Programming the Basic Computer and Assembler

Module 2
Programming the Basic Computer
By:
Mitul Patel

Topics
Introduction
MachineLanguage
AssemblyLanguage
Assembler
Programloops
Programming Arithmetic and logicoperations
Subroutines
I-OProgramming

Computer Instruction
(Memory Reference Instruction)
And Memory to AC
Branch Uncondionary
Sym
AND
ADD
LDA
STA
BUN
BSA
I=0 I=1
0xxx
1xxx
2xxx
ISZ
3xxx
4xxx
5xxx Dxxx
6xxx
8xxx
9xxx
Axxx
Bxxx
Cxxx
Exxx
Description
Add Memory to AC
Load To AC
Store Of AC
Return Address
Skip If Zero

(Register Reference Instruction)
7800
7400
Clear AC
Clear E
Comp. AC
Comp. E
Cir. Right AC and E
Cir. Left AC and E
Increment AC
Skip If AC is Positive
Skip If AC is Negative
Skip If AC is Zero
Skip If E is 0
Halt Computer
7200
7100
7080
7040
7020
7010
7008
7004
7002
7001
CLA
CLE
CMA
CME
CIR
CIL
INC
SPA
SNA
SZA
SZE
HLT

(Input Output Reference Instruction)
F800
F400
F200
F100
F080
F040
INP
OUT
SKI
SKO
ION
IOF
Input to AC
Output From Ac
Skip on Input Flag
Skip On Output Flag
Interrupt On
Interrupt Off

Machine Language
Program:
• A program isalist of instructions or statements
for directing the computerto performa
required data-processingtask.
• What is instructions??
• Programcan be machine dependent or machine
independent.

Categories of Program
1. Binary Code
2. Octal or Hexadecimal code
3. Symbolic code
4. High-level programming language

Categories of programs
1. Binarycode
• Thisisasequenceof instructions and operandsin binary
that list the exactrepresentation of instructions as
they appear in computermemory.
Location Instruction Code
0 0010 0000 0000 0100
1 0001 0000 0000 0101
10 0011 0000 0000 0110
11 0111 0000 0000 0001
100 0000 0000 0101 0011
101 1111 1111 1110 1001
110 0000 0000 0000 0000

2. Octalor hexadecimalcode
• Thisisanequivalenttranslation of the binary codeto
octalor hexadecimalrepresentation.
Location Instruction
000 2004
001 1005
002 3006
003 7001
004 0053
005 FFE9
006 0000

3. Symboliccode
• The user employs symbols (letters, numerals, or special characters) for the
operationpart, the addresspart, andother partsof the instruction code.
• Each symbolic instruction can be translated into one binary coded instruction
by a special program called an assembler and language is referred to as an
assemblylanguageprogram.
Location Instruction Comment
000 LDA 004 Loadfirst operandinto AC
001 ADD 005 Addsecondoperandto AC
002 ST
A 006 Storesumin location006
003 HL
T Haltcomputer
004 0053 Firstoperand
005 FFE9 Secondoperand (negative)
006 0000 Storesumhere

A.L.P to add two numbers
ORG 0 /Origin of program is location 0
LDA A /Load operand from location A
ADD B /Add operand from location B
STA C /Store Sum in location C
HLT /Halt Computer
A, DEC 83 /Decimal operand
B, DEC -23 /Decimal operand
C, DEC 0 /Sum stored in location C
END /End of symbolic program
Module 2: Programmingbasic
computer

4. High-levelprogramminglanguages
• These are special languages developed to reflect the procedures used in the
solution of aproblem rather than be concerned with the computer hardware
behavior.E.g.Fortran, C++,Java,etc.
• The program is written in a sequence of statements in a form that people
prefer to think in when solvingaproblem.
• However, each statement must be translated into a sequence of binary
instructions before the program canbeexecutedin acomputer.
• The program that translates a high level language program to binary is called a
compiler.
INTEGER A, B, C
DATA A, 83 B,-23
C = A + B
END

Rules of the Language
• Each line of an assembly language is arranged
in three columns called fields. The fields
specify the following information:
1. The label field may be empty or it may
specify a symbolic address.
2. The instruction field specifies a machine
instruction or a pseudo instruction.
3. The comment field may be empty or it may
include a comment.

