computer architecture chapter 6

8/2/2019 Computer Architecture Chapter 6

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Computer System ArchitectureCCEE 313

Dr. Abir A. EL ABED

Chapter 6Programming the Basic Computer


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25 Instruction Set of the basic

computer

Memory Reference Instruction

Has 3 parts: a mode bit, an operation

code of 3 bits, and a 12-bit address

Register Reference Instruction

16-bit operation code

First digit of a register reference

instruction is always 7

Input-output Instruction

First digit is always F

6-1 Introduction

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6-2 Machine Language

Program = list of statements for directing the computer to perform a task

Various types of programming languages may be write for computer

Computer can execute only the binary form of statements => Programs written in other

language must be translated to binary

Program Categories

1) Binary Code (Tab. 6-2)

A sequence of instructions and operand in binary that list the exact representation ofinstructions as they appear in computer memory

2) Octal or Hexadecimal Code (Tab. 6-3)

Equivalent translation of the binary code to octal or hexadecimal representation

3) Symbolic Code (Tab. 6-4)

User employs symbols (letters, numerals, or special characters) for operation, address

and other parts of instruction code

Each symbols can be translated into binary coded instruction

Translation done by an assembler

4) High Level Language (Tab. 6-6)

C, Fortran,

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6-3 Assembly Language Programming language defined by a set of rules

The rules for writing assembly language program

Documented and published in manuals (from the computer manufacturer)

Rules of the Assembly Language

Each line of an assembly language program is arranged in three columns called fields

Fields specify the following information:

1. Label field : empty or symbolic address

2. Instruction field : specifies a machine instruction or a pseudoinstruction

3. Comment field : empty or comment

Symbolic Address (Label field)

One, two, or three, but not more than three alphanumeric characters The first

character must be a letter; the next two may be letters or numerals

A symbolic address is terminated by a comma (recognized as a label by the

assembler), (ex. Table 6-5)

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Instruction Field

1. A memory-reference instruction(MRI)

Ex) ADD OPR

2. A register-reference or input-output instruction(non-MRI)

CLA (register-reference), INP (input-output)

3. A pseudoinstruction with(ORG N) or without(END) an operand

Comment field

Comment filed must be preceded by a slash(recognized by assembler as

comment)

Translation to Binary

Assembler = the translation of the symbolic(= assembly) program into binary

Address Symbol Table = Hexadecimal address of symbolic address

MIN = 106, SUB = 107, DIF = 108

6-3 Assembly Language

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6-4 The Assembler

Assembler = a program that accepts a symbolic language program and

produces its binary machine language equivalent

Input symbolic program is called source program

Resulting binary program is called object program

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6-4 The Assembler

Representation of Symbolic Program in Memory : Tab. 6-11

To starting the assembly process, symbolic program must be stored inmemory

A program consists of symbols => its representation in memory use analphanumeric character code

In basic computer, each character is represented by 8-bit code

Each character is assigned two hexadecimal digits which can be convertedto 8-bit code

Line of Code : PL3, LDA SUB I (Carriage return CR)

The assembler recognizes a CR code as the end of a line of code

Code for CR is produced when the return key is depressed

The following line PL3, LDA SUB I is stored in 7 consecutive memorylocations (Tab 6-11)

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o An input program is scanned by the assembler twice to produce the equivalentbinary program

o Two Pass Assembler

1) 1st pass : Generate user defined address symbol table with their binaryequivalent symbol

- Binary translation is done during the 2nd pass

- To track the location of instructions, the

assembler uses location counter (LC)

- LC stores the value of the memory locationassigned to the instruction or operand

presently being processed

- ORG pseudoinstruction initializes LC to the

value of first location

- LC is incremented by 1 after processing each

line of code

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2) 2nd pass : Binary translation

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MRI: Memory Reference Instruction


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6-7 Subroutines A set of common instruction that can be used in a programmany times

Each time that a subroutine is used in the main part of the program, a branch

is executed to the beginning of the subroutine

After the subroutine has been executed, a branch is made back to the main

program

A branch can be made to the subroutine from any part of the main program

Problem: how the subroutine knows which location to return to?

All computer provide special instructions to facilitate subroutine entry and

return

In basic computer, the link between the main program and a subroutine is

the BSA instruction (Branch and Save return Address)

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Program: shifting a word 4 times

- Load the value of X into the

accumulator AC

- BSA instruction at location 101

- Subroutine SH4 must return to

location 102 after finishes its

task

- When BSA executed, controlunit stores the return address

102 into the location defined

by the symbolic address SH4

(which is 109)

- And value of SH4 + 1 is

transferred into the programcounter

- After execution of instruction SH4, HEX 0 the memory location 109 contains the binary

equivalent of 102


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Computation in the subroutine circulatesthe content of AC four times to the left

To accomplish a logical shift operation,the four low-order bits must be set tozero => this is done by masking FFF0 withthe content of AC

BUN SH4 I returns the computer to themain program (to the address stored in

location SH4 == location 102)

STA X store the result in the accumulatorAC into X

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- The first memory location of each subroutine serves as a link between the main programand the subroutine

- The procedure for branching to a subroutine and returning to the main program is

referred to as a subroutine linkage


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6-8 Input-Output Programming

- Users write programs with symbols defined by programming language

- Symbols are strings of characters and each character is assigned an 8-

bit code

- A binary-coded character enters the computer when an INP (input)

instruction is executed

- A binary-coded character is transferred to the output device when an

OUT (output) instruction is executed

- Output device detects the binary code and types the corresponding

character

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- SKI instructions the input flag to see if a character is available for transfer

- INP transfers the binary-coded character into AC(0-7)- OUT instruction print out the character to the terminal

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Interrupt Program

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