Using Page 2 in SCREEN 9 Gives "Illegal Function Call" on EGA (21956)

SUMMARY

Question:

Why do I get an "Illegal Function Call" error message when I reference Page 2 in SCREEN mode 9 on a computer with an EGA card? The following is an example:

   SCREEN 9,1,2,2  'EGA mode=9, 1=colorswitch on, apage=2, vpage=2
   COLOR 2,0
   LINE (1,1)-(100,100)
				

Response:

Screen 9 does have two pages available, but the pages are referenced as "0" and "1". Note that "2" is not a valid page number in screen 9. The program will work correctly if coded as follows:

   SCREEN 9,1,1,1  'EGA mode=9, 1=colorswitch on, apage=1, vpage=1
   COLOR 2,0
   LINE (1,1)-(100,100)