Comparch 03 CPU

CHAPTER 3: CPU
1
Computer
Architecture and Organization
Computer Architecture and Organization
LEARNING OBJECTIVES
 Present an overview of essential characteristics of machine
instructions.
 Describe the types of operands used in typical machine
instruction sets.
 Describe the various types of addressing modes common in
instruction sets.
Computer Architecture and Organization 2

WHAT IS AN INSTRUCTION
SET?
 The operation of the processor is determined by the instructions it
executes, referred to as machine instructions or computer instructions.
 The collection of different instructions that the processor can execute is
referred to as the processor’s instruction set.
 Elements of a Machine Instruction
 Operation code: Specifies the operation to be performed (e.g., ADD, I/O).
 Source operand reference: operands that are inputs for the operation.
 Result operand reference: The operation may produce a result.
 Next instruction reference: address of the next instruction.
 Source and result operands can be in one of four areas: Main or virtual
memory, Processor register, immediate and I/O Device

INSTRUCTION
REPRESENTATION
 Within the computer, each instruction is represented by a sequence of
bits.
 The instruction is divided into fields
 It’s difficult to deal with binary representations of machine instructions.

 Common practice: use a symbolic representation of machine instructions.
E.g. ADD, SUB, MUL…etc.

INSTRUCTION TYPES
 Any program written in a high-level language must be translated into
machine language to be executed.
 Thus, the set of machine instructions must be sufficient to express any
of the instructions from a high-level language.
 With this in mind we can categorize instruction types as follows:

 Data processing: Arithmetic and logic instructions.
 Data storage: Movement of data into or out of register and or memory
locations.
 Data movement: I/O instructions.
 Control: Test and branch instructions.

NUMBER OF ADDRESSES
 What is the maximum number of addresses one might need in an
instruction?
 In most architectures, many instructions have one, two, or three operand
addresses, with the address of the next instruction being implicit
(obtained from the PC).
 With 3 addresses, 2 source operand locations and a destination operand

location.
 ADD A, B, C //A = B+C
 Not common:- require a relatively long instruction format to hold the three
address references.

NUMBER OF ADDRESSES
 With two-address instructions, and for binary operations, one address
must do double duty as both an operand and a result.
 SUB Y, B //Y = Y – B
 The two-address format reduces the space requirement but also alters the
value of an operand,
 MOVE instruction is used to move one of the values to a result or temporary
location before performing the operation.
 MOVE A, Y , SUB A, B , answer in A
 Reduces length of instruction but requires some extra work
 Temporary storage to hold some results
 Simpler yet is the one-address instruction. For this to work, a second

address must be implicit (AC).
 C = A – B => LOAD A, SUB B, STOR C
 Common on early machines

NUMBER OF ADDRESSES
 Write a three, two and one address instructions that could be used to
compute
Y = (A - B)/[C + (D * E)]
 Note that there are four instructions and that the original expression had
five operands.
 Try not to alter the value of any of the operand locations.
 Try to compute the expression with smallest no of instructions possible.

CON…
 It is, in fact, possible to make do with zero addresses for some instructions.
 Zero-address instructions are applicable to a special memory organization called

a stack.
 The stack is in a known location and, often, at least the top two elements are in
processor registers.
 zero-address instructions would reference the top two stack elements.
10
SO HOW MANY ADDRESSES?
 The number of addresses per instruction is a basic design decision.
 More addresses
 More complex instructions
 Fewer instructions per program
 Fewer addresses
 Less complex instructions
 More instructions per program
 Require less complex processor
 It also results in instructions of shorter length.
 Results in longer execution times

INSTRUCTION SET DESIGN
The design of an instruction set is very complex because it affects so many
aspects of the computer system.
The most important of these fundamental design issues include the following:
 Operation list: How many and which operations to provide, and how
complex operations should be.
 Data types: The various types of data upon which operations are performed.
 Instruction format: Instruction length (in bits), number of addresses, size of

various fields, and so on.
 Registers: Number of processor registers that can be referenced by
instructions, and their use.
 Addressing: The mode or modes by which the address of an operand is
specified.
TYPES OF OPERAND
 Addresses
 Numbers
 Integer/floating point
 Characters
 ASCII etc.
 Logical Data
 Bits or flags

