TK 2123

Ass. Prof. Dr Masri Ass. Prof. Dr Masri AyobAyob

TK 2123

Lecture 14: Instruction Set Architecture Level (Level 2)

April 20, 2023 Prepared by: Dr Masri Ayob 2

ISA Level

OS level – ISA level- Microarchitecture level. Historically, ISA level was developed before

any of the other levels (originally the only level).

Sometimes referred as “the architecture” of a machine or “assembly language” (incorrect).

Interface between software and hardware. The ISA level defines the interface between

the compilers and hardware.


ISA Level

The ISA level is the interface between the compilers and the hardware.


ISA Level

Is defined by how the machine appears to a machine language programmer. No person does machine language

programming. Redefined:

ISA-level code is what a compiler outputs.

The compiler writer has to know: Memory model Registers Data type Addressing mode Instruction set .


Memory Models

What order do we read numbers that occupy more than one byte

e.g. (numbers in hex to make it easy to read)

12345678 can be stored in 4x8bit locations as follows


Memory Models (example)

Address Value (1) Value(2) 184 12 78 185 34 56 186 56 34 186 78 12

i.e. read top down or bottom up?


Memory Models

The problem is called Endian The system on the left has the most

significant byte in the smallest address: Big-endian

The system on the right has the least significant byte in the smallest address: Little-endian


Example of C Data Structure


Standard…What Standard?

Pentium (80x86), VAX are little-endian IBM 370, Motorola 680x0 (Mac), and

most RISC are big-endian Internet is big-endian.


What is an Instruction Set?

The complete collection of instructions that are understood by a CPU Machine Code Binary Usually represented by assembly codes


Elements of an Instruction

Operation code (Opcode) Do this…i.e. the task to be performed

Source Operand reference To this..i.e. the data to be operated on

Result Operand reference Put the answer here…..

Next Instruction Reference When you have done that, do this...


Where have all the Operands Gone?

Main memory (or virtual memory or cache)

CPU register I/O device


Instruction Cycle State Diagram


Instruction Representation

In machine code each instruction has a unique bit pattern

For human consumption (well, programmers anyway) a symbolic representation is used

e.g. ADD, SUB, LOAD Operands can also be represented in this way

ADD A,B Usually there are not enough bits in one byte

to store enough instructions and addresses: Instructions stored in more than one byte.


Simple Instruction Format


Number of Addresses (a)

3 addresses Operand 1, Operand 2, Result a = b + c; May be a forth - next instruction

(usually implicit) Not common Needs very long words to hold

everything


Number of Addresses (b)

2 addresses One address doubles as operand and

result a = a + b Reduces length of instruction Requires some extra work

Temporary storage to hold some results


Number of Addresses (c)

1 address Implicit second address Usually a register (accumulator) Common on early machines


Number of Addresses (d)

0 (zero) addresses All addresses implicit Uses a stack e.g. push a push b add pop c


How Many Addresses

More addresses More complex (powerful?) instructions More registers

Inter-register operations are quicker Fewer instructions per program

Fewer addresses Less complex (powerful?) instructions More instructions per program Faster fetch/execution of instructions


Design Decisions (1)

Operation issues: How many ops? What can they do? How complex are they?

Data types Instruction formats

Length of op code field Number of addresses


Design Decisions (2)

Registers Number of CPU registers available Which operations can be performed on

which registers? Addressing modes (later…)

RISC v CISC


Addressing Modes

Instructions can be categorized according to their method of addressing the hardware registers and/or memory. Implied Addressing Register Addressing Immediate Addressing Direct Addressing Register Indirect Addressing Combined Addressing Modes.

The various ways of addressing data in an instruction are known as addressing mode.


Implied Addressing

The addressing mode of certain instructions is implied by the instruction’s function. For example:

the STC (set carry flag) instruction deals only with the carry flag

the DAA (decimal adjust accumulator) instruction deals with the accumulator.


