hello assembly

section .bss
        Buff resb 1
section .data
section .text
        global _start
_start:
        nop   ; this no op keeps the debugger happy
Read:   mov eax, 3   ; mov 3 to eax which is read syscall no
        mov ebx, 0   ; fd 0 stdin
        mov ecx, Buff ; mov address of Buff to ecx
        mov edx, 1 ; mov read length to edx
        int 80h ; call syscall
        cmp eax, 0 ; compare eax with 0 eax is return value of syscall
        je Exit  ; jump if prev instruction is equal
        cmp byte [Buff], 61h   ; compare Buff with 'a'  [Buff] is the value Buff point to
        jb Write   ; jump to Write label if com below 'a'
        cmp byte [Buff], 7Ah ; compare Buff with 'z'
        ja Write   ; if above 'z' not lowercase 'z' at this point, we have a lowercase
        sub byte [Buff], 20h   ; subtract 20h from buff
Write:  mov eax, 4  ; write syscall
        mov ebx, 1  ; stdout fd
        mov ecx, Buff ; address of buff to ecx
        mov edx, 1  ; number of chars to write
        int 80h  ; interupet syscall 
        jmp Read  ; jump back to Read label
Exit:   mov eax, 1 ; exit syscall
        mov ebx, 0 ; exit number 0
        int 80h   ; make syscall