Gogotron

mov ah, 0x0e		; initialize ah
mov al, 'a'		; first letter
label:			; enter loop
	int 0x10	; print char
	xor al, 32	; switch from uppercase to lowercase or vice-versa,
	inc al		; next letter

	mov bl, al	; This checks if the last 5 digits of al are 11010 
	and bl, 31	; meaning 26, without modifying it. This is the case 
	cmp bl, 26	; for 'Z' and 'z' but not for any other ascii letter.