; this is a program in 8086 assembly language that 
; accepts a character string from the keyboard and 
; stores it in the string array. the program then converts  
; all the lower case characters of the string to upper case.  
; if the string is empty (null), it doesn't do anything. 

name "upper"

org 100h


jmp start


; first byte is buffer size, 
; second byte will hold number 
; of used bytes for string, 
; all other bytes are for characters: 
string db 20, 22 dup('?')

new_line db 0Dh,0Ah, '$'  ; new line code. 

start:

; int 21h / ah=0ah - input of a string to ds:dx,  
; fist byte is buffer size, second byte is number  
; of chars actually read. does not add '$' in the 
; end of string. to print using int 21h / ah=09h 
; you must set dollar sign at the end of it and  
; start printing from address ds:dx + 2. 

lea dx, string

mov ah, 0ah
int 21h

mov bx, dx
mov ah, 0
mov al, ds:[bx+1]
add bx, ax ; point to end of string. 

mov byte ptr [bx+2], '$' ; put dollar to the end. 

; int 21h / ah=09h - output of a string at ds:dx. 
; string must be terminated by '$' sign. 
lea dx, new_line
mov ah, 09h
int 21h


lea bx, string

mov ch, 0
mov cl, [bx+1] ; get string size. 

jcxz null ; is string is empty? 

add bx, 2 ; skip control chars. 

upper_case:

; check if it's a lower case letter: 
cmp byte ptr [bx], 'a'
jb ok
cmp byte ptr [bx], 'z'
ja ok

; convert to uppercase: 

; upper case letter do not have 
; third bit set, for example: 
; 'a'             : 01100001b 
; 'a'             : 01000001b 
; upper case mask : 11011111b 

; clear third bit: 
and byte ptr [bx], 11011111b

ok:
inc bx ; next char. 
loop upper_case


; int 21h / ah=09h - output of a string at ds:dx. 
; string must be terminated by '$' sign. 
lea dx, string+2
mov ah, 09h
int 21h

; wait for any key press.... 
mov ah, 0
int 16h


null:
ret  ; return to operating system.