Programming basic computer

Programming Basic Computer
• The list of instructions or statements that
directs the computer hardware to perform a
specific task is called a program.
• There are many programming languages but,
the computer basically understands programs
that are given in binary (0s, 1s) form. This
implies that all other programs should be
translated to binary before they can be
executed by the computer..

Classification of Programs
• Binary Code: These are instruction and data that are in 0s
and 1s. They are stored in the computer's main memory as
they do not need any conversions.
• Octal / Hexadecimal code: These are codes or instructions
that are in octal or hexadecimal representation.
• Symbolic Code: This type of programming uses symbols
(numbers, letters or special characters) in place of binary
code.
• High level programming language: These are codes that
are directed at solving problems and not the details of
computer hardware behaviours. e.g. Java, C#, Pascal etc.
They need special software called interpreter or compiler
to convert them into binary code

Machine Language
• These are programs that are written in binary
and stored directly in memory. The octal (3
bits) or hexadecimal (4 bits) that can be easily
converted to binary can also be referred to as
machine language.

Assembly Language
• This uses symbols (English language words) to
write instructions. This is also a low level
language because of the one-to-one
relationship between symbolic instruction and
its binary equivalent. The symbols are referred
to as mnemonics. An assembler is a special
software that converts assembly language
program into binary language program.

Basic symbols (mnemonics) of
assembly language
Symbol Example Explanation
AND AND A Perform AND logic operation on A with value of AC
ADD ADD A Add the value of variable A to AC, and carry bit to E flip flop
LDA LDA M Load value of M into AC
STA STA M Store value of AC in M
BUN BUN TT Branch unconditionally to location TT
BSA BSA TT Save the return address in location TT and branch to TT+1
ISZ ISZ T Increment T and skip next instruction (if T = 0)
CLA CLA Clear Accumulator (AC)
CLE CLE Clear E
CMA CMA Complement AC (first complement)
INC INC Increment AC
SPA SPA Skip next instruction if AC is positive
SNA SNA Skip next instruction if AC is negative
HLT HLT Halt computer

Mnemonics of Assembly Language
Mnemonics Example Explanation
INP INP Get information from some input device
OUT OUT Put information to some output device
SKI SKI Skip next instruction if input flag is ON
SKO SKO Skip next instruction if output flag is ON
ION ION Turn interrupt ON
IOF IOF Turn interrupt OFF
CALL CALL SUB Call subroutine which starts at location SUB
RET RET Return to main program
MOV MOV A, B Transfer the value of register B to register A

Pseudo instruction (Assembler
Directive)
• These are false (pseudo) instructions that do not have
equivalent binary form but serve various functions. They do
not refer to an operation that will be performed by the
program during execution, rather it is a message to the
assembler to help the assembler in the assembly process.
E.g.
• ORG T, T is the memory location where the first instruction
of the program must be stored.
• END, Specifies the end of the program
• DEC T, T is a decimal number that needs to be converted
into binary by the assembler
• HEX T, T is a hexadecimal number that needs to be
converted into its equivalent binary using 4bits.

Rules of the Assembly Language
• Each line of code must be divided into four fields.
i.e. Label, instruction, operand & comment.
• Label: This is a one to three alphanumeric
characters (symbolic address) that specifies the
location of the instruction in memory. It should
be terminated with a comma (,) to enable the
assembler recognize it as a label. The first
character should be an alphabet and the rest
either alphabets or numerals. A line with ORG or
END should not have a label.

• A symbolic address in the instruction field
should specify the memory location of an
operand. This symbolic address must appear
again as a label, later in the program.
• Instruction field. This specifies a mnemonic
(pseudo) instruction.

• Comment: Explains what each line of code
does for easy understanding and explanation.
Each comment must be preceded by /. This
helps the assembler recognize the beginning
of a program. It can be left empty.

Programming in Assembly Language
• Write a program in assembly language to store
15 at memory location T
Code Comment
ORG 0 /Origin of the program in location zero
LDA A /Load operand (15) from A
STA T /Store operand (15) in memory location T
HLT /Halt computer
A, DEC 15 /Decimal operand with value 15
T DEC 0 /Value 15 will be stored at T
END /End of symbolic program

• Write a program in assembly language to add
two decimal numbers (14 and -7) in memory
locations A and B.
NB. We first have to bring the first operand to
AC. Using the ADD instruction, we add the
second operand with the content of AC. The
result is then stored in AC. However to store
the result we use STA.

• . Label Code Description
ORG 0 /Origin of program in location zero
BUN T /Branch unconditionally to location T
LDA A /Load operand in accumulator from memory
location A
ADD B /Add operand from memory location B
STA SUM /Store the sum in location SUM
HLT /Halt Computer
A DEC 14 /Decimal operand with value 14
B DEC -7 /Decimal operand with value -7
SUM DEC 0 /Sum of A and B will be stored in AC, initial value of
SUM is 0
END /End of symbolic program.

Flow chart
• Flow chart for adding two decimal numbers
Specify the location where the program must be stored
in memory
Specify the memory location of first operand
Specify the memory location of second operand
Add second operand with first operand , already in AC
Specify the memory location where the result from AC
is to be stored
Load the first operand in AC

Assembly language to subtract two
numbers A – B (14 - 10)
Label Code Description
ORG 100 /Origin of program is 100
LDA SUB /Load subtrahend to AC (B)
CMA /Complement AC (-B)
INC /Increment AC
ADD MIN /Add minuend to AC (A)
STA DIF /Store the difference in memory location DIF
HLT /Halt computer
MIN, DEC 14 /Minuend with decimal 14 (A)
SUB DEC 10 /Subtrahend with decimal 10 (B)
DIF DEC 0 /Result of the difference is stored here
END /End of symbolic program.