TYPES OF OPERATION
 Data Transfer
 Arithmetic
 Logical
 Conversion
 I/O
 System Control
 Transfer of Control

DATA TRANSFER
 Specify
 Source
 Destination
 Amount of data
 Mode of addressing
 May be different instructions for different movements

 e.g. IBM 390……………….(L, LH, LD,ST,STH…..etc)
 Or one instruction and different addresses

 e.g. VAX……….(MOV)

ARITHMETIC
 Add, Subtract, Multiply, Divide
 Signed Integer
 Floating point
 May include
 Increment (a++)
 Decrement (a--)
 Negate (-a)
 Absolute

LOGICAL
 Bitwise operations based upon Boolean operations
 AND, OR, NOT, EQUAL

SHIFT AND ROTATE
OPERATIONS
Examples of Shift and Rotate Operations

10100110
CONVERSION
 Change the format or operate on the format of data.
 E.g. Binary to Decimal
INPUT/OUTPUT
 For i/o operation
SYSTEM CONTROL
 Executed only while the processor is in a certain privileged state or is
executing a program in a special privileged area of memory.
 Typically, these instructions are reserved for the use of the operating
system.

TRANSFER OF CONTROL
 Branch …..AKA Jump
 Can be conditional or unconditional
 e.g. BRZ,BRP,BRN,BRO

TRANSFER OF CONTROL
 Skip
 Implies that one instruction be skipped
 e.g. increment and skip if zero

TRANSFER OF CONTROL
 Subroutine call
 Procedure call

Use of Stack to Implement Nested Subroutines of Figure 12.8
ADDRESSING MODES
 Immediate
 Direct how the processor can determine which address
mode is being used in a particular instruction?
 Indirect
 Register
 Register Indirect
 Displacement (Indexed)
 Stack

IMMEDIATE ADDRESSING
Instruction
 The simplest form of addressing Opcode Operand
 Operand is part of instruction

 Used to define & use constants or set initial values of variables
 e.g. ADD 0101
 Add 5 to contents of accumulator
 5 is operand
 Pros
 Fast:- No memory reference to fetch data
 Cons
 Limited range: - the size of the number is restricted to the size of the address
field

DIRECT ADDRESSING
 The address field contains the effective address of the operand
EA = A
 e.g. ADD A
 Add contents of memory address A to accumulator
 Look in memory at address A for operand
 Was common in earlier generations of computers

 Single memory reference to access data
 No additional calculations to work out effective address
 Limited address range

 restricted to the size of the address field

INDIRECT ADDRESSING
 The address field refer to the address of a word in memory, which in
turn contains a full-length address of the operand.
EA = (A)
 Look in A, find address (A) and look there for operand
 e.g. ADD (A)

 Add contents of cell pointed to by contents of A to accumulator
 Pros
 Increased address space
 Cons
 Instruction execution requires two memory references to fetch the operand:
 one to get its address and a second to get its value.

REGISTER ADDRESSING
 Address field refers to a register rather than a main memory address
EA = R
 Pros
 Only a small address field is needed in the instruction
 Very fast: - No time-consuming memory references are required.
 Cons
 Address space is very limited
 Limited number of registers

REGISTER INDIRECT
ADDRESSING
 C.f. indirect addressing
EA = (R)
 Operand is in memory cell pointed to by contents of register R
 Pros
 Large address space (2n)
 One fewer memory access than indirect addressing
 Cons
 Extra memory access

DISPLACEMENT
ADDRESSING
 Powerful mode of addressing combines the capabilities of direct
addressing and register indirect addressing.
EA = A + (R)
 Address field hold two address fields
 A = base value
 R = register that holds displacement
 or vice versa

RELATIVE ADDRESSING
 A version of displacement addressing
 R = Program counter, PC
 EA = A + (PC)
 i.e. the next instruction address is added to the address field to produce
the EA.
 locality of reference

BASE-REGISTER
ADDRESSING
 EA = A + (R)
 A holds displacement
 R holds pointer to base address
 R may be explicit or implicit
 e.g. segment registers in 80x86

