How can I write this program using assembly language

Question

1-Write programs to test that a character read from the keyboard and transfer control to label ok_here, if the character is:

(i) a valid lowercase letter ( 'a' <= character <= 'z' )

(ii) either an uppercase or lowercase letter ('A' <= character <= 'Z' OR 'a' <= character <= 'z')

(iii) is not a lowercase letter, ie character < 'a' or character > 'z'. The programs should display appropriate messages to prompt for input and indicate whether the character satisfied the relevant test.

i)

.Model  small

.stack 100h

.code 

Main proc

Mov ah,1h

Int 21h

Cmp AL,'a'

JE then

Tmp else

Then:ok_here

JMP End if

Else mov  AH,4CH

Int 21h

End if

ii)

mov AH,1h

int 21h

cmp Al,'a'

JE  then

Cmp AL,'A'

JE  then

Cmp AL,'z'

JE  then

JMP else

Then ok_here

Jmp End if

Else mov AH,4CH

Int 21h

End if

2-Write a program that reads an uppercase letter, converts it to lowercase and displays the lowercase equivalent. the program let the user repeat this process as often as desired. The user is asked to enter 'y' to carry out the operation, after each iteration.

.model small

.stack 100h

.data

CR  EQU 0DH

LF EQU 0AH

MSG1 DB 'enter an uppercase letter'

MSG2 DB CRLF ' the lowercase equivalent'

CHAR DB ?

.code

Main proc

Mov AX @data

Mov DS,AX

LEA DX,MSG1

Mov AH,@H

Int 21h

SAR AL.20h

Mov CHAR,AL

LGA DX ,MSG2

Mov AH,02H

Int 21h

Mov AH,4CH

Int 21h

Main Endp

End main

Is my code correct?

Answer 1

No.

Your first program only tests whether AL is lowercase 'a' or not. It has also a syntax error, because you are using ok_here as an instruction, not a label (the label would be Then ). Besides, I guess that the Tmp instruction is actually the JMP instruction.

For your first program to be correct, you must test if the value in AL is equal or greater than 'a' AND is equal or less than 'z':

CMP AL,'a'
JB notok
CMP AL,'z'
JA notok
ok_here: ;if we reach this point, AL is a lowercase letter

Your second program does a similar thing: only goes to ok_here if AL is 'A', 'a' or 'z'. You only use the JE instruction, which jumps if AL is equal to the second operand of the CMP instruction. To fix it, use the previous code to test if AL is lowercase, and if it is so, repeat the test but this time with 'A' and 'Z'. That is: four CMP's and four conditional jumps.

Also, your program lacks the strings to tell the user to input a character and the printed messages telling if the character meets the criteria or not.