Yamaha PSR640 Step By Step Procedures page 12

Songs on disk can be played back in five ways; SINGLE, SINGLE REPEAT, ALL,
ALL REPEAT, and RANDOM. To have Songs play continuously, select 'ALL
REPEAT' from the 'Play Mode' screen.
1) Insert the desired Song disk into the Floppy Disk Drive.
2) Press the [SONG] button.
3) Use the [BACK] and [NEXT] buttons to display the 'Song UTILITY' page.
4) Turn the Data Dial to select 'Play Mode' in the screen.
5) Press the [NEXT] button.
6) Turn the Data Dial to select 'ALL REPEAT' in the screen.
7) Press the [EXIT] button to return to the main screen.
8) Press the [START/STOP] button to start the first Song. All Songs on the
disk will now be played continuously.
Owner's Manual, p. 76-77
V. Playing Songs Continuously