HUMBOLDT-UNIVERSITÄT ZU BERLIN
INSTITUT FÜR INFORMATIK
COMPUTER ARCHITECTURE
Lecture 9
MICROPROGRAMMED CONTROL UNIT
Sommersemester 2002
Leitung: Prof. Dr. Miroslaw Malek
www.informatik.hu-berlin.de/rok/ca
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MICROPROGRAMMED CONTROL UNIT
Microprogramming is an orderly method of designing the control unit
of a conventional computer (Wilkes 1951). The term
icroprogramming is based on the analogy between sequence of
transfer required to execute a machine instruction and the sequence
of individual instructions in conventional user program.
Each step is called microinstruction and complete set of steps
required to process a machine instruction is called the microprogram.
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BASIC LAYOUT OF A MICROPROGRAMMED CONTROL
UNIT
Control memory stores a microprogram corresponding to each op-code of a
conventional instruction.
IR
Op
OR
Control
memory
Microinstruction
register
Micro op Address
Address
Control address
register
Conditions
FFs
Control signal
to various functional
subunits
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Decoder
BASIC EXECUTION SEQUENCE
Control sequence for execution of the instruction "Add contents
of a memory location addressed in absolute mode to register R1."
Step
Action
1
PCout, MARin, Read, Clear Y, Set carry-in to ALU, Add, Zin
2
Zout, PCin, Wait for MFC
3
MDRout, IRin
4
Address-field-of-IRout, MARin, Read
5
R1out, Yin, Wait for MFC
6
MDRout , Add, Zin
7
Zout , R1in, End
Control sequence for an unconditional branch instruction.
Step
Action
1
PCout, MARin, Read, Clear Y, Set carry-in to ALU, Add, Zin
2
Zout, PCin, Wait for MFC
3
MDRout, IRin
4
PCout, Yin
5
Address-field-of-IRout , Add, Zin
6
Zout , PCin, End
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MICROCODE EXECUTION
1. Op-code is decoded.
2. Microinstructions are retrieved from control memory (control address register
and the decoder serve as the address register and selection mechanism or
control unit).
3. The control address register locates the microinstruction to be retrieved from
control memory.
4. The microinstruction register holds the retrieved microinstruction - micro opcode and address of the next microinstruction in the control memory
5. Current microinstruction is executed.
6. The address of the next microinstruction is entered into the control memory to
retrieve the next microinstruction.
7. If all microinstructions were executed, then store next op-code of conventional
instruction in the control address register, if not, execute remaining
microinstruction.
8. Conditional jumps are implemented by letting the states of some conditional
flip-flops modify the address of theCAnext
microinstruction to be retrieved.
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WILKES' IMPLEMENTATION OF MICROPROGRAM
CONTROL (1951)
Matrix A
Matrix B
Decoding tree
Timing
pulse
Control lines
to gates in
arithmetic unit,
control registers, etc.
Control
address
register
From operation
field of
microinstruction
register
Address for
next microinstruction
From
conditional
flipflop
Delay
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VARIATIONS IN WILKES' IMPLEMENTATION
1. Microinstruction address selection.
2. Instruction set strategies (control word organization).
Ad 1. Possible modifications to address selection:
1) The control address register is modified to function as a counter when the next
microinstruction follows the current address and as a register when a branch is
required (similar to Wilkes' scheme).
- control address acts as a counter or a register
2) Addition of a register to hold the address of the next microinstruction (µPC).
3) Control memory may be organized in two storage matrices each with its own
decoding tree, thus providing faster operation at lower cost, since the size of the
decoder is reduced.
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MICROPROGRAMMED CONTROL UNIT WITH A
MICROPROGRAM COUNTER (µPC)
Instruction
register
IR
Opcode
AC = 0
AC < 0
MQ(n-1)
COUNT=n-2
V
Decoding
circuits
K
Branch address
MUX
Load branch
address
Condition
select
Increment
Reset
µPC
8
Control memory
(256 x 36 bits)
3
8
36
CMDR
25
Control signals
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MICROPROGRAMMED CONTROL UNIT WITH TWO
STORAGE MATRICES
IR
Op-code
Control-address register
Control memory
(address) matrix
Control memory read register
Control memory
(command) matrix
Control signals
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VARIATIONS IN WILKES' IMPLEMENTATION
Ad 2. Variations on control organisation
1) No encoding
0
Control fields
1
2
13
Control lines
2) Some encoding
0
Decoder 0
Control fields
1
2
Decoder 1
Decoder 2
Control lines
3) Complete encoding
Single control field
Decoder
Control lines
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Nanodata's Computer
Bus 1
Bus 8
Bus 2
Bus 3
Bus 9
Shifter
Main memory
(256 x 18 bits)
32 18-bit general registers
(local store)
Main
ALU
Secondary
(index) ALU
32 18-bit external registers
(ext. store)
Bus 1
Bus 5
Bus 10
Bus 6
I/O
devices
Bus 7
Control signals
32 6-bit registers (F store)
3
10
Bus 11
Bus 12
NSDR
360
Control memory 2
(nano store)
10
Branch
address
Page address
nPC
Nanoprogram
counter
7
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Control memory 1
(control store)
EXAMPLE OF IBM FORMATS
90-bit microinstruction format of the IBM S/360 Model 50
0 1
6
19
30
CM addressing information
Parity of bits 0 to 30
31
Parity of bits 32 to 55
61
65
68
Unused
56
60
Parity of bits 57 to 89
83
89
CM addressing
information
horizontal
vs.
vertical
= Control fields
Unused
32-bit microinstruction format of the IBM S/370 Model 145
0
8
control field
(op-code)
24
16
Operand 1
Operand 2
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31
CM addressing
information
SIZE OF CONTROL STORE VS.
