Failing to convert upper to lower and vice versa user input string

Question

I am trying to write an assembly program on emu8086 to convert upper to lower and vice versa user inputted string but it's not changing any character of the string:

.data

buffer db 10,?, 10 dup('')

.code

; Read the input

mov ah, 0ah int 21h

mov si,2

mov ah,0 mov al, [si]

;convert

start:

cmp al, 0

je show

; Lowercase to uppercase

cmp al, 'a'

jl upper

cmp al, 'z'

jg upper

sub al, 32

jmp check

upper: cmp al, 'A'

jl next_bit

cmp al, 'Z'

jg next_bit

add al, 32

jmp check

check:

mov [si], al jmp next_bit

;incrementing si register to get the next character

next_bit: inc si

mov al,[si]

jmp start

;display result

show:
mov si,2

lea dx,si

mov ah, 9

int 21h

jmp end

end: mov ah, 4ch

int 21h

Answer 1

1. mov si, offset buffer is wrong. The text starts at buffer+2.
2. cmp al, 0 is wrong. Your buffer is not zero initialized and you should use the returned byte count anyway.
3. Your code will never convert upper case to lower case since the ; Uppercase to lowercase block is unreachable.
4. next_bit is a misnomer. You probably meant next_byte.
5. lea dx, si is wrong. You have already incremented si to point past the string. You should reload dx with buffer+2. (Should be a syntax error too.)
6. The text is not terminated by a $ so int21/09 can not print it as is.
7. jmp check is not wrong :) But it's certainly useless jumping to the next line.