine to generate
microoperations that execute the
instruction.
17

 Subroutine:
• Sequence of microinstructions used by other routines
to accomplish particular task
 Example:
• Subroutine to generate effective address of operand
for memory reference instruction
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 Branching from one routine to another depends on
status bit conditions.
 Working on the base of condition.
e.g :
{ if a>50;
cout<<message;
----------
----------
----------
}
 Unconditional branch
• Fix value of status bit to 1
e.g :
Goto X;
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Each computer instruction has its own
microprogram routine stored in a given
location of the control memory
Mapping:
• Transformation from instruction code bits to
address in control memory where routine is
located
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 Each routine must be able to branch to the next routine
in the sequence. An initial address is loaded into the
CAR when power is turned on; this is usually the
address of the first microinstruction in the instruction
fetch routine. Next, the control unit must determine the
effective address of the instruction.
 Address sequencing capabilities required in control
unit
• Incrementing CAR
• Unconditional or conditional branch, depending on status bit
conditions
• Mapping from bits of instruction to address for control memory
• Facility for subroutine call and return
22

1940
opcode 940
 1 is the opcode.
 940 is the Address.
---------------------------------------------
------------------------------------------
---------------------------------------
CONTROL UNIT
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 Computer instruction format:
 Four computer instructions:
(EA is Effective Address)
I Opcode Address
15 14 11 10 0
ADD 0000 AC  AC + M[EA]
BRANCH 0001 if (AC < 0) then (PC  EA)
STORE 0010 M[EA]  AC
EXCHANGE 0011 AC  M[EA], M[EA]  AC
Symbol OP-code Description
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• F1,F2,F3 : Micro-operation fields.
• CD : Condition for branching
• BR : Branch Field
• AD : Address Field
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CD Condition Symbol Comments
00 Always = 1 U Unconditional branch
01 DR(15) I Indirect address bit
10 AC(15) S Sign bit of AC
11 AC = 0 Z Zero value in AC
BR Symbol Function
00 JMP CAR  AD if condition = 1
CAR  CAR + 1 if condition = 0
01 CALL CAR  AD, SBR  CAR + 1 if condition = 1
CAR  CAR + 1 if condition = 0
10 RET CAR  SBR (Return from subroutine)
11 MAP CAR(2-5)  DR(11-14), CAR(0,1,6)  0
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 Sample Format :
 Label May be empty or may specify symbolic address
terminated with colon
 Micro-ops Consists of 1, 2, or 3 symbols separated by commas
 CD One of {U, I, S, Z}
U: Unconditional Branch
I: Indirect address bit
S: Sign of AC
Z: Zero value in AC
 BR One of (JMP, CALL, RET, MAP)
 AD One of (Symbolic address, NEXT, empty).
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MicroProgrammed Explained .

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