The Cosmac VIP 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 VIP 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 VIP firmware uses the most common CDP 1861 resolution of 64 x 32 with interrupt routine entry point at @8146 and display memory at the last RAM page, i.e. for a 4 KB RAM system 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.
High Resolution via Chip 10 Support
A high resolution mode (128 x 64) on top of the CDP 1861 was described in Viper Volume 1, issue 7. For more details this volume can be found on the RCA Cosmac VIP yahoo group.
Since the 1861 video chip is incapable of more than 64 elements of horizontal resolution, a hardware add-on is necessary to get the 128-element horizontal resolution offered by CHIP-10. This device is conceptually simple: a pair of 128-bit memories, with address counters and control. One of the pair accepts and stores two successive 64-bit lines of video output data from the 1861 at its standard 1.76 MHz rate. Meanwhile, the other memory is providing the high-resolution video output, at a 3.52 MHz rate; it outputs the same information twice, on two successive TV scan lines. The stored video came, of course, from the 1861 on the previous two scan lines; the twin memory systems swap roles every two TV scan lines. This approach involves essentially zero modification of the original VIP hardware, and instant reversion to the standard 64X32 format whenever desired (e.g. to use the ROM operating system).