Testing the used display adaptor.

Step 1. MDA or better?
40h:63h contains B4 for hercules, D4 for CGA or better.

Step 2. EGA present?
40h:87h is zero if no ega or better is present.

Step 3. VGA present?
EGA/VGA BIOS is located at C000h (in case of doubt, test segments
C000, C800, D000, D800, E000 and E800).
VGA bios contains the word 'VGA' (usually several times).