bogus

[ORG 0x100]

jmp @Principal

@WriteCh:

pusha
push es

mov dx, 0xB800
mov es, dx
mov cx, ax
mov al, 80
mul ah	
add al, cl
adc ah, 0
shl ax, 1
mov di, ax
mov ax, bx
stosw

pop es
popa
ret

@Principal:

mov al, 25                ;col
mov ah, 41                ;line
mov bh, 0x2F
mov bl, 'A'
call @WriteCh

@Fim:

mov ax, 0x4C00
int 0x21

;Formula -> 2*(lines*80+cols)