WIDTH OF MICROINSTRUCTIONS
DEC VAX (Models 730, 750, 780)
IBM 370 (Models 135, 145, 155, 165)
IBM 360 (Models 30, 40, 50, 65)
Control store size (Kbits)
750
(6Kx80)
500
730
(16Kx24)
400
135
(24Kx16)
155
(6Kx69)
145
(12Kx32)
300
40
(4Kx52)
200
100
30
(4Kx50)
0
20
40
60
Microinstruction width (bits)
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165
(4Kx105)
780
(4Kx105)
65
(2.75Kx87)
50
(2.75Kx85)
80
100
SUMMARIZING
Next address of microcode execution may be specified by:
1) Address field of the current microinstruction
2) µPC (microprogam counter)
3) Address from address control store
Microinstruction formats:
1) No encoding
2) Some encoding
3) Complete encoding
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FIRMWARE ENGINEERING =
MICROPROGRAMMING + ...
Practical application of scientific knowledge in the design and construction of microprograms.
Firmware engineering, in analogy to software engineering, covers specification and design of
microprograms, construction techniques, debugging, testing, verification and maintenance.
DESIGN
DEVELOPMENT
PERIODS
Engineering practice during microcode design:
1) Specification (what is to be accomplished; at what cost)
2) Construction (microassemblers, HLL‘s for firmware are needed,
difficulty of counting HLL-microcode)
3) Verification
4) Testing
5) Debugging
6) Maintenance
HARDWARE
Earlier it was thought that firmware would
narrow hardware/software gap;
what happened it introduced two gaps:
FIRMWARE
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SOFTWARE
DESIGN OF MICROPROGRAMS (FIRMWARE)
1) SPECIFICATION
- What is to be accomplished and at what cost?
2) CONSTRUCTION
- there are no widely accepted HLL‘s for microprogramming
- microassembler is used
- need for HLL-to-microcode translation (very difficult in case of horizontally
microprogrammed machine)
- microcode optimization is NP(nonpolinomial)-hard problem
- microassembler allows to write more understandable and maintainable code.
- mnemonic is defined for each microoperation
- HLL optimization: minimize size of control memory, execution time and length of control
word
Microprogram support system is a collection of utilities that supports the construction,
debugging, testing and maintenance of microprograms. It has: microassembler, simulator
for testing and loading dynamic & static microprograms
3) VERIFICATION
- attempts to prove the absence of errors
4) TESTING
- attempts to discover errors, easier to test than software because of number of
microinstructions is usually not larger than 3000.
5) DEBUGGING
- to correct errors, after locating them first
6) MAINTENANCE
- repair and updating of microprograms
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ADVANTAGES & APPLICATIONS
OF MICROPROGRAMMING
1) THE SYSTEMATIZATION OF CONTROL
2) IMPROVEMENT IN PERFORMANCE
a) a high degree of parallelism in data paths e.g., multiple bit microinstructions are
performed in one cycle
b) a high degree of decision logic (in table search and sorting routines)
3) COMPUTER-SERIES COMPATIBILITY
Compatibility of instruction sets between smaller and larger machines of a series,
e.g., Intel286, 386, Pentium, IBM Systems/309x, Motorola 68000 series
4) EMULATION
Emulation is the combined software/hardware interpretation of the machine instruction of
one machine by another. Target‘s machine architecture is mapped onto the host machine.
EMULATOR - a set of microprograms that interpret a particular instruction set or language
L1. Computer C1 emulates computer C2 if it can interpret machine language L2.
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ADVANTAGES & APPLICATIONS
OF MICROPROGRAMMING (2)
5) MICRODIAGNOSTICS
Microprogramming diagnostic routines have allowed refinements and increased the speed
of detecting and localizing faults,including error detection and correction of microstorage
itself.
a) software diagnostics
b) hardware diagnostics (test generation methods)
c) microdiagnostics
6) SOFTWARE SUPPORT
eases programming
7) SPECIAL-PURPOSE DEVICES
e.g, special processors for data communication, data aquisition, device controllers
8) DYNAMIC MICROPROGRAMMING
This allows routines to be easily microprogrammed. Computer can be restructured to
represent any instruction vocabulary by use of writable control memory (WCM). It allows
the instruction set of the machine to be changed and be TAILORED to specific
applications.
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TAXONOMY OF CONTROL UNITS
CONTROL UNIT
Decoder
one level
decoder
simple tree
decoder
dual tree
decoder
Sequencer
Synchronous
Asynchronous
control unit
Clocks
Hard wired
Fixed
counter
Microprogrammed
Fixed counter
with variable
modulus
Flexible
counter
Vertical
Ring
counter
Horizontal
residual
control
packed
Technology
nano control
unpacked
decoder
Control Store
ROM
PLA
R/W
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PROM
ERROM
EAROM
Technology