Lecture 15
Microarchitecture Level:
Level 1
Microarchitecture Level
• The level above digital logic level.
• Job: to implement the ISA level above it.
• The design depends on:
—ISA being implemented
—Cost
—Performance goal
Microarchitecture Level
• Many modern ISAs, e.g. RISC, have simple
instructions that can usually be executed in
a single clock cycle.
• Other complex ISAs, e.g. Pentium IV, may
require many clock cycles to execute single
instruction.
Basic Elements of Processor
•
•
•
•
•
ALU
Registers
Internal data paths
External data paths
Control Unit
Types of Micro-operation
•
•
•
•
Transfer data between registers
Transfer data from register to external
Transfer data from external to register
Perform arithmetic or logical operations
Functions of Control Unit
• Sequencing
—Causing the CPU to step through a series of
micro-operations
• Execution
—The CU causes each micro-op to be
performed.
• This is done using Control Signals
Control Signals
• Clock
—One micro-instruction (or set of parallel microinstructions) per clock cycle
• Instruction register
—Op-code for current instruction
—Determines which micro-instructions are
performed
• Flags
—State of CPU
—Results of previous operations
• From control bus
—Interrupts
—Acknowledgements
Model of Control Unit
Control Signals - output
• Within CPU
—Cause data movement
—Activate specific functions
• Via control bus
—To memory
—To I/O modules
Example Control Signal Sequence Fetch
• MAR <- (PC)
—Control unit activates signal to open gates
between PC and MAR
• MBR <- (memory)
—Open gates between MAR and address bus
—Memory read control signal
—Open gates between data bus and MBR
Example: Data Paths and Control Signals
Internal Organization
• Usually a single internal bus
• Gates control movement of data onto and
off the bus
• Control signals control data transfer to
and from external systems bus
• Temporary registers needed for proper
operation of ALU
Microprogrammed Control
• A microprogram has a sequence of
instructions in a microprogramming
language.
—These are very simple instructions that
specify micro-operations.
Microprogrammed Control
• A microprogrammed control unit is a simple
logic circuit that is capable of:
—Sequencing through microinstructions
—Generating control signals to execute
each microinstruction.
• As in a hardwired control unit, the
control signal generated by a
microinstruction are used to cause
register transfers and ALU operations.
Control Unit Organization
Micro-programmed Control
• Use sequences of instructions to control
complex operations.
• Called micro-programming or firmware
Micro-programmed Control
• All the control unit does is generate a set
of control signals.
• Each control signal is on or off.
• Represent each control signal by a bit.
• Have a control word for each microoperation.
• Have a sequence of control words for each
machine code instruction.
• Add an address to specify the next microinstruction, depending on conditions.
Micro-programmed Control
• Today’s large microprocessor
—Many instructions and associated register-level
hardware
—Many control points to be manipulated
• This results in control memory that
—Contains a large number of words
– co-responding to the number of instructions to be
executed
—Has a wide word width
– Due to the large number of control points to be
manipulated
Micro-program Word Length
• Based on 3 factors
—Maximum number of simultaneous microoperations supported
—The way control information is represented or
encoded
—The way in which the next micro-instruction
address is specified
Micro-instruction Types
• Each micro-instruction specifies single (or
few) micro-operations to be performed
— (vertical micro-programming)
• Each micro-instruction specifies many
different micro-operations to be
performed in parallel
—(horizontal micro-programming)
Vertical Micro-programming
•
•
•
•
Width is narrow
n control signals encoded into log2 n bits
Limited ability to express parallelism
Considerable encoding of control
information requires external memory
word decoder to identify the exact control
line being manipulated
Horizontal Micro-programming
• Wide memory word
• High degree of parallel operations possible
• Little encoding of control information
Typical Microinstruction Formats
Compromise
• Divide control signals into disjoint groups
• Implement each group as separate field in
memory word
• Supports reasonable levels of parallelism
without too much complexity
Organization of
Control Memory
Control Unit
Store a set of
microinstructions
For vertical
microinstructions
only
Control Unit Function
1. Sequence logic unit issues read command
2. Word specified in control address register is
read (from control memory) into control buffer
register
3. Control buffer register contents generates
control signals and next address information
4. Sequence logic loads new address into control
address register based on next address
information from control buffer register and ALU
flags
Next Address Decision
• Depending on ALU flags and control buffer
register
—Get next instruction
– Add 1 to control address register
—Jump to new routine based on jump
microinstruction
– Load address field of control buffer register into
control address register
—Jump to machine instruction routine
– Load control address register based on opcode in IR
Advantages and Disadvantages of
Microprogramming
• Advantage:
—Simplifies design of control unit
– Cheaper
– Less error-prone
• Disadvantage:
—Slower
Microprogramming is the dominant technique for
implementing control unit in pure CISC.
Hard Wired Control Unit
• Disadvantage:
—Complex sequencing & micro-operation logic
—Difficult to design and test
—Inflexible design
—Difficult to add new instructions
• Advantage:
—Faster
Hardwired control unit is typically use for
implementing control unit in pure RISC.
