The VELF uses the CDP 1861 for video output. For details on the CDP 1861 please see the data sheets for this chip or the 'summary' on the CDP 1861 page. The VELF controls the CDP 1861 via OUT 1 to switch the display off, INP 1 to switch the display on and EF1 to indicate the display status.
The VELF firmware uses the most common CDP 1861 resolution of 64 x 32 with interrupt routine entry point at @8144 and display memory at @0F00-@0FFF.
The display status is an active low output signal occurring for a period of four horizontal cycles prior to the beginning and end of the 128 line display window. The signal is used by the firmware interrupt routine to indicate the last 'row' of the display area.