Fields of ALP
Label
(Empty/Symbo
lic address)
Instruction
(Machine/
Pseudo)
Comment
(Empty/include comment)
ORG 0 /Origin of program is location 0
LDA A /Load operand from location A
ADD B /Add operand from location B
STA C /Store Sum in location C
HLT /Halt Computer
A, DEC 83 /Decimal operand
B, DEC -23 /Decimal operand
C, DEC 0 /Sum stored in location C
END /End of symbolic program
computer

Fields of A.L.P
• The instruction field in an assembly language
program may specify one of the following
items:
1. A memory reference instruction(MRI)
2. A register-reference or input-output
instruction.(non-MRI)
3. A pseudo instruction with or without an
operand.

Pseudo Instruction
• A pseudo instruction is not a machine instruction but rather an
instruction to the assembler giving information about some phase
of thetranslation.
Symbol Information for the Assembler
ORG N Hexadecimal number N is the memory location for the
instruction or operand listed in the following line
END Denotes the end of symbolic program
DEC N Signed decimal number N to be converted to binary
HEX N Hexadecimal number N to be converted to binary

Instruction Field of Program
Symbol Information for the Assembler
CLA NON_MRI
ADD OPR DIRECT ADDRESS MRI
ADD PTR I INDIRECT ADDRESS MRI

Assembly language program subtract Two number
ORG 100 ORIGIN LOCATION IS 100
LDA SUB LOAD TO AC
CMA COMPLEMENT AC
INC INCREMENT AC
ADD MIN ADD TO AC
STA DIF STORE DIFFERENCE
HLT HALT COMPUTER
MIN, DEC 83 MINUEND
SUB, DEC –23 SUMTRAHEND
DIF, HEX 0 DIFFERENCE STORE HERE
END END PROGRAM

translated program of subtraction
HEXADECIMAL
LOCATION CONTENT SYMBOLIC PROGRAM
ORG 100
100 2107 LDA SUB
101 7200 CMA
102 7020 INC
103 1106 ADD MIN
104 3108 STA DIF
105 7100 HLT
106 0053 MIN, DEC 83
107 FFE9 SUB, DEC -23
108 0000 DIF, HEX 0
END

A.L.P. to subtract 2 numbers
Location Instruction Symbol Location
ORG100 MIN 106
100 LDASUB SUB 107
101 CMA DIF 108
102 INC
103 ADDMIN
104 ST
ADIF
105 HL
T
106 MIN, DE
C83
107 SUB, DE
C-23
108 DIF
, HEX0
END
1
2
computer

computer

Assembler
• Anassemblerisaprogramthat accepts asymbolic
language program and produces its binary machine
language equivalent.
• Theinput symbolic program is called the source
program and the resulting binary program is called
the object program.
• Theassembleris aprogram that operates on
character strings and producesanequivalent binary
interpretation.

Line of Code: PL3, LDA SUB I
Memory word symbol hex-code binary represent
1 PL 50 4C 0101 0000 0100 1100
2 3, 33 2C 0011 0011 0010 1100
3 L D 4C 44 0100 1100 0100 0100
4 A 41 20 0100 0001 0010 0000
5 SU 53 55 0101 0011 0101 0101
6 B 42 20 0100 0010 0010 0000
7 I CR 49 OD 0100 1001 0000 1100

First Pass of an Assembler
• During the first pass, it generates a table that
co-relates all user supplied address symbols
with their binary equivalent value.
• The binary translation is done during the
second pass.
• To keep track of the location of instructions,
the assembler uses a memory word called a
location counter(LC).
• Initial value of LC is given by ORG.

