Chapter 4 Register Transfer

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Chapter 4Register Transfer Microoperations
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Describing Digital Systems

• Digital systems are very complex.
• They are usually designed and built of different
  modules that perform specific tasks.
• The best way to define the operation of these
  modules is by specifying the registers they
  contain and the operations possible on the data
  in these registers.

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Example

• We can define an adder as a module that
• Interacts with three registers
• Two source registers.
• One destination register.
• Performs the addition operation on the contents
  of the source registers and places the result in
  the destination register.

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Microoperations

• Operations that specify every modification to the
  data in the registers are called
  microoperations.
• They are simple operations performed on
  information stored in one or more registers.
• Each assembly language instruction can be broken
  down into multiple microoperations.

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Example

• The ADD B instruction of the 8085 is made up of
  the following microoperations
• Move the contents of the accumulator to
  temporary register W.
• Move the contents of the B register to temporary
  register X.
• Add W to X.
• Move result of addition back to the accumulator
  and set the appropriate flags.
• Increment the program counter.

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Designing a Hardware System

• When designing a hardware system, one must
  specify the following items about the system
• The set of register it contains and their
  functions.
• The set of microoperations that can be performed
  on the information in these registers.
• The sequencing and control of these
  microoperations.
• The microoperations and the control will be
  translated into the hardware that make up the
  system.

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Register Transfer Language

• We can use plain English to specify these
  registers, operations and their control.
• However, the result will be very long and verbose
  descriptions.
• It would be much easier to use symbols to
  represent (abstract) some of the information.
• This abstraction is called a Register Transfer
  Language RTL.
• Register Transfer operations are performed on
  data stored in registers and the results are
  transferred either back to the same registers or
  to other registers.

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Register Transfer Language

• Register Transfer Languages are abbreviated,
  concise representations of the actual
  description.
• They contain ALL the necessary information in
  shortened descriptions.
• They contain ONLY the necessary information.
• These languages require that the designer learns
  a few symbols to represent the different entities
  and operations in the digital system.
• The result is a very powerful tool for expressing
  hardware design.

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Register Transfer Language

• The language can be classified into some main
  statement types
• Register Transfer Operations.
• Bus and Memory Transfer Operations.
• Arithmetic Operations.
• Logic Operations.
• Shift Operations.

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Register Transfer Operations

• These are operations that simply denote the
  transfer of the binary data from one register to
  another.
• First, some definitions
• Special purpose registers are represented using
  capital letters that specify their function
• PC Program Counter, AR Address Register,
• General purpose registers are either represented
  using an alphabetically ordered list, or an
  enumerated list.
• A, B, C,
• R1, R2, R3,
• Bits in an n-bit register are numbered from 0
  (least significant) to n-1 (most significant).

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Register Transfer Operations

• Register transfers are represented in the
  following manner
• Destination ? Source
• This means, all of the bits of the source
  register are copied into the destination
  register.
• For example,
• R2 ? R1

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Conditions

• The previous operation requires that the
  destination register must have some control which
  would allow the loading of the register at the
  required point in time.
• This LOAD input would only be activated under
  certain conditions
• If (P 1)
• R2 ? R1
• This, we write in the following way
• P R2 ? R1

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Hardware Implications

• Translating into hardware
• P R2 ? R1
• There has to be two registers called R1 and R2.
• R2 must have parallel load capability.
• There has to be n wires connecting the output of
  R1 to the inputs of R2.
• There has to be a control signal called P
  connected to the load input of R2.

Control Unit
P
R2
LD
n
R1
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Bus Transfer Operations

• How do we connect all of the registers in the
  Microprocessor to each other?
• Separate connection paths?
• Too many wires.
• Common bus scheme.
• This is the most often used technique to reduce
  the number of connections needed.
• What is a bus?
• A bus consists of a set of common wires that are
  accessed by multiple modules or sub-systems.
• And, a mechanism to control who is allowed to use
  it.

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Bus Control

• To allow the multiple modules to access the bus
  without interfering with each other, some
  mechanism is required to control who uses the bus
  when.
• One common way of controlling access to the bus
  from several registers is by utilizing a set of
  multiplexers.

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Controlling Access to the Bus
Common Wires
n
n
Control Unit
n-bit 4 X 1 MUX
S0
S1
n
n
n
n
R1
R2
R3
R4
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Controlling access to the bus

• The previous diagram assumes that only the
  registers will access the bus.
• If other modules may use the bus, Tri-state
  buffers must be used to isolate the bus from the
  modules not using it.
• The tri-state buffers can be added after the
  outputs of the multiplexers.
• OR
• It is possible to replace the multiplexers with
  the buffers. However, controlling them would
  require the same amount of logic as the
  multiplexers.

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Controlling Access to the Bus
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Bus Transfer Representation

• In RTL, busses are usually referred to as BUS1,
  BUS2,
• A transfer involving the bus can be designated in
  one of two ways
• BUS ? C, R1 ? BUS
• OR
• R1 ? BUS ? C
• Even if there is only one bus and all operations
  utilize it, it is useful to explicitly show the
  bus in the operations.
• It is a common resource that is constantly under
  contention.

