They're stored in the serial buffer, which can store up to 64 bytes. To be on the safe side, I only send 32 bytes at a time (waiting==32). As soon as the arduino has processed those 32 bytes (which can be any of A through H, each corresponding to a direction of travel), it sends the command to resume to processing, which sends the next 32 bytes. In practice, there's no noticeable delay - the motors are in constant movement.
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