Prints the alphabet, in alternating caps (aBcD...)

boot.asm

; bios setup
mov ah, 0x0e

; print
mov bl, 0 ; offset
mov bh, 97 ; lowercase
mov ch, 65 ; uppercase

loop:
  ; lowercase
  mov al, bh
  add al, bl
  int 0x10
  inc bl

  ; uppercase
  mov al, ch
  add al, bl
  int 0x10
  inc bl

  cmp al, ('Z')
  je exit
  jmp loop

exit:
  jmp $

; end
times 510-($-$$) db 0
db 0x55, 0xaa