Register Addressing

The accumulator is implied as a second operand. For example,

the instruction CMP E may be interpreted as 'compare the contents of the E register with the contents of the accumulator.


Immediate Addressing

These instructions have data assembled as a part of the instruction itself. For example, the instruction CPI 'C' may be

interpreted as ‘compare the contents of the accumulator with the letter C.


Direct Addressing

These instructions directly specify the memory address of the operand.

Example: JMP 1000H causes a jump to the address

1000H by replacing the current contents of the PC with the new value 1000H.

LDA 2000H will load the contents of memory location 2000H into the accumulator.


Register Indirect Addressing

These instructions reference memory via a register pair.

For example: MOV M,C moves the contents of the C register into

the memory location pointed by the H and L register pair.

The instruction LDAX B loads the accumulator with the byte of data specified by the address in the B and C register pair .


Combined Addressing Modes

Some instructions use a combination of addressing modes. A CALL instruction, for example, combines

direct addressing and register indirect addressing.

The direct address in a CALL instruction specifies the address of the desired subroutine;

the register indirect address is the stack pointer. The CALL instruction pushes the current contents of the program counter into the memory location specified by the stack pointer. .


Discussion of Addressing Modes

A comparison of addressing modes.


Timing Effects of Addressing Modes

Addressing modes affect both: the amount of time required for executing an

instruction. the amount of memory required for its storage.

For example, instructions that use implied or register addressing, execute very quickly since they deal directly with the processor’s hardware or with data already present in hardware registers.

the entire instruction can be fetched with a single memory access.

The number of memory accesses required is the greatest factor in determining execution timing.


Timing Effects of Addressing Modes

More memory accesses require more execution time. A CALL instruction, for example, requires

five memory accesses: three to access the entire instruction and two more to push the contents of the program counter onto the stack.


Types of Operand

Addresses Numbers

Integer/floating point Characters

ASCII etc. Logical Data

Bits or flags


Pentium Data Types

8 bit Byte 16 bit word 32 bit double word 64 bit quad word Addressing is by 8 bit unit A 32 bit double word is read at

addresses divisible by 4


Types of Instruction Operation

Data Transfer (data movement) Arithmetic Logical Conversion I/O System Control Program Control


Data Transfer (Data Movement) Specify

Source Destination Amount of data

May be different instructions for different movements e.g. IBM 370

Or one instruction and different addresses e.g. VAX

E.g. MOV A,BMove 8-bit data from register B to

accumulator A.


Data Transfer (Data Movement) Data Movement Instructions are the most

frequently used and computer designers provide a lot of flexibility to these instructions. E.g. Intel 8085 provides data transfer

between: Register-to-register Register-to-memory Memory-to-register Stack operation.


Arithmetic

Add, Subtract, Multiply, Divide E.g. ADD C

Add the content of register C to the content of accumulator A.

Signed Integer Floating point ? May include

Increment (a++) Decrement (a--) Negate (-a)


Shift and Rotate Operations


Logical

Bitwise operations AND, OR, NOT Example:

ANA B A= A AND B


Conversion

E.g. Binary to Decimal


Input/Output

May be specific instructions E.g. IN 12 Read input data from port

location 12 May be done using data movement

instructions (memory mapped) May be done by a separate controller

(DMA)


Systems Control

Privileged instructions CPU needs to be in specific state

Ring 0 on 80386+ Kernel mode

For operating systems use


Program Control Instructions

Branch/Jump e.g. branch to x if result is zero

JZ Skip

e.g. increment and skip if zero ISZ Register1

Branch xxxx Subroutine call

E.g. CALL sum Return from subroutine

E.g. RET


Nested Procedure Calls


Use of Stack


The Pentium 4’s primary registers.


The Pentium 4 Instruction Formats

The Pentium 4 instruction formats.


Example of 8051 Instructions

More……


Thank youQ&A

TK 2123

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