1. Block diagram of a Microcoded Control unit
Starting address from opcode
Clk
Other
External source
4 to 1 mux for choosing the source of micro-PC …
Microprogram
Controller
MUX
Inc
Branch Address
Micro-PC
Microprogram
Memory
Micro-Branch Control
Micro-IR …
Control Signals

2. Review

3. CS501 Advanced Computer Architecture
Lecture22
Dr.Noor Muhammad Sheikh

4. Block diagram of a Microcoded Control unit
Starting address from opcode
Clk
Other
External source
4 to 1 mux for choosing the source of PC …
Microprogram
Controller
MUX
Holds the address of next micro-instruction
Inc
Branch Address
Micro-PC
Microprogram
Memory
Micro-Branch Control
Micro-IR …
Control Signals

5. Block diagram of a Microcoded Control unit
Starting address from opcode
Clk
Other
External source
4 to 1 mux for choosing the source of PC …
Microprogram
Controller
MUX
Holds the address of next micro-instruction
Inc
Branch Address
Micro-PC
Microprogram
Memory
Micro-Branch Control
contains control words
Micro-IR …
Control Signals

6. Block diagram of a Microcoded control unit
Starting address from opcode
Clk
Other
External source
4 to 1 mux for choosing the source of PC …
Microprogram
Controller
MUX
Holds the address of next micro-instruction
Inc
Branch Address
Micro-PC
Microprogram
Memory
Micro-Branch Control
contains control words m b
Holds current micro-instruction
Micro-IR …
Control Signals

7. Fields in the micro-instruction
Micro-IR

8. Fields in the micro-instruction
Micro-IR
c bits control
signal field
m bits branch
address field
b bits branch
control field

9. Loading the micro-PC Simple increment Lookup table External source
The micro-PC can be loaded from one of the four possible
sources
Simple increment
Lookup table
External source
Branch address

10. Microprogram Memory
contains micro-routines for all the instructions in the ISA
Input: address
Output: control word
Faster, smaller

11. Layout of a typical microprogram memory
Micro- Address
Memory
Contents
Microcode for instruction fetch …
Microcode for load instruction
Microcode for add instruction
Microcode for br instruction
2n-1
Microcode for reset instruction
Layout of a typical microprogram memory

12. Example: Control Signals for the sub Instruction
100 … 1
101
102
203
204
205
Branch Control
Address
PC out
Cout
Cout
R2Bus
LMAR LC
LPC
LIR LA
Bus2R
INC4
Read
LMBR
MARout
SUB
RAE
RBE
RCE
END

13. Example: Control Signals for the sub Instruction
100 … 1
101
102
203
204
205
Branch Control
Address
PC out
Cout
Cout
R2Bus
LMAR LC
LPC
LIR LA
Bus2R
INC2
Read
LMBR
MARout
SUB
RAE
RBE
RCE
END
Microcode for instruction fetch

14. Example: Control Signals for the sub Instruction
100 … 1
101
102
203
204
205
Branch Control
Address
PC out
Cout
Cout
R2Bus
LMAR LC
LPC
LIR LA
Bus2R
INC2
Read
LMBR
MARout
SUB
RAE
RBE
RCE
END
Microcode for instruction fetch
Microcode for sub instruction

15. Branching Controls in Microcontroller
Neg flag
Zero flag N Z 2
External Address
Branch Address 2
From IR
Brn 2 2
MUX
Brp
inc 2
Brz 2
Micro-PC
Brnz
MicroController
Branch
Microprogram
Memory
MUX Control
Control Signals BR 2 ….

16. 4-1 Multiplexer
External Address
Branch Address
The multiplexer supplies one of the four possible values to the micro-PC
From IR 2
MUX
Mux Control
inc
Mux Control
Select 00
Increment micro-PC 01
Opcode from IR 10
External address 11
Branch address
Micro-PC

17. Normal flow of execution
4-1 Multiplexer
External Address
Branch Address
The multiplexer supplies one of the four possible values to the micro-PC
From IR 2
MUX
Mux Control
inc
Mux Control
Select 00
Increment micro-PC 01
Opcode from IR 10
External address 11
Branch address
Micro-PC
Normal flow of execution

18. 4-1 Multiplexer
External Address
Branch Address
The multiplexer supplies one of the four possible values to the micro-PC
From IR 2
MUX
Mux Control
inc
Mux Control
Select 00
Increment micro-PC 01
Opcode from IR 10
External address 11
Branch address
Micro-PC
Normal flow of execution
Initial loading of microroutine

19. 4-1 Multiplexer
External Address
Branch Address
The multiplexer supplies one of the four possible values to the micro-PC
From IR 2
MUX
Mux Control
inc
Mux Control
Select 00
Increment micro-PC 01
Opcode from IR 10
External address 11
Branch address
Micro-PC
Normal flow of execution
Initial loading of microroutine
For resetting etc.

