equivalent of waitvbl (from Amos and sdlBasic) needed

[nitrofurano]

i tried this code

Code:

#include <print64.bas> 10 ink 7:paper 0:border 2: bright 1:flash 0:cls 20 let x=10:let y=10 30 printat64(y,x):print64("X") 100 let a$=inkey$ 200 if a$="w" or a$="W" or a$="7" or a$=chr$(11) then printat64(y,x):print64(" "):y=y-1:printat64(y,x):print64("X"):end if 210 if a$="s" or a$="S" or a$="6" or a$=chr$(10) then printat64(y,x):print64(" "):y=y+1:printat64(y,x):print64("X"):end if 220 if a$="a" or a$="A" or a$="5" or a$=chr$(8) then printat64(y,x):print64(" "):x=x-1:printat64(y,x):print64("X"):end if 230 if a$="d" or a$="D" or a$="8" or a$=chr$(9) then printat64(y,x):print64(" "):x=x+1:printat64(y,x):print64("X"):end if 1000 goto 100

but it runs too fast, and i only need to synchronize it with the display frequence (50hz or 60hz)
i think zxspectrum uses this with some z80 interrupt instruction? which one, and how can we use it inside 'asm/endasm'?