BYTE CONVERSION CODE: ASCII to HEXADECIMAL

section .data
message db "Enter a byte of HEXADECIMAL VALUE ",10 ;Declare a message
message_length equ $-message ;Store length of message


section .bss

%macro PRINT 2 ;PRINT macro, This macro will take 2 parameters
mov rax,1
mov rdi,1
mov rsi,%1 ;Source to print
mov rdx,%2 ;Length of what to print
syscall
%endmacro

%macro READ 2 ;READ macro, This macro will take 2 parameters
mov rax,0
mov rdi,0
mov rsi,%1 ; Source: Where to store read value
mov rdx,%2 ;How many characters to be read + 1 (enter key)
syscall
%endmacro



counter resb 1
read_ascii resb 2 ;this will store the read value
converted_hex resb 1 ;this will store converted hexadecimal value

section .text
global _start
_start:

PRINT message,message_length ;Print the message
READ read_ascii,3                 ;Read the value 3= 2 characters to be read+ 1(enter key)

mov rsi,read_ascii                 ;Give source (value to be converted)
mov rdi,converted_hex         ;Give destination to store result
CALL ASCII_HEX ;Call the conversion Procedure

mov rax,60                 ;EXIT
mov rbx,0
syscall


ASCII_HEX:
xor bl,bl         ;bl will store the result. So,initializing rbx with zero
mov byte[counter],2 ;counter = number of characters to convert
loop: ;loop label
mov al,byte[rsi] ;Take the byte of value to be converted
cmp al,39h ;Refer ASCII table. This is needed for deciding value of character between (0 to 9) or (A to F)
jbe sub_30 ;If character<39 it is between 0 to 9. So,subtract only 30 from it
sub al,0x07 ;If character>39 it is between A to F. So,subtract only 7 + 30 from it
sub_30:
sub al,0x30 ;Now al contains converted value read from rsi
rol bl,4         ;Rotation is needed for storing new converted value which is of 4 bits
add bl,al         ;Store in bl
inc rsi ;Point to next character
dec byte[counter] ;Decrement Counter
jnz loop         ;If Counter!=0 jump to loop label
mov byte[rdi],bl ;Store result in destination
RET         ;Return