First Pass of an assembler
LC←0
Scannext line of code
Store
addressin
symbol table
together
with value
of LC
Increment LC
Set
LC
ORG
END
Goto
second
pass
Firstpass
1
2
no no
no
Label
yes
yes
yes
computer

Assembly language program subtract Two
number
ORG 100 ORIGIN LOCATION IS 100
LDA SUB LOAD TO AC
CMA COMPLEMENT AC
INC INCREMENT AC
ADD MIN ADD TO AC
STA DIF STORE DIFFERENCE
HLT HALT COMPUTER
MIN, DEC 83 MINUEND
SUB, DEC –23 SUMTRAHEND
DIF, HEX 0 DIFFERENCE STORE HERE
END END PROGRAM

Translated Program of Subtraction
HEXADECIMAL
LOCATION CONTENT SYMBOLIC PROGRAM
ORG 100
100 2107 LDA SUB
101 7200 CMA
102 7020 INC
103 1106 ADD MIN
104 3108 STA DIF
105 7100 HLT
106 0053 MIN, DEC 83
107 FFE9 SUB, DEC -23
108 0000 DIF, HEX 0
END

Address symbol Table
MEMORY SYMBOL HEX-CODE Binary rep.
WORD ( LC )
1 M I 4D 49 0100 1101 0100 1001
2 N , 4E 2C 0100 1110 0010 1100
3 (LC) 01 06 0000 0001 0000 0110
4 S U 53 55 0101 0011 0101 0101
5 B , 42 2C 0100 0010 0010 1100
6 (LC) 01 07 0001 0001 0000 0111
7 DI 44 49 0100 0100 0100 1001
8 F 46 2C 0100 0110 0010 1100
9 (LC) 01 08 0000 0001 0000 1000

Second Pass
• Machine instructions are translated during
the second pass by means of table-lookup
procedure.
• The assembler used four tables:
1. Pseudo instruction table.
2. MRI table
3. Non-MRI table
4. Address symbol table.

Pseudo instruction table
• Four symbols ORG,END,DEC and HEX.
• Each entry refers the assembler to a
subroutine that processes the pseudo
instruction when encountered in the
program.

MRI Table
And Memory to AC
Branch Uncondionary
Sym
AND
ADD
LDA
STA
BUN
BSA
3-bit Op-code
000
001
010
ISZ
011
100
011
110
Description
Add Memory to AC
Load To AC
Store Of AC
Return Address
Skip If Zero

Non-MRI Table
(12 Register Reference Instruction + 6 I/O reference
instruction)
7800
7400
Clear AC
Clear E
Comp. AC
Comp. E
Cir. Right AC and E
Cir. Left AC and E
Increment AC
Skip If AC is Positive
Skip If AC is Negative
Skip If AC is Zero
Skip If E is 0
Halt Computer
7200
7100
7080
7040
7020
7010
7008
7004
7002
7001
CLA
CLE
CMA
CME
CIR
CIL
INC
SPA
SNA
SZA
SZE
HLT

Non-MRI Table
(6 Input Output Reference Instruction)
F800
F400
F200
F100
F080
F040
INP
OUT
SKI
SKO
ION
IOF
Input to AC
Output From Ac
Skip on Input Flag
Skip On Output Flag
Interrupt On
Interrupt Off

Address Symbol Table
• Generated in First Pass.

Second Pass of an Assembler
• Second Pass
computer

Program Loops
• What is program loop?
DIMENSION A(100)
INTEGER SUM,A
SUM = 0
DO 3 J=1,100
3 SUM = SUM + A(J)

Symbolic Program to Add 100 Numbers
Location
100
101
102
103
104
105 Lop,
106
107
108
109 ADS,
110 PTR,
111 NBR,
112 CTR,
113 SUM,
114
…..
150
…..
1B3
1B4
Org 100
LDA ADS
STA PTR
LDA NBR
STA CTR
CLA
ADD PTR I
ISZ PTR
ISZ CTR
BUN LOP
HEX 150
HEX 0
DEC –100
HEX 0
HEX 0
ORG 150
……
DEC 75
DEC 23
END
/ Origin is HEX 100
/ Load First Address Of Operand
/ Store in Pointer
/ Load Minus 100
/ Store in Counter
/ Clear Ac
/ Add To Ac
/ Inc Pointer
/ Inc Counter
/ Loop Again
/ First Address Of Operand
/ Reserved for Pointer
/ Initialized Counter
/ Reserved For Counter
/ Sum is Stored Here
/Origin of operands is HEX 150
…..
/ First Operand
/Last Operand
/End of Symbolic Program

