Compress string lowercase for uppercase in Assembly

I made this code that reads a sentence and determines whether the first letter is uppercase or not, now I have to scan the whole sentence by changing where it is lowercase to uppercase, I'm having trouble with how to do this.

segment .data
msn db "Entre com uma frase:",sl
tam equ $-msn

msn1 db "Sua frase  comeca com  letra maiuscula:", sl
tam1 equ $-msn1

msn2 db "Sua frase nao comeca com  letra maiuscula:",sl
tam2 equ $-msn2

msn3 db " ",sl
tam3 equ $-msn3

sl equ 10
segment .bss
buf resb 100
buf2 resb 2


segment .text
global main
main:

     ;exibe string
      mov edx,tam    ;tamanho
      mov ecx,msn   ;ponteiro
      call print_str


     ;ler strim
      mov edx,100     ;tamanho da string
      mov ecx,buf    ;destino
      mov ebx, 0     ;Teclado
      mov eax, 3     ;read 
      int 0x80

      mov al,[buf +0]
      mov [buf2],al

      cmp byte[buf2,0],65
      jl minusculo 
      cmp byte[buf2,0],90
      jg minusculo 

      mov edx,tam1
      mov ecx, msn1  
      call print_str

      jmp fim


minusculo:

      mov edx,tam2
      mov ecx, msn2    
      call print_str

fim:      
      mov edx,tam3
      mov ecx,msn3
      call print_str


; ----------------- AREA DOS PROCEDIMENTOS ------------------

print_str:
    mov ebx, 1          ; Descritor (monitor = 1)
    mov eax, 4          ; Servico (print)
    int 0x80            ; Executa (exibe)
    ret             ; Devolve ao chamador