Re: Double Lores mode?

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <47lrr7$52u@urvile.MSUS.EDU> schefflr@news.msus.edu (Bill
Scheffler) writes:
>	I heard there is a double low resolution mode for the Apple II.  
>I know there isn't a direct command (ie:GR2) to access it.  So my 
>question is:  what's the command (poke xxxx, ect.).  Thanks.

Double lores graphics is turned on by activating the 80-column display, and
turning off annunciator 3, and then flipping the switches to turn on ordinary
lores.  The following will display the double lores screen.

     10  PRINT  CHR$ (4);"PR#3": PRINT  CHR$ (12);
     20  POKE 49246,0
     30  GR

What you see will be dependent on what kind of Apple you have.  An Apple IIe
will show you alternating vertical black and colored stripes.  An Apple IIGS or
(I think) and Apple IIc will clear the entire double lores screen.

On an Apple IIe, that's about all BASIC can do with the double lores screen,
unless you call a machine-language double-lores drawing routine.  You might
be able to make do with the PLOT command and POKEing the PAGE1/PAGE2 switch,
but that's slow and inconvenient.

On an Apple IIc or IIGS, however, the above lines activate double lores
plotting mode.  The PLOT, HLIN, and VLIN commands accept X coordinates from 0
to 79 (instead of the normal 0 to 39), and you can draw on the entire screen.
For example,

     39  REM (Lines 10-30 are as above)
     40  FOR X = 0 TO 39
     50  COLOR=  RND (1) * 16
     60  HLIN 0,79 AT X: HLIN 0,79 AT 39 - X
     70  VLIN 0,39 AT X: VLIN 0,39 AT 39 - X
     80  VLIN 0,30 AT X + 40: VLIN 0,39 AT 79 - X
     90  NEXT : GOTO 40

Remember, this won't work on an Apple IIe (you'll get an error message when
the program hits the number 79 in line 60).


I think I remember reading somewhere that there's a bug in the SCRN function
in double lores mode.

               - Neil Parker
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker

     "Evolution is vastly overrated."  -- Ambassador Delenn, _Babylon_5_