Flow chart for subtracting two
numbers A – B = A + (-B)
Specify the memory location where the program must be stored
Specify memory location of subtrahend (B)
Load the subtrahend in AC
Complement value stored in AC
Increment value in AC by 1 (get - B)
Specify location where minuend is stored in memory (A)
Add the minuend to value in AC (A +(-B))
Specify the location where the result will be stored in memory
Calculating
2s complement
The negative of a
number is obtained
by calculating its
2s complement. This is
done by adding 1 to its
1s complement

Assembly language- Looping
• Program loop (looping) is a sequence of instructions
that are executed repeatedly, each time with a
different set of data and the new result stored. E.g.
2.....
Int x, y;
X = 10; y = 20;
If (x<y)
system.out.println(“x is
less than y”);
x = x +2;
If (x == y)
System.out.println(“now x
is equal to y”);
}
}
1)........
Int x;
X = 3;
If (x < 10)
x = x +1;
else
System.out.println(x);
}
}
3...
int x;
for(x=0; x<10; x++)
System.out.println (“x is:”+ x);
}
}

Program to add 50 numbers
• To add 50 numbers we will have to use
looping by repeatedly performing addition (50
times) with each of the 50 operands and
storing the result in AC each time. We assume
that the operands are stored in consecutive
memory location so that we can get the
operand by incrementing the address

Assembly language program to add 50 numbers
Line no Label Code Description
1 ORG 50 /Origin of program is HEX 50
2 LDA ADS /Load address of first operand
3 STA PTR /Store address in memory location called (PTR) pointer
4 LDA NBR /Load -50 into AC
5 STA CTR /Store the value in AC at CTR (counter)
6 CLA /Clear accumulator
7 LOP, ADD PTR I /Add an operand to AC
8 ISZ PTR /Increment pointer
9 ISZ CTR /Increment counter
10 BUN LOP /Repeat loop again, AC gets nos added and store result
11 STR SUM /Store sum
12 HLT /Halt computer

Assembly language program to add 50 numbers
Line no. Label Code Description
13 ADS, HEX 50 /Address of first operand
14 PTR, HEX 0 /Location reserved for pointer
15 NBR, DEC -50 /Initialise counter
16 CTR, HEX 0 /Location reserved for counter
17 SUM, HEX 0 /Sum is stored here
18 ORG 100 /Origin of operands is from memory location 100
19 DEC 30 /First operand (i.e. 30)
.
.
68 DEC 14 /Last operand (50th number is 14)
69 END /End of symbolic program

Flow chart for adding 100 numbers
• .
Specify the memory location where program is to be stored
Specify the memory location where the address of 1st operand is stored
Load this address into AC
Store address (in AC) in a memory location. Let the location be PTR
Similarly, store -50 (counter variable) in a memory loc. called CTR
Set AC 0
Take address from PTR. Fetch the operand stored at
this address and add it to the value in AC
Increment PTR by 1
Increment CTR by 1
Is CTR = 0 ?
Store the value in AC at location named SUM
No
Yes

Compute X+Y and store the result at Z
(OR) Logic operations.
Label Code Description
ORG 0 /Origin of program at memory location 0
LDA X /Load first operand a
CMA /Complement A to get Ā
STA TMP /Store Ā in a temporary location TMP
LDA Y /Load second operand B
CMA /Complement B to get B̄
AND TMP /AND with Ā to get Ā.B̄
CMA /Complement (Ā.B̄) to get (A + B)
HLT /Halt computer
X, 1011 /First operand is stored here
Y, 1000 /Second operand stored here
TMP, 0000 /Temporary location to store Ā
END

Flow chart for logic OR operation
Load X into AC
Complement value in AC
Store the value of AC in TMP
Load Y into AC
AND value with TMP
Store value of AC in Z

Imperative, Declarative and Directive
Statements
• Imperative statement. Indicates actions to be performed
during execution of the assembled program. E.g.
LDA A, STA, ADD, ISZ C
• Declarative statement. Declares constants or storage
locations or area in a program. E.g
T DS 30
declares a storage area of 30 words and indicates that the
storage area must be called T. T is the first word of the
operand. To access the 6th word for example, use T + 5.
Alternative an index can be used. T(k) where k is the index
number of item to be accessed. E.g T(6) the seventh
number as it starts from 0 i.e. T(0) is the first element.

Directive
• Assembler directive statement, directs the
assembler to take certain actions during the
process of assembling a program. E.g. ORG
’10’ indicates that the first word of the
program must be placed at memory location
‘10’. Also END indicates that no more
assembly language statements remain to be
processed.

Program to show all codes
.
Label Code comments
ORG 0 (Directive) /Store program at location 0
LDA A (Imperative) /Load the value stored at memory location
A
STA B (Imperative) /Store the value at A in the location B
A DC ‘1500’ (Declarative) /Declare a constant 1500 named A
B DS 1 (Declarative) /Declare a storage location called ‘B’
capable of storing 1 word
END (Directive) /End program

Programming basic computer

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