Module 5 part1

MODULE-V
Part-I
Processor Logic Design
1

 Contents
• Register Transfer Logic
 Inter Register Transfer
 Arithmetic Microoperations
 Logic Microoperations
 Shift Microoperations
• Conditional Control Statements
2

3
Registers
 Register
• a collection of binary storage elements
• included a set of flip-flop
• n-bit register store n-bit binary information
 Frequently used to perform simple data
storage and data movement and processing
operations

4
Register and Load Enable
Register with load enable
by clock gating

5
Register transfer
 Large digital systems are often designed by modular,
hierarchical approach
 Large digital systems are partitioned into two types of modules
• Data path: performs data-processing operations
• Control unit: determine the sequence of those operations

6
Register transfer
 The registers are assumed to be basic components of the
digital system
 Register transfer operation: movement on the data stored
in register and the processing performed on the data
 The basic components for describing a digital system in
register transfer logic is:
• The set of registers in digital systems and their
functions.
• The operations performed on the data
• Control on the sequence of operations

Register transfer Operations
 Microoperations: operations executed on data stored in one
or more registers.
 For any function of the computer, a sequence of
micro operations is used to describe it
 The result of the operation may be:
• replace the previous binary information of a register or
• transferred to another register
7

Register Transfer Language
 Register Transfer Language (RTL) : a symbolic notation to
describe the microoperation transfers among registers
Next steps:
• Define symbols for various types of microoperations,
• Describe the hardware that implements these
microoperations
 A statement in a register transfer language consists of a
control function and a list of micro operations.
8

 The type of microoperations in digital systems can be
classified into four categories:
• Interregister –transfer microoperations do not change the
information content when the binary information moves from one
register to another.
• Arithmetic micro operations perform arithmetic on numbers stored
in registers.
• Logic microoperations perform operations such as AND and OR on
individual pairs of bits stored in registers.
• Shift microopeartions specify operations for shift registers.
9

10
InterRegister Transfer
 Computer registers are designated by capital letters
(sometimes followed by numerals) to denote the function
of the register
 R1: processor register
 MAR: Memory Address Register (holds an address
for a memory unit)
 PC: Program Counter
 IR: Instruction Register
 SR: Status Register

 The individual flip-flops in an n-bit register are numbered
in sequence from 0 to n-1 (from the right position toward
the left position)
11
R1
Register R1
7 6 5 4 3 2 1 0
Showing individual bits
A block diagram of a register

12
PC
Numberingof bits
Partitioned intotwoparts
15 0
PC(H)
PC(L)
07815
LowerbyteUpperbyte
Other ways of drawing the block diagram of a register:

13
 Information transfer from one register to another is described
by a replacement operator: R2 ← R1
 This statement denotes a transfer of the content of register R1
into register R2
 The transfer happens in one clock cycle
 The content of the R1 (source) does not change
 The content of the R2 (destination) will be lost and replaced by
the new data transferred from R1
 We are assuming that the circuits are available from the outputs
of the source register to the inputs of the destination register,
and that the destination register has a parallel load capability

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 The condition which determines when the transfer is to occur
is called a control function.
 Conditional transfer occurs only under a control condition
 Representation of a (conditional) transfer
P: R2 ← R1
 A binary condition (P equals to 0 or 1) determines when the
transfer occurs
 The content of R1 is transferred into R2 only if P is 1

 The control function is included with the statement is as
follows:
x’T1: A ← B
 The control function is terminated with a colon.
 It shows that the transfer operation be executed by the
hardware only when the Boolean function x’T1 = 1
• i.e,when variable x=0 and timing variable T1 = 1
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Hardware implementation for the
above statement is as follows
17

18
7-3 Register Transfer Operations
More register transfer operation executed at the same time:
K3: R2 ←R1, R1 ←R2

 Consider the two statements:
 T1: C ← A
 T5: C ← B
• Destination register receives information from two
sources, but not at the same time.
19

Bus and Memory Transfers
 Paths must be provided to transfer information from one register
to another
 A Common Bus System is a scheme for transferring information
between registers in a multiple-register configuration
 A bus: set of common lines, one for each bit of a register,
through which binary information is transferred one at a time
 Control signals determine which register is selected by the bus
during each particular register transfer
21

Memory Transfer
 Memory read : Transfer from memory
 Memory write : Transfer to memory
 Data being read or wrote is called a memory word (called
M)
 It is necessary to specify the address of M when writing
/reading memory
 This is done by enclosing the address in square brackets
following the letter M
 Example: M[0016] : the memory contents at address
0x0016
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 Assume that the address of a memory unit is
stored in a register called the Address
Register AR
 Lets represent a Data Register with DR,
then:
 Read: DR ← M[AR]
 Write: M[AR] ← DR
27

 The transfer of information from a bus into one of many
destination registers is done:
• By connecting the bus lines to the inputs of all
destination registers and then:
• activating the load control of the particular destination
register selected
 We write: R2 ← C to symbolize that the content of register
C is loaded into the register R2 using the common system
bus
 It is equivalent to: BUS ←C, (select C)
R2 ←BUS (Load R2)
29

Arithmetic Microoperations
 The basic arithmetic microoperations are: addition,
subtraction, increment, decrement, and shift
 Addition Microoperation:
R3 ←R1+R2
 Subtraction Microoperation:
R3 ←R1-R2 or :
R3 ←R1+R2+1
31

 One’s Complement Microoperation:
R2 ←R2
 Two’s Complement Microoperation:
R2 ←R2+1
 Increment Microoperation:
R2 ←R2+1
 Decrement Microoperation:
R2 ←R2-1
32

Logic microoperations
 Logic microoperations specify binary operations for a
string of bits stored in registers.
 These operations consider each bit in the registers
seperately and treat it as a binary variable.
 Exclusive –OR operation symbolized by the statement:
F ← A ⊕ B
 If the contents of register A is 1010 and B is 1100, the
information transferred to register F is 0110:
 1010 content of A
 1100 content of B
 0110 content of F ← A ⊕ B
35

 16 different possible logic operations that
can be performed with two binary variables,
 The symbol ∨ will be used to denote OR
microoperation and the symbol ∧ to denote
an AND microoperation.
 Complement microoperation is the sameas
1’s complement and use a bar on top of the
letter that denotes the register.
36

Shift Microoperations
 Used for serial transfer of data
 Also used in conjunction with arithmetic, logic, and other
data-processing operations
 The contents of the register can be shifted to the left or to
the right
 As being shifted, the first flip-flop receives its binary
information from the serial input
 Three types of shift: Logical, Circular, and Arithmetic
38

CONDITIONAL CONTROL STATEMENTS
 Conditional control statement is symbolized by an if-then –
else statement as follows:
 P: If (condition) then [microoperation(s)] else
[microoperation(s)]
• It means that if the control condition stated within the
parentheses after the word if is true, then the
microoperation enclosed within the parentheses after
the word then is executed.
• If the condition is not true, the microoperation listed
after the word else is executed.
• In any case the control function P must occur for
anything to be done.
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 Eg:
 T2: If(C=0) then (F ← 1) else (F ← 0)
 F is assumed to be a 1-bit register(flip-flop) that can be set
or cleared.
 If register C is a 1-bit register, the statement is equivalent
to the following two statements:
C’T2: F ←1
CT2: F ←0
• Only one of the microoperation will be executed during T2,
depending on the value of C
47

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