1 computer architecture cs 215 registers. 2 registers & counters register collection of storage...
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Computer ArchitectureCS 215
Registers
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Registers & Counters
Register Collection of storage elements Set of flip-flops
Counter Register that cycles through a
sequence of states (values)
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Independent data lines
Shared Clock
pulse Write
enable
2-bit Register
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In1
In0CPW
Q1
Q0
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Register Design Models Large numbers of states and input
combinations as n becomes large State diagram/state table model is not
feasible Options?
Add predefined combinational circuits to registers
Design individual cells using the state diagram/state table model and combine them into a register
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Register StorageExpectations:
Store information for multiple clock cycles
“store” or “load” control signalReality:
D flip-flop loads information on every clock cycle
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Register StorageOptions:
1. Signal to block the clock to the register
2. Signal to control feedback of the output of the register back to its inputs
3. Use other SR or JK flip-flops which for (0,0) applied store their state
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Clock Gating
ClockLoad
Gated Clock to FF Gated Clock = Clock + Load
Advantages?
Disadvantages?
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2 to 1 Mux used to control load operation
Note feedback loops
Load-Controlled Feedback
CD Q
C
D Q
ClockIn0
In1
A1
A0
Y1
Y0
Load
2-to-1 MultiplexersBuild this!
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Register Transfer OperationsMovement and processing of data
stored in registersComponents
set of registers operations control of operations
Microoperations Elementary operations Load, count, shift, add, bitwise "OR",
etc.
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Register Notation
Letters and numbers denotes a register Parentheses ( ) denotes a range of register bits Arrow () denotes data transfer Comma separates parallel operations Brackets [ ] specifies a memory address
R 7 6 5 4 3 2 1 0
15 8 7 0 15 0
PC(H) PC(L) R2
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Conditional Transfer K1: (R2 R1)
If (K1 =1) then (R2 R1)
R1 R2
K1
Clock
Loadn
Clock
K1Transfer Occurs Here
No Transfers Occur Here
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Microoperations Logical Groupings:
Transfer - move data from one set of registers to another
Arithmetic - perform arithmetic on data in registers Logic - manipulate data or use bitwise logical
operations Shift - shift data in registers
Logical operations Logical OR Logical AND Logical Exclusive OR Not
Arithmetic operations+ Addition– Subtraction* Multiplication/ Division
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Control ExpressionsAppear to the
left of operations separated by a colon
Specify the logical condition for the operation to occur
Example:
X’ K1 : R1 R1 + R2
X K1 : R1 R1 + R2’ + 1 Variable K1 enables the add or
subtract operation. If X =0, then X’ =1 so K1 = 1,
activating the addition of R1 and R2.
If X = 1, then X K1 = 1, activating the addition of R1 and the two's complement of R2 (subtract).
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Arithmetic Microoperations
Symbolic Designation Description R0 R1 + R2 Addition R0 R1 Ones Complement R0 R1 + 1 Two's Complement R0 R2 + R1 + 1 R2 minus R1 (2's Comp) R1 R1 + 1 Increment (count up) R1 R1 – 1 Decrement (count down)
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Logical MicrooperationsSymbolic
Designation Description
R0 R1 Bitwise NOT
R0 R1 R2 Bitwise OR (sets bits)
R0 R1 R2 Bitwise AND (clears bits)
R0 R1 R2 Bitwise EXOR (complements bits)
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Shift MicrooperationsSymbolic
Designation Description
R1 sl R2 Shift Left
R1 sr R2 Shift Right
R1 Operation
10010010 R1 sl R2
01100100 R1 sr R2
Note: These shifts "zero fill". Sometimes a separate flip-flop is used to provide the data shifted in, or to “catch” the data shifted out.
Other shifts are possible (rotates, arithmetic)
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Register Transfer Structures
Multiplexer-Based Transfers - Multiple inputs are selected by a multiplexer dedicated to the register
Bus-Based Transfers - Multiple inputs are selected by a shared multiplexer driving a bus that feeds inputs to multiple registers
Three-State Bus - Multiple inputs are selected by3-state drivers with outputs connected to a bus that feeds multiple registers
Other Transfer Structures - Use multiple multiplexers, multiple buses, and combinations of all the above
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Multiplexer-Based Transfers
Multiplexers connected to register inputs produce flexible transfer structures (Note: Clocks are omitted for clarity)
Load
R0n
MUX
S
K2
0
1
Load
Load
n
n
K1R2
R1
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Shift Registers
Move data laterally within the register Simplest case, the shift register is a set of
D flip-flops connected in a row Data input, In, is called a serial input or the shift
right input Data output, Out, is often called the serial output The vector (A, B, C, Out) is called the parallel
output.
DQDQDQDQIn
CP
A B C Out
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Parallel Load Shift Registers By adding a MUX between each
shift register stage, data can be shifted or loaded
If SHIFT is low, A and B are replaced by the data on DA and DB lines, else data shifts right on each clock
D QD Q
A B
CP
SHIFT
IN
DA DB
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