INDEXED ADDRESSING
 A = base
 R = displacement
 EA = A + (R)
 Good for accessing arrays
 EA = A + (R)
 (R)++

STACK ADDRESSING
 Top of stack pointer is maintained in a register.
 In some processors, the top two elements stored in the processor’s registers
 Operand is (implicitly) on top of stack

 e.g.
 ADD Pop top two items from stack and add
 Pros
 No memory reference
 Cons
 Limited applicability

SUMMARY OF ADDRESSING

36
PART 2
Chapter 2: CPU
Computer Architecture and Organization

LEARNING OBJECTIVES
 Distinguish between user-visible and control/status registers, and
discuss the purposes of registers in each category.
 Summarize the instruction cycle.
 To know the data flow in instruction cycle
 To know the difference between RISC and CISC architecture.
 To understand the micro-operations involved in instruction cycle.
 Discuss the inputs and outputs of Control Unit and how it works.
 To understand the two implementations of control unit.

INSTRUCTION FORMAT
Reading
Assignments
Page 469
PROCESSOR
ORGANIZATION
What is the requirements placed on the processor, (the things that it must
do) ?
 Fetch instruction
To do these things, it should be clear that the
processor needs to store some
 Interpret instruction data temporarily.
 Fetch data
 Process data In other words, the processor needs

a small internal memory.
 Write data.
Registers

REGISTER ORGANIZATION
Registers: - are a very fast, small, and most expensive storage inside the
CPU.
The registers in the processor perform two roles:
 User-visible registers: Enable the machine- or assembly language
programmer to minimize main memory references by optimizing use of
registers.
 Control and status registers:
 Used by the control unit to control the operation of the processor and by
privileged, operating system programs to control the execution of programs.
There is no clean separation of registers into these two categories.

USER-VISIBLE REGISTERS
Is one that may be referenced by means of the machine language that the
processor executes. (4 categories )
 General purpose: can be assigned to a variety of functions by the
programmer (with small restrictions)
 Data: used only to hold data
 Address: somewhat general purpose, or they may be devoted to a
particular addressing mode (Segment pointers, Index registers, Stack
pointer).
 Condition codes: are bits set by the processor hardware as the result of
operations.
 partially visible to the user
 read by implicit reference, but the programmer cannot alter them.
 Condition Codes pros and cons ? Check your reference book.

REGISTERS DESIGN ISSUES….
1. Use completely general-purpose registers or to specialize their use?
 Make them general purpose
 Increase flexibility and programmer options
 Increase instruction size & complexity
 Make them specialized

 Smaller (faster) instructions format (register is given implicitly)
 limits the programmer’s flexibility
2. How Many Registers?

 Between 8 – 32 registers appears optimum
 Fewer = more memory references
 More does not reduce memory references and takes up processor real
estate
 Some RISC systems use 100 or more registers.
REGISTERS DESIGN
ISSUES….
3. How big? (register length)
 Large enough to hold full address (for addresses registers)
 Large enough to hold full word (for data registers
 Some machines allow two contiguous registers to be used as one for
holding double-length values
 C programming
 double int a;
 long int a;

CONTROL AND STATUS
REGISTERS
 Employed to control the operation of the processor.
 Most of these, on most machines, are not visible to the user.
 Some of them may be visible to machine instructions executed in a
control or operating system mode.
 Program Counter (PC)

 Instruction Register (IR)
what do these all do?
 Memory Address Register (MAR)
 Memory Buffer Register (MBR)

PROGRAM STATUS WORD
(PSW),
PSW : a register or set of registers, which contains condition codes plus
other status information.
 Sign: Contains the sign bit of the result of the last arithmetic operation.
 Zero: Set when the result is 0.
 Carry: Set if an operation resulted in a carry (addition) into or borrow
(subtraction) out of a high-order bit.
 Equal: Set if a logical compare result is equality.
 Overflow: Used to indicate arithmetic overflow.
 Interrupt Enable/Disable: Used to enable or disable interrupts.
 Supervisor: Indicates whether the processor is executing in supervisor
or user mode.
CONT…
 Other registers (related to status  The allocation of control
and control) information between registers
 Process control blocks and memory.
 Interrupt vector register  must decide how much control
 System stack pointer
information should be in
registers and how much in
 Page table pointer memory.
Control and status register
organization design issues
 OS support
 N.B. CPU design and operating
system design are closely linked

EXAMPLE
MICROPROCESSOR
REGISTER ORGANIZATIONS

INDIRECT CYCLE
 Processor’s instruction cycle, Fetch – Execute – Interrupt
 If indirect addressing is used, then additional memory accesses are
required.
 We can think of the fetching of indirect addresses as one more
instruction stages.

DATA FLOW
 The exact sequence of
events during an
instruction cycle
depends on the design
of the processor.
 But, what happens to
MAR,MBR,PC,IR
 ?????

CONT…

CONT…

CPU IS ARCHITECTURE: CISC
vs RISC
 Two Instruction set architectures
 Complex Instruction Set Computers (CISC)
 Reduced Instruction Set Computers (RISC)
Example: Multiply the content of two memory locations and store the result to
another memory location?
 The CISC Approach
 The primary goal of CISC architecture is to complete a task in as few lines of assembly
as possible.
 This is achieved by building processor hardware that is capable of understanding and
executing a complex instruction.
MULTI [loc 3] , [loc 1], [loc 2]
 When executed, this instruction loads the two values from memory into separate
registers, multiplies the operands in the execution unit, and then stores the product in
the appropriate register (R1).
A single instruction takes
more than 1 clock cycle to finish
CONT…THE CISC APPROACH
 Instructions for CISC arc closely resembles a command in a higher level
language.
 Potential advantage ? the compiler has to do very little work to translate a high-level
language statement into assembly.
 Because the length of the code is relatively short, very little RAM is required to
store instructions.
 The emphasis is put on building complex instructions directly into the hardware.
 The CISC approach attempts to minimize the number of instructions per
program, sacrificing the number of cycles per instruction.

THE RISC APPROACH
 RISC processors only use simple instructions that can be executed
within one clock cycle.
 LOAD A, [loc 1]
 LOAD B, [loc 2]
 PROD A, B
 STORE [loc 3] , A
 Because there are more lines of code, more RAM is needed to store the
assembly level instructions.
 The compiler must also perform more work to convert a high-level
language statement into code of this form.
 The RISC approach reduces the cycles per instruction at the cost of the
number of instructions per program.

 Because each instruction requires only one clock cycle to execute, the entire
program in RISC will execute in approximately the same amount of time as

of CISC.
 Potential advantage ? RISC "reduced instructions" require less transistors of
hardware space than the complex instructions, leaving more room for general
purpose registers.
 Because all of the instructions execute in a uniform amount of time (i.e. one
clock), pipelining is possible.
 What happen to the registers after we finish the current program?
 In CISC:- processor automatically erases the registers. If one of the operands
needs to be used for another computation, the processor must re-load the data
from the memory bank into a register.
 In RISC:- the operand will remain in the register until another value is loaded in
its place.
CISC RISC
The original ISA Redesigned on the original ISA
Instruction can take several clock cycle Instruction take one clock cycle
Hardware-centric design Software-centric design

More efficient use of RAM than RISC Heavy use of RAM
Complex and variable length instructions Simple standardized instructions
Sequential instruction execution Parallel instruction execution
Complexity in CPU (micro-programs) Complexity in compiler
Small code size Large code size

Pipeline is difficult Pipeline is easy
More addressing modes Fewer addressing mode
Have less registers Have more registers

Micro-operation and Control
Unit
 What are micro-operations
 Grouping rule
 Examples
 Input and output of CU
 Implementation of CU

MICRO-OPERATIONS
 The execution of a program consists of the sequential execution of
instructions.
 Each instruction is executed during an instruction cycle made up of
shorter subcycles
 (fetch, indirect, execute, interrupt).
 The execution of each subcycle involves one or more shorter operations,

that is, micro-operations.
 Micro-operations are the functional, or atomic, operations of a
processor.
 The prefix micro refers to the fact that each step is very simple and
accomplishes very little.

OP’S IN FETCH CYCLE
 Four registers are involved: MAR, MBR, PC, IR
 Simple fetch cycle actually consists of 3 steps and 4 OP
 Each 𝝁OP involves the movement of data into or out of a register.

RULES OF 𝝁OP GROUPING
1. The proper sequence of events must be followed.
 Thus (MAR (PC)) must precede (MBR Memory) because the memory read operation
makes use of the address in the MAR.
2. Conflicts must be avoided.

 One should not attempt to read to and write from the same register in one time unit, because
the results would be unpredictable.
 For example, the micro- operations (MBR Memory) and (IR  MBR) should not occur
during the same time unit.
Note: As long as each 𝝁OP follows the above rules and do not interfere with one
another, several of them can take place during one step, to saving time.
OP’S IN INDIRECT CYCLE
 let us assume a one-address instruction format, with direct and indirect
addressing allowed.
 The address field of the instruction is transferred to the MAR.
 This is then used to fetch the address of the operand.
 Finally, the address field of the IR is updated from the MBR, so that it
now contains a direct rather than an indirect address.

OP’S IN INTERRUPT CYCLE
 The nature of this cycle varies greatly from one machine to another.

OP’S IN EXECUTE CYCLE
 This cycle is different, because of the variety of opcodes, there are a
number of different sequences of micro-operations
 ADD R1, X BSA X
 t1: MAR  (IR(address))
t1: MAR  (IR(address))
 t2: MBR  Memory MBR  (PC)
 t3: R1  (R1) + (MBR) t2: PC  (IR(address))
Memory  (MBR)
 ISZ X
t3: PC  (PC) + I
 t1: MAR  (IR(address))
 t2: MBR  Memory
 t3: MBR  (MBR) + 1
 t4: Memory  (MBR)
 If ((MBR) = 0) then (PC  (PC) + I)

THE INSTRUCTION CYCLE
 Defines the complete sequence of micro-operations
 We assume a new 2-bit register called the instruction cycle code (ICC).
 The ICC designates the state of the processor in terms of which portion
of the cycle it is in:
 00: Fetch
 01: Indirect
 10: Execute
 11: Interrupt
 At the end of each of the four cycles, the ICC is set appropriately.

CONTROL OF THE
PROCESSOR
Define basic elements of processor ?
Describe micro-operations processor performs ?
Determine functions control unit must perform ?
 The functional requirements of the control unit: (those functions that the
control unit must perform)
 Sequencing: The control unit causes the processor to step through a series of
micro-operations in the proper sequence, based on the program being
executed.
 Execution: The control unit causes each micro-operation to be performed

with the use of control signals

EXTERNAL SPECIFICATIONS OF
THE CONTROL UNIT
 Clock ---This is how the control unit “keeps time.”
 One micro-instruction (or set of parallel micro-instructions) per clock cycle
IN
 Instruction register
PU
 Op-code and addressing mode for current instruction TS
 Determines which micro-instructions to perform
 Flags
 State of processor
 Results of previous ALU operations
 Control signals from control bus

 Interrupts
 Acknowledgements

CONT…
The outputs are as follows:
 Control signals within the processor:
 These are two types: Cause data movement & Activate specific ALU
functions
 Control signals to control bus:
 These are also of two types: control signals to memory, and control signals to
the I/O modules.

CONTROL SIGNALS
 Three types of control signals are used:
 Those that activate an ALU function;
 Those that activate a data path; and
 Those that are signals on the external system bus or other external interface.
 All of these signals are ultimately applied directly as binary inputs to

individual logic gates
 Let us consider again the fetch cycle to see how the control unit
maintains control.

CONTROL SIGNALS IN FETCH
CYCLE
 The first step is to transfer the contents of the PC to the MAR.
 CU activate the control signal that opens the gates between the bits of the PC
and the bits of the MAR.
 The next step is to read a word from memory into the MBR and increment
the PC.
 A control signal that opens gates, allowing the contents of the MAR onto the
address bus;
 A memory read control signal on the control bus;
 A control signal that opens the gates, allowing the contents of the data bus to be
stored in the MBR;
 Control signals to logic that add 1 to the contents of the PC and store the result
back to the PC.
 Following this, the control unit sends a control signal that opens gates
between the MBR and the IR.
CONTROL SIGNALS IN FETCH
CYCLE

A CONTROL SIGNALS
EXAMPLE

For simplicity, the data and control paths for incrementing the PC and for
loading the fixed addresses into the PC and MAR are not shown.

INTERNAL PROCESSOR
ORGANIZATION

CU IMPLEMENTATION
Control unit can be implemented using
 Hardwired implementation
 Microprogrammed implementation
Hardwired Control Unit

 When the control signals are generated by hardware using conventional logic
design techniques, the CU is said to be hardwired.
 Can be viewed as a state machine that changes from one state to another in
every clock cycle, depending on the inputs of CU.
 The outputs of the state machine are the control signals.
 The sequence of the operation carried out by this machine is determined by
the wiring of the logic elements and hence named as “hardwired”.

SUMMARY OF HARDWIRED
CU
 Fixed logic circuits that correspond directly to the Boolean expressions are
used to generate the control signals.
 Hardwired control is faster than micro-programmed control.
 A controller that uses this approach can operate at high speed.
 RISC architecture is based on hardwired control unit
Limitation of this implementation

 Complex sequencing & micro-operation logic
 Difficult to design and test
 Inflexible design
 Difficult to add new instructions
MICRO-PROGRAMMED
IMPLEMENTATION OF CU
 The control signals associated with operations are stored in special
memory units inaccessible by the programmer as Control Words.
 Control signals are generated by a program are similar to machine
language programs.
 Micro-programmed CU is slower in speed because of the time it takes
to fetch microinstructions from the control memory.
 Is the dominant technique for implementing control units in pure CISC
architectures processors
SOME IMPORTANT TERMS IN
MICRO-PROGRAMMED CU
 Control Word : is a word whose individual bits represent various control
signals.
 Micro-instruction : Individual control words in this micro-routine are
referred to as microinstructions.
 Micro-program : A sequence of micro-instructions is called a micro-
program, which is stored in a ROM.
 Micro-routine : Group of micro-instructions each stored in control
word that belongs to same routine.
 Control Memory : the micro-routines for all instructions in the
instruction set of a computer are stored in a special memory called the
Control Memory.

ADVANTAGES AND DISADVANTAGES
OF
Micro-programmed implementation of CU
 Advantage
 Cheaper and less error prone to implement.
 Simple to design and test.
 Flexible design….. easy to add new features and
 Disadvantage
 Somewhat slower than a hardwired implementation
To know more about this implementation refer to Chapter 16 Micro-programmed

Control (page 586) in your textbook.

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CHAPTER 3: CPU

 Present an overview of essential characteristics of machine

 Describe the types of operands used in typical machine

 Describe the various types of addressing modes common in

Computer Architecture and Organization 2

Computer Architecture and Organization 3

 It’s difficult to deal with binary representations of machine instructions.

Computer Architecture and Organization 4

 With this in mind we can categorize instruction types as follows:

Computer Architecture and Organization 5

 With 3 addresses, 2 source operand locations and a destination operand

Computer Architecture and Organization 6

 Simpler yet is the one-address instruction. For this to work, a second

Computer Architecture and Organization 7

Computer Architecture and Organization 8

 Zero-address instructions are applicable to a special memory organization called

Computer Architecture and Organization 11

 Instruction format: Instruction length (in bits), number of addresses, size of

Computer Architecture and Organization 13

Computer Architecture and Organization 14

 May be different instructions for different movements

 Or one instruction and different addresses

Computer Architecture and Organization 15

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Examples of Shift and Rotate Operations

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 Operand is part of instruction

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 Was common in earlier generations of computers

 Limited address range

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 e.g. ADD (A)

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 Operand is (implicitly) on top of stack

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 To know the data flow in instruction cycle

 To know the difference between RISC and CISC architecture.

 To understand the micro-operations involved in instruction cycle.

 To understand the two implementations of control unit.

 Process data In other words, the processor needs

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 Make them specialized

2. How Many Registers?