Something

section .data

letter db 'A'

global _start

_write:
	mov edx, 1
	mov ecx, letter
	mov ebx, 1
	mov eax, 4 ; ssize_t sys_write(unsigned int fd, const char * buf, size_t count)
	int 0x80
	inc BYTE [letter]
	
	
_start:
	cmp BYTE [letter], 'Z'
	jle _write
	cmp BYTE [letter], '[' ; Z + 1
	je _add
	cmp BYTE[letter], 'z'
	jle _write
	xor ebx, ebx
	mov eax, 1
	int 0x80

_add:
	add BYTE [letter], 6