Tasks Done By Microprogrammed
Control Unit
• Microinstruction sequencing
• Microinstruction execution
• Must consider both together
Design Considerations
• Size of microinstructions
• Address generation time
—Determined by instruction register
– Once per cycle, after instruction is fetched
—Next sequential address
– Common in most designed
—Branches
– Both conditional and unconditional
Sequencing Techniques
• Based on current microinstruction,
condition flags, contents of IR, control
memory address must be generated.
• Based on format of address information
—Two address fields
—Single address field
—Variable format
Branch Control
Logic:
Two Address
Fields
Branch Control
Logic: Single
Address Field
Branch Control
Logic: Variable
Format
Execution
• The cycle is the basic event
• Each cycle is made up of two events
—Fetch
– Determined by generation of microinstruction
address
—Execute
– Effect is to generate control signals
– Some control points internal to processor
– Rest go to external control bus or other
interface
Control Unit
Organization
A Taxonomy of Microinstructions
• Classification of microinstruction:
—Vertical/horizontal
—Packed/unpacked
—Hard/soft microprogramming
—Direct/indirect encoding
How to Encode
• K different internal and external control
signals
• Wilkes’s (first proposed the use of
microprogrammed control unit in 1951):
—K bits dedicated
—2K possible combination of control signals.
• Not all used
—Two sources cannot be gated to same
destination
—Register cannot be source and destination
—Only one pattern presented to ALU at a time
—Only one pattern presented to external control
bus at a time.
How to Encode
• Require Q < 2K which can be encoded with
log2Q < K bits
• Not done
—As difficult to program as pure decoded
(Wilkes) scheme.
—It is complex and therefore slow control logic
module.
• Compromises
—More bits than necessary are used.
—Some combinations that are physically
allowable are not possible to encode.
Microinstruction Encoding
• In practice, microprogrammed control
units are not designed using:
—a pure unencoded or
—horizontal microinstruction format.
• At least some degree of encoding is used
to reduce control memory width and to
simplify the task of microprogramming.
Specific Encoding Techniques
•
•
•
•
Microinstruction organized as set of fields
Each field contains code
Activates one or more control signals
Organize format into independent fields
—Field depicts set of actions (pattern of control
signals)
—Actions from different fields can occur
simultaneously
• Alternative actions that can be specified
by a field are mutually exclusive
—Only one action specified for field could occur
at a time
Microinstruction Encoding
Direct Encoding
Microinstruction Encoding
Indirect Encoding
The Data Path:
Example
The data path is the
part of CPU
containing ALU, its
input and its output.
Mic-1
The Data Path: Example( cont…)
ALU
functions is
determined
by the 6
control lines.
Useful combinations of ALU signals and the function
performed.
Microinstruction Control: The Mic-1
• To control the datapath of Mic-1, we need
29 signals:
—9 signal to control writing data from the C bus
into registers.
—9 signal to control enabling registers onto the
B bus for ALU i/p.
—8 signal to control the ALU and shifter
functions.
—2 signal (not shown) to indicate memory
read/write via MAR/MDR.
—1 signal (not shown) to indicate memory fetch
via PC/MBR.
Microinstruction Control: The Mic-1
• The values of 29 control signals specify
the operations for one cycle of the data
path.
• A data path consists of:
1. Gating values out of registers and onto the B
bus.
2. Propagating the signals through the ALU and
shifter.
3. Driving them onto the C-bus.
4. Writing the results in the appropriate
register(s).
5. In addition, if a memory read data signal is
asserted, the memory operation is started at the
end of the data cycle, after MAR has been loaded.
Microinstruction Control: The Mic-1
•
Issues:
— It may be desirable to write the o/p on the C bus
into many registers.
— It is never desirable to enable more than one
register write onto the B bus at a time – physical
damage.
By adding a 4-to-16 decoder, we can reduce
the number of bits needed to select among the
possible sources for driving the B bus.
We use 9+4+8+2+1=24 signals to control data
path for one cycle.
How to determine the
next cycle?
Need 2 additional fields:
NEXT_ADDRESS field
JAM field.
The microinstruction format for the Mic-1
36 signals
Address of a
potential next
microinstruction.
Determine how the
next microinstruction
is selected.
Microinstruction Control: The Mic-1
•
At each cycle, the sequencer must
produce:
— The state of every control signal in the system.
— The address of the next microinstruction.
•
•
The control store is a memory that holds
microinstructions.
In this example, we have 512x36-bit
control word. We need:
— MicroProgram Counter (MPC) as a pointer to the
control store memory address.
— MIR (MicroInstruction Register) to hold the
current microinstruction.
Microinstruction Control: The Mic-1
•
The operation of Mic-1:
1. MIR is loaded from the word in the control store
pointed by MPC.
2. Once MIR is et up, the various signals propagate out
into the data path.
3. A register content is put out onto the B bus and the
ALU knows which operation to perform.
4. The ALU perform the task after the inputs stable.
5. After ALU, N, Z and shifter outputs are stable, the N
and Z values are then saved in a pair of 1-bit flipflops.
6. Load the registers from bus C; and load N and Z flipflops onto MPC to determine the next
microinstruction.
The complete
block
diagram of our
example
microarchitect
ure, the
Mic-1.
Thank you
Q&A