From QB64 Wiki
The WAIT statement waits until the value read from an I/O port has certain bits set.
- WAIT port%, andMask%[, xorMask%]
- The WAIT statement reads a value from port% using INP.
- If xorMask% is specified, the value is XOR'd with xorMask%. It has the effect of "toggle these bits".
- The value is then AND'd with andMask%. It has the effect of "check if these bits are set".
- If the final value is non-zero, WAIT returns. Otherwise, another value is read from port% and checked again.
- The WAIT statement returns immediately if port% is not supported.
- Waiting for vertical retrace
' Either statement can be used to try to reduce screen flickering. ' If both statements are used, try changing the order. WAIT &H3DA, 8 ' finishes whenever the screen isn't being written to WAIT &H3DA, 8, 8 ' finishes whenever the screen is being written to