Programming Arithmetic and Logic
Operations
• Addition, Subtraction, Multiplication, Division
• AND,CMA,CLA – Logical Operations.
• Basic computer have only arithmetic instruction,
such as ADD.
• Operations that are implemented in a computer
with one machine instruction are said to be
implemented by hardware.
• Operations implemented by a set of instructions
that constitute a program are said to be
implemented by Software.
• Software implementation of arithmetic operations.

Logic Operations
• Three machine instructions: AND,CMA and CLA that
perform any logic operations.
• For example OR operation is not avaiable.
DeMorgan’s Law: x + y = (x’y’)’
LDA A Load first operand A
CMA Complement to get A’
STA TMP Store in a temporary location
LDA B Load Second Operand B
CMA Complement to get B’
AND TMP AND with A’ to get A’^ B’
CMA Complement again to get A OR B

Shift Operations
• There are three types of shift operations: logical, rotate and
arithmetic
• A logical shift moves bits to the left or right.
• The bits which 'fall off' the end of the word are discarded and the
word is filled with 0's from the opposite end.
• A rotate operation is a circular shift in which no bits are discarded.
A rotate right of the 8 bit binary number 1000 1011 gives
1100 0101, as shown below.
• An arithmetic right shift is similar to a logical right shift, except
that the leftmost bits are filled with the sign bit of the original
number instead of 0's.

Shift Operations
• Circular-Shift operations are machine
instructions in the basic computer.
• The logical shift-right operation is implemented
by:
CLE
CIR
• For logical shift-left operation
CLE
CIL

Subroutine
• A set of common instructions that can be used
in a program many times is called a subroutine.
• 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.
• In the basic computer, the link between the
main program and a subroutine is a BSA
instruction.

Demonstrates the use of subroutines

Input-Output Programming
• 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.

Programs to Input and Output One character
(a) Input a Character:
CIF, SKI /CHECK INPUT FLAG
BUN CIF /FLAG=0
INP /FLAG=1
OUT /PRINT CHARACTER
STA CHR /STORE CHARACTER
HLT
CHR, ____ /STORE CHARACTER
(b) Output one character:
LDA CHR /LOAD INTO AC
COF, SKO /CHECK OUTPUT FLAG
BUN COF /FLAG=0
OUT /FLAG=1
HLT
CHR, HEX0057 /CHARACTER IS “W”

Program to Compare Two Words
LDA WD1 /LOAD FIRST WORD
CMA
INC /FORM 2’S COMPLEMENT
ADD WD2 /ADD SECOND WORD
SZA /SKIP IF AC IS ZERO
BUN UEQ /UNEQUAL ROUTINE
BUN EQL /EQUAL ROUTINE
_____
_____
WD1,
WD2,

Exercise 1
• The following program is stored in memory unit of the basic computer.
Show the contents of AC,PC and IR(in hexadecimal) at the end, after
each instruction is executed. All numbers below are in hexadecimal.
computer
Location Instruction
010 CLA
011 ADD 016
012 BUN 014
013 HLT
014 AND 017
015 BUN 013
016 C1A5
017 93C6

Exercise 2
A line of code in an assembly language program
is as follows:
DEC -35
a. Show that four memory words are required
to store the line of code and give their binary
content.
b. Show that one memory word stores the
binary translated code and give its binary
content.

Exercise 3
• Show how the MRI and Non-MRI tables can be
stored in memory.

Exercise 4
• List the assembly language program(of the
equivalent binary instructions generated by a
compiler from the following fortran program.
Assume integer variables.
SUM=0
SUM=SUM + A + B
DIF = DIF – C
SUM = SUM + DIF
computer

Programming the Basic Computer and Assembler

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