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

36 APPLE II COMPUTER GRAPHICSFor I/e 's OnlyThe APPLE lie uses three additional soft switches to help control the display.These switches are referred to by the names 80COL, 80STO RE, andALTCHARSET.80COL determi nes whether the text is displayed with the normal 40columns per line or with 80 columns per line. (You must have an 80column card installed to display 80 columns per line!) To set the 40column mode, POKE a value (any value) into 491 64 ($C00C). To enablethe 80 column mode, POKE location 49165 ($C00D).80STORE determines whether addresses referencing memory locationswithin the first page of text (1024 to 2047) actually refer to memory on themain memory boara, or memory on the auxil iary 80 column board. Doyou have the feeling that you missed something? Let's back up a bit.Allowing an 80 column display doubles the number of characters whichare displayed on one page of text. This requires twice the amount ofmemory. The Apple lie designers had to fi nd an additional 1024 bytes ofmemory somewhere, without disturbing the existing memory ranges usedin the APPLE II +. The solution was to "clone" Page 1 of Text memory andput the duplicate memor/ on the 80 column board. But now there are twopost office boxes with the address 1024, two boxes addressed 1025, and soon through 2047. When your c " omputer "postman" is told to deliver avalue to one of these addresses (for example, address 1777), he or shemust know which of the two boxes having that address is to receive thevalue. The 80STORE switch acts as a mini ZIP code; it specifies which ofthe two banks of memory, main or auxiliary, is intended . POKEing anyvalue into 49152 ($C000) will route all addresses to memory on the mainboard, and POKEing into 49153 ($C©©1) routes addresses from 1 ©24through 2©47 to the memory on the 8© column board .ALTCHARSET switches the character generator (the thing that takes thenumeric code for an A and makes it appear as A on the on the screen)between the standard character set (American characters) and some onboard,alternate character set. For, though it may come as a surprise to8©COL8©STOREALTCHARSET$C!iJ!iJC ( 49164)$C!iJ!iJD ( 49165)$C!il!il!il ( 49152)$C!il!ill ( 4915 3)$C!iJ!iJE ( 49166)$C!iJ!iJF ( 49167)4© Columns8© ColumnsStore in main memoryStore in auxiliary memoryStandard character setAlternate character setFigure 5-4. Summary of Apple lie so switches.