williams-et-al-1983-apple-ii-computer-graphics

34 APPLE II COMPUTER GRAPHICS• Switch 4 selects between the Low"Res and Hi-Res modes. Switch 4 hasno visible effect unless switc h 1 is set to display graphics.The APPLE lie has several additional display-oriented soft switches.Those switches are discussed later in this chapter under the heading "Forlle's Only."Toggling So SwitchesDismembering your computer to look fo r these switches will not help youlocate them; remember, they are not a physical switch, but only exist inthe system software. So how do you flip a switch that does not exist? Whenthe computer is turned on, the operating system automatically togglesswitch one and switch three to display the primary page of text, but fromthere it is up to you.Each switch may be fli pped in the desired direction by referring to theassociated address. To set switch 1 from BASIC to display graphics, youcould PEEK at the value which is in location 49232, or POKE a new valuein there. Neither the value POKEd nor the the value PEEKed is of anyconsequence; POKE 49232,0, X = PEEK (49232), PRINT PEEK (49232), andPOKE 49232, 175 all set switch 1 to graphics mode. It is the mere act ofreferring to that location which sets the switch. From Monitor you can setthe graphics switch by typing C050 , so that Monitor willdisplay the value in location C050, or type C050:0 to place a zero in thatlocation. Again, it is only the reference to the location which is important.Try setting the switches to view the fi rst page of Hi-Res graphics withoutthe four lines of text at the bottom and without clearing the screen. Referto Figure 5-2 fo r the associated addresses for each switch. Your sequenceof commands should look something like this. From the BASIC prompt ( >or ])type:POKE 49234, (Set fu ll screen)POKE 49236, (Set page 1)POKE 49239, (Set Hi-Res mode)POKE 49232, (Set graphics)The order is not important, but if you set the graphics mode fi rst (POKE49232,0), then the remainder of the POKEs will not be visible to you sincethe text is no longer being displayed.To set the same display sequence from Monitor, type:C52: C54: C57: C5: