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Chapter 3. Instruction Set Architectures. Instruction Set Architecture. ISA Includes the information needed to interact with the microprocessor. Does not include information as to how microprocessor is designed or implemented

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Chapter 3

Chapter 3

Instruction Set Architectures

Instruction set architecture
Instruction Set Architecture

  • ISA Includes the information needed to interact with the microprocessor.

  • Does not include information as to how microprocessor is designed or implemented

  • Includes microprocessor instruction set, which would be the set of all assembly languages instructions.

  • Also includes the complete set of accessible registers.

3 1 levels of programming languages
3.1 Levels of Programming Languages

  • Programming languages are divided into three categories.

  • High level languages hide the details of the computer and operating system.

  • Are also referred to as platform-independent.

  • Examples include C++, Java, and Fortran

3 1 levels of programming languages contd
3.1 Levels of Programming Languages (contd)

  • Assembly language is an example of a lower level language.

  • Each microprocessor has its own assembly language

  • A program written in the assembly language of one microprocessor cannot be run on a different microprocessor

3 1 levels of programming languages contd1
3.1 Levels of Programming Languages (contd)

  • Backward compatibility used in order to have old programs that ran on a old microprocessor, can run on a newer model.

  • Assembly language can manipulate the data stored in a microprocessor.

  • Assembly language are not platform independent

3 1 levels of programming languages contd2
3.1 Levels of Programming Languages (contd)

  • Lowest level of languages are machine language.

  • Contains binary values to cause microprocessor to perform operations.

  • Microprocessor understands the machine language, and thus it is in this state that it executes a instruction set.

  • High level language and assembly language are converted to machine language.

3 1 2 compiling and assembling programs
3.1.2 Compiling and Assembling Programs

  • High-level language programs are compiled

  • Assembly languages are assembled.

3 1 2 compilers
3.1.2 Compilers

  • Compiler checks statement in a program is valid.

  • If every instruction is syntactically correct, then the compiler generates a object code.

  • Linker combines object code as an executable file.

  • Executable file copied into memory, and microprocessor then runs the machine code contained in that file

3 1 2 compilers contd
3.1.2 Compilers (contd)

  • A high-level language statement is usually converted to a sequence of several machine code instructions.

  • Every high-level language statement might have more then one valid conversion of a statement.

3 1 2 assemblers
3.1.2 Assemblers

  • Every statement in assembly language however corresponds to one unique machine code instruction.

  • The assembler converts source code to object code, and then the linking, and the loading of procedures occur.

Compiler vs assembler
Compiler vs Assembler

Assembly language

Program for processor X

Compiler for Pentium

Windows Pc



Object files

Assembler for

Processor X


Object code

Processor X

Object code

Pentium Linker

Other processor X

Object files

Process X



Executable file

Processor X

Executable file


Pentium PC

Computer with

Processor X

3 2 a closer look at assembly language
3.2 A closer look at Assembly Language

  • Assembly language is very important part of a instruction set architecture.

  • Assembly instructions can be grouped together based on their functions.

3 2 1 1 data transfer instructions Data Transfer Instructions

  • Related with moving data from one place to another.

  • Not be mistaken as the idea that data is literally “moved”, instead it is copied from one location to another.

  • This particular type of instruction set does not modify data.

3 2 1 1 data transfer instructions contd Data Transfer Instructions (contd)

Instructions related with this category perform the following transfers:

  • Load data from memory into microprocessor.

  • Store data from the microprocessor into memory.

  • Move data within the microprocessor.

  • Input data to the microprocessor.

  • Output data from the microprocessor.

3 2 1 2 data operation instructions Data Operation Instructions

  • Data operation instructions modify their data values.

  • They require one or two operands, and then they store the result

  • Arithmetic instructions make up a large part of the data operation instructions.

  • Logic instructions perform basic logical operations on data.

  • Shift instructions shift bits of data values in a register.

3 2 1 3 program control instructions Program Control Instructions

  • For Assembly languages, the jump or branch instruction is commonly used to go to another part of the program.

  • An assembly language instruction set may include instructions to call and return from subroutines.

  • Microprocessor can also be designed to accept interrupts, which basically causes a microprocessor to stop its current process, and execute another set of instructions.

3 2 2 data types
3.2.2 Data Types

Numeric data can be represented as integers:

  • Unsigned integers of n-bit values can range from 0 to 2^n-1.

  • Signed n-bit integers can have values between –2^n-1 to 2^n-1-1

3 2 2 data types contd
3.2.2 Data Types (contd)

Other types include:

  • Float: Microprocessor may have special registers only for floating point data, and its corresponding instruction set.

  • Boolean: Instructions can perform logical operations on these values.

  • Characters: Stored as binary values. Operations include concatenation, replacing characters, or character string manipulation.

3 2 3 addressing modes
3.2.3 Addressing Modes

  • Microprocessor needs memory address to access data from the memory.

  • Assembly language may use several addressing modes to accomplish this task.

3 2 3 addressing modes contd
3.2.3 Addressing Modes (contd) Direct Mode

  • Instruction includes memory access.

  • CPU accesses that location in memory.


    LDAC 5

    Reads the data from memory location 5, and

    stores the data in the CPU’s accumulator.

3 2 3 addressing modes contd1
3.2.3 Addressing Modes (contd) Indirect Mode

  • Address specified in instruction contains address where the operand resides.


    LDAC @5 or LDAC (5)

    Retrieves contents of location 5, uses it to access memory addrss

3 2 3 addressing modes contd2
3.2.3. Addressing Modes (contd) Register Direct and Register Indirect Modes

  • Does not specify a memory address. Instead specifies a register.


    LDAC R

    Where R is a register containing the value 5.

    The instruction copies the value 5 from register and into the CPU’s accumulator.

3 2 3 addressing modes contd3
3.2.3 Addressing Modes (contd) Immediate Mode

  • The operand specified in this mode is the actual data it self.


    LDAC #5

    Moves the value 5 into the accumulator.

3 2 3 addressing modes contd4
3.2.3 Addressing Modes (contd) Implicit Mode

  • Does not exactly specify an operand. Instruction implicitly specifies the operand because it always applies to a specific register.


    CLAC Clears the accumulator, and sets value to zero. No operands needed.

3 2 3 addressing modes contd5
3.2.3 Addressing Modes (contd) Relative Mode

  • Operand supplied is an offset, not the actual address. Added to the contents of the CPU’s program counter register to generate the required address.


    LDAC $5 is located at memory location 10, and it takes up two blocks of memory. Thus the value retrieved for this instruction will be 12 + 5, and will be stored in the accumulator

3 2 3 addressing modes contd6
3.2.3 Addressing Modes (contd) Index Mode and Base Address Mode

  • Address supplied by the instruction is added to the contents of an index register.

  • Base address mode is similar except, the index register is replaced by a base address register.


    LDAC 5(X) where X = 10

    Reads data from location (5 + 10) = 15 and stores it in the accumulator.

3 2 3 addressing modes contd7
3.2.3 Addressing Modes (contd)

h) 0: LDAC 5(X)

instruction gets value from index register

X: 10

then adds contents of X(10) to offset (5) to get

to get address (15)

15: 30 stores value in CPU


a) 0: LDAC 5 (instruction gets data from location 5)

5: 10 stores value in CPU

  • 0: LDAC @5 (instruction gets address from location 5)

    5: 10 (then gets data from location 10)

    10: 20 stores value in CPU

    c) 0: LDAC R

    instruction gets address from register R

    R: 5 stores value in CPU

    d) 0: LDAC R

    instruction gets address from register R

    : 5

    then gets data from location 5

    5: 10 stores value in CPU

    e) 0: LDAC # 5 stores value from instruction in CPU

    f) 0: LDAC (implicit)

    instruction gets value from stack

    stack stores value in CPU

    g) 0: LDAC $5

    1: instruction adds address of next instruction (1) to

    5: offset (5) to get address (6)

    6: 12 stores value in CPU

3 2 4 instruction formats
3.2.4 Instruction Formats

  • Assembly language in machine language is represented as binary value called instruction code.

  • Binary value for each instruction is in different format.

  • Representation of operation to be performed is called opcode.

3 2 4 instruction formats contd
3.2.4 Instruction Formats (contd)


ADD = 1010 A = 00

MOVE = 1000 B = 01

LOAD = 0000 C = 10

STORE = 0001 D = 11

PUSH = 0100

POP = 1100

3 2 4 instruction formats examples contd
3.2.4 Instruction Formats Examples (contd)

A) ADD A,B,C (A=B+C)

1010 00 01 10

B) MOVE A,B (A = B) 1000 00 01

ADD A,C (A = A + C) 1010 00 10

C) LOAD B (Acc = B) 0000 01

ADD C (Acc = Acc + C) 1010 10

STORE A (A = Acc) 0001 00

D) PUSH B (Stack = B) 0101

PUSH C (Stack = C,B) 0110

ADD (Stack = B + C) 1010

POP A (A = stack) 1100

Fewer bits requires less hardware, but more instructions. Fewer bits allows faster execution.

4 bits


2 bits

Operand #1

2 bits

Operand #2

2 bits

Operand #3

4 bits


2 bits

Operand #1

2 bits

Operand #2

4 bits


2 bits


4 bits


3 3 instruction set architecture design
3.3 Instruction Set Architecture Design

To design a optimal microprocessor, the following questions and issues have to be addressed in order to come up with an optimized instruction set architecture for the CPU:

  • Completeness; does the instruction set have all of the instructions a program needs to perform its required task.

  • Issue of orthogonality, the concept of two instructions not overlapping, and thus not performing the same function.

  • The amount of registers to be added. More registers enables a CPU to run faster, since it can access and store data on registers, instead of the memory, which in turn enables a CPU to run faster. Having too many registers adds unnecessary hardware.

  • Does this processor have to be backward compatible with other microprocessors.

  • What types and sizes of data will the microprocessor deal with?

  • Are interrupts needed?

  • Are conditional instructions needed?

3 4 creating a simple instruction set
3.4 Creating a simple Instruction Set

Designing a simple microprocessor fit for

maybe a microwave will involve integrating

the following models:

  • Memory model

  • Register model

  • Instruction set

3 4 1 memory model
3.4.1 Memory Model

  • Microprocessor can access 64 K or 2^16 byes of memory

  • Each byte has 8 bits or 64K x 8 of memory.

  • I/O is treated as memory access, thus requires same instruction to access I/O as it does to access memory

3 4 2 registers
3.4.2 Registers

  • Three registers in this microprocessor

  • First register is 8-bit accumulator where the result is stored. Also provides one of the operands for instructions requiring two operands.

  • Second register R is a 8-bit register that provides the second operands, and also stores in result so that the accumulator can gain access to it.

  • Third register is a Z register which is 1 bit. It is either 0 or 1. If a result of a instruction is 0 then the register is set to 1 otherwise it is set to 0.

3 4 3 instruction set
3.4.3 Instruction Set

16 instructions, 8-bit each:

3 4 3 instruction set contd
3.4.3 Instruction Set (contd)

Note: LDAC uses direct addressing mode. MOVR uses the implicit addressing mode. JUMP uses immediate addressing mode.

3 4 4 implementation
3.4.4 Implementation

1 + 2 + … + n, or

Total = 0

For I = 1 TO N do (Total = Total + I);

Break Down:

1: Total = 0, I = 0

2: I = I + 1

3: Total = Total + I

4: If I  n THEN GOTO 2

3 4 4 implementation contd
3.4.4 Implementation (contd)

CLAC Clear Accumulator

STAC total Store value 0 to address total

STAC i Store value 0 to address i

Loop: LDAC i Load contents of address i into accumulator

INAC Add 1 to the accumulator

STAC i Store result from accumulator back to address i

MVAC Move result from accumulator into Register R

LDAC total Load Total into accumulator

ADD Add contents of Register R and accumulator and store it in accumulator

STAC total Store Total back to address total

LDAC n Load n into accumulator

SUB Subtract R (R = i) from AC (AC = n)

JPNZ Loop If result is not zero then jump back to loop:

3 4 5 analysis of instruction set and implementation
3.4.5 Analysis of Instruction Set, and Implementation

  • Cannot have value greater then 255, therefore n has to be less then or equal to 22

  • Is it complete? For simple hardware, maybe. Not enough to be implemented in a PC.

  • Fairly orthogonal; however by eliminating OR and implementing by AND and NOT, we can reduce the amount of hardware used.

  • Not enough registers.

3 5 8085 microprocessor instruction set archtecture
3.5 8085 Microprocessor Instruction Set Archtecture

  • Processor has practical applications. Examples include the Sojourner robot.

  • Contains several registers including the accumulator register, A.

  • Other registers include B,C,D,E,H,L.

  • Some are accessed as pairs. Pairs are not arbitrary. B and C, D and E, H and L.

  • SP is a 16 bit stack pointer register pointing to the top of the stack.

3 5 8085 microprocessor instruction set archtecture1
3.5 8085 Microprocessor Instruction Set Archtecture

Contains five flags known as flag registers:

  • Sign flag, S indicates sign of a value

  • Zero flag, Z, tells if a arithmetic or logical instruction produced 0 for a result.

  • Parity flag, P, is set to 1 if result contains even number of 1’s

  • Carry flag, CY, is set when an arithmetic operation generates a carry out.

3 5 8085 microprocessor instruction set archtecture2
3.5 8085 Microprocessor Instruction Set Archtecture

  • Auxiliary carry flag, generates a carry out from a lower half of a result to a upper half.


    0000 1111 + 0000 1000 = 0001 0111

  • IM register used for enable and disable interrupts, and to check pending interrupts.

3 5 2 8085 microprocessor instruction set
3.5.2 8085 Microprocessor Instruction Set

Contains a total of 74 instructions.

3.5.2 Data operation instruction for the 80855 microprocessor

3.5.2 Data operation instruction for the 80855 microprocessor

3 5 3 a simple 8085 program
3.5.3. A Simple 8085 Program

1: i = n, sum = 0

2: sum = sum + i, i = i - 1

3: IF i  0 then GOTO 2

4: total = sum

3 5 3 a simple 8085 program contd
3.5.3 A Simple 8085 Program (contd)

i = n




Loop: ADD B


JNZ Loop

STA total

sum = A  A = 0

sum = sum + i

i = i - 1

IF i  0 THEN GOTO Loop

total = sum

3 5 4 analyzing the 8085 isa
3.5.4 Analyzing the 8085 ISA

  • Instruction set more complete then the simple CPU, however not sufficient enough for a PC.

  • Able to use subroutines, and interrupts

  • It is fairly orthogonal.

  • Has sufficient number of registers