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Memory Transfer Operations

• A memory read operation has the following stages
• The address is sent to the memory system
• The memory system produces the word stored in the
  specified location
• The memory word is read back into the digital
  system.
• In most digital systems
• A special register called the address register
  (AR) holds the address during the memory
  operation.
• A special register called the data register (DR)
  holds the data before or after the operation.

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Memory Transfer in RTL

• In RTL, a memory transfer is represented in a
  manner similar to a simple register transfer,
  except that the source or destination is replaced
  by MAR.
• Read DR ? MAR
• Write MAR ? DR
• MAR means the memory location whose address is
  in the address register AR.

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Arithmetic Microoperations

• The five main arithmetic microoperations that one
  encounters when designing any digital systems
  are
• Addition.
• Subtraction.
• Increment.
• Decrement.
• Shift.
• Logical Shift.
• Arithmetic Shift.

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Arithmetic Microoperations

• Addition
• R3 ? R1 R2
• Subtraction
• R3 ? R1 R2 1
• Increment
• R2 ? R2 1
• Decrement
• R2 ? R2 - 1

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Arithmetic Microoperations

• What about Multiplication and Division?
• Multiplication and Division are either
  implemented directly in hardware
• Then a separate single microoperation is used to
  specify them.
• Or, they may be implemented by repeated addition
  or repeated subtraction.
• Then a sequence of addition, subtraction, and
  shift microoperations are used to specify them.

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Hardware Implications

• Binary Adder/Subtractor

B3
A3
B2
A2
B1
A1
B0
A0
M
FA
FA
FA
FA
S3
C4
S2
S1
S0
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Hardware Implications

• The above circuit can implement all of the
  following operations

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Hardware Implications

• What is missing?
• Any operation that needs to use the input carry.
• Add with Carry
• Subtract with Borrow
• The input carry is being used to form the 2s
  complement.
• If the above operations are needed, a completely
  different circuit must be used.

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Hardware Implications Arithmetic Circuit
Cin
A0
S1
S0
B0
A1
B1
A2
B2
A3
B3
0
D0
D1
D2
D3
Cout
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Hardware Implications Arithmetic Circuit
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Logic Microoperations

• Logic microoperations specify binary operations
  for strings of bits stored in registers.
• These operations treat the values in the
  registers simply as a string of unrelated bits.
• These operations process the data in the
  registers bit-wise. i.e. bit by bit.

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Logic Microoperations

• There are many possible logic operations.
• However, the following are the most common (and
  useful).
• Clear (set to 0) F ? 0
• AND F ? A ? B
• OR F ? A ? B
• XOR F ? A ? B
• NOT F ? A
• Set (set to 1) F ? 1

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Hardware Implications

• Logic circuit (per bit).

Ai
Bi
S0
0
1
2
3
4 X 1 MUX
Control Unit
S1
Ei
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Hardware Implications

• The above circuit can implement all of the
  mentioned operations.

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Shift Microoperations

• Shift microoperations are mainly used for serial
  transfer of data.
• They are also used in conjunction with
  arithmetic, logic and other operations.
• The contents of a register can be shifted to the
  left or to the right.
• At the same time, the first bit (from the right
  or the left) receives its input from the serial
  input.
• In all shift operations, the source and
  destination registers must be the same.

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Types of Shift Operations

• Logical shift.
• A shift where the serial input is set to 0.
• shl and shr.
• R1 ? shl R1
• R1 ? shr R1
• Circular shift (rotate).
• A shift where the serial input is set to the last
  bit.
• cil and cir.
• R1 ? cil R2
• R1 ? cir R2

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Types of Shift Operations

• Arithmetic shift right
• Shifting to the right divides the number by 2.
• The serial input is set to the sign bit to
  preserve the sign of the number.
• ashr.
• R1 ? ashr R1

R0
R1
R2
Rn-2
Rn-1
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Types of Shift Operations

• Arithmetic shift left
• Shifting to the left multiplies the number by 2.
• The serial input is set to 0.
• The sign bit (Rn-1) is replaced by bit Rn-2.
• If Rn-2 is not equal to Rn-1, shift overflow
  occurs.
• ashl.
• R1 ? ashl R1

0
R0
R1
R2
Rn-2
Rn-1
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Hardware Implications
A0
A1
A2
A3
0
A3
0
A3
A0
0
cil
cir
shl
shr
ashl
ashr
0 Shift Right 1 Shift Left
s
s
s
s
0
1
0
1
0
1
0
1
2X1 MUX
2X1 MUX
2X1 MUX
2X1 MUX
H0
H1
H2
H3
A Register Output H register Input
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Arithmetic Logic Shift Circuit - ALU

• Taking all of the above together, we can design a
  composite circuit that implements all of the
  arithmetic, logic and shift operations.
• To facilitate this and not introduce too much
  connection complexity, most computer designs
  employ temporary registers.
• The information is transferred to these registers
  before an ALU operation.
• The ALU operates on the contents of these
  registers.
• The results of the ALU operation are stored in
  the destination register directly.