20. 4-1 Multiplexer
External Address
Branch Address
The multiplexer supplies one of the four possible values to the micro-PC
From IR 2
MUX
Mux Control
inc
Mux Control
Select 00
Increment micro-PC 01
Opcode from IR 10
External address 11
Branch address
Micro-PC
Normal flow of execution
Initial loading of microroutine
For resetting etc.
Implementing conditional &
unconditional branches

21. How to form a branch Condition unconditional not zero zero positive
negative
Address
From IR
External Address
Branch Address

22. Microcode branching Examples

23. Microcode Branching Examples
Action
Equivalent C
construct
200 00 …
xxx
No branch,goto next address in sequence-201
{…};
201 01 1
To the address supplied by opcode
{…}; goto initial address;
202 10
To external address if Z flag is set
{…}; if Z then goto Ext. Add.
203 11
To 300 if N flag is set, else to 204
{…}; if N then goto Label1;
204
000
406
To 206 if N is false, else to 205
While (N) {...};
205
404
Branch to 204
While contd…
Control Signals
Branch Address
Address
Mux control
Branch
brnz
brz
brp
brn

24. Horizontal microcode scheme
Microprogram
Memory
Micro-PC
…..
Cin
PCout
INC2

25. Vertical Microcode Scheme
Microprogram
Memory
Micro-PC
DataPath
n to 2n decoder
PCout
INC2 …

26. Microcoded 1-bus SRC design

27. A Unibus Data Path Implementation
General purpose registers
(32-bits each)
ALSU C A R0
R31 PC IR
32 lines
<31..0>
A Unibus Data Path Implementation
MAR
MBR …
add
sub
shl
Other ALSU functions
Internal processor bus R1
Repeat
To external CPU bus

28. Hard wired control unit for the SRC
31 … 27
26…24 …
opcode ra
Instruction register
Repeat
0 NOP
5 to 32
decoder
1 LOAD 1 2 3 4
2 LOADR
3 STORE
4 STORER
5 LOADA …
31 STOP
Enable
Logic ‘1’
bit 27
bit 31

29. Hard wired control unit for the SRC
Repeat
…..
31…27 26…22 21…17 16…12 11 IR
opc
ra rb rc
Control signals to register file
RAE 5 5 5
RCE
RBE
LR0
LR1
LR31
BUS2R
….. R0 R1
R31
R0out R0 5
5-to-32 decoder
R1out R1
…..
R31out
R31
R2BUS

30. Microcoded Control Unit for the SRC
Starting address from opcode
Clk
Other
External source …
SRC Micro-controller
MUX
Inc
Branch Address
Micro-PC
Microprogram
Memory
Micro-Branch Control
Micro-IR …
Control Signals

31. The SRC Microcontroller
The microcontroller for the SRC microcoded control unit employs the logic for handling exceptions and reset process
Since the SRC does not have any condition codes, we use the CON and n signals instead of N and Z flags to control branches

32. The SRC Microcontroller2
External Address
Branch Address 2
From IR 2 2
4-1
MUX 2 2
inc
MicroController
Micro-PC
MuxControl
Branch
Br(CON=0)
Br(n=1)
Br(n=0)
END

33. Microcode for some SRC instructions

34. The shl Instruction The signals for instruction fetch are not shown here as they are same for all instructions
RTL
300 00 …
xxx
n IR<4…0>;
301 11 1
303
n!=0:Micro-PC ;
302
N R[rc]<4…0>
C R[rb];
304
306
n=0:Micro-PC
305
C<31..0>0C<31..1>,n n-1,
Micro-PC 304
100
R[ra] C; Micro-PC ;
Branch
Address
MuxControl
Branch
Br(CON=0)
Br(n=1)
Br(n=0)
End
PCout
LMAR
Control Signal

35. Alternative approaches to microcoding
Bit ORing
Nanocoding
Writable Microprogram Memory
Subroutines in Microprogramming