Banking Made Easy or, Should You Use›DEPOSIT Instead of POKE to Access›Banked Memory?››by Anthony Ramos››You 130XE and RAMBO XL owners know›how great those 16K memory banks are›for a RAM disk.  They're also very›handy when it's time to use›Atariwriter Plus, and other software›that uses them to your advantage. ›But what about when you want to roll›up your sleeves and use them›yourself?  You can't just POKE around›as though it's a continous chunk of›memory.  The "hhh*LVd" disk reading›string won't do the job either.  But›here's some information you can›really bank on, from Mapping the›Atari:››When a bank is opened for access, it›appears through an "access window" in›the main memory, at locations›16384-32767.  If you enable bank›switching, you cause the normal RAM›in this area to be replaced by the›bank you've chosen.  You can›configure the system to one of four›modes: both processors using main›memory, CPU (6502 main processor)›using banked RAM, video (ANTIC) using›banked RAM, or both using banked›memory.  In all cases, the only›memory affected is the area›16384-32767.››In CPU extended RAM mode, only the›CPU accesses the extra memory.  All›ANTIC cycles operate in the main 64K›memory.  This means you can use the›extended memory for programs and›data, while using the main bank for›display lists and screen data.››In the video extended RAM mode, all›ANTIC references to the area›16384-32767 will be directed to the›secondary bank; all CPU references›will occur in the main bank.  This›allows you to access the entire RAM›memory for programs and data in the›main area, while locating display›lists and screen data in the›secondary bank.›In the general extended RAM mode,›both the CPU and ANTIC process in the›bank switched memory, and the main›area in 16384-32767 is not used at›all.››Location 54017 controls which bank is›accessed, and by which processors. ›Default at powerup is both chips›using main memory, or compatibility›mode.  To access the banks, POKE›54017 with 193+(Mode*16)+(Bank*4),›with the modes and banks as follows:››Mode 0: both chips access banked›memory;›Mode 1: CPU accesses main memory,›ANTIC accesses banked memory;›Mode 2: CPU accesses banked memory,›ANTIC accesses main memory; and›Mode 3: both chips access main memory›(Compatibility mode).››Bank 0: first bank;›Bank 1: second bank;›Bank 2: third bank; and›Bank 3: fourth bank.››An example: If you plan to use the›banked memory for a character set,›you must first load it in using the›6502 main processor.  Poke 54017 with›193+16*2+0 to access bank one.  Then›use the familiar POKEs to 852-853 and›856-857 and "hhh*LVd" to load the set›into the area 16384-32767.  Now POKE›54017 with 193+1*16 to make ANTIC›access the banked area while the 6502›uses main memory as usual, and POKE›756, address/256 of your set.  Make›sure that any statement which affects›location 54017 is not in the area›16384-32767, or the computer will›crash.  You can check the memory›location of a BASIC statement with›the line PRINT ADR(" ").  The book›"Mapping the Atari" goes into more›detail.››If you don't want to get this›involved, here are some machine›language strings which give you the›same power over the banks that you›are accustomed to with the "straight"›memory.  Remember, though, that these›commands will alter or completely›erase the RAM disk if present.››When you power up the computer, the›banks are not clear, but full of›garbage data.  BCLEAR fills all four›banks with zeros for a fresh start. ›BPOKE and BPEEK work like POKE and›PEEK, except you access the banks›like a continous block of memory›locations from 0-65535.  These›routines take care of the›bank-switching location 54017, but›they must not reside in the area›16384-32767.  Put the lines at the›top of your program and you will have›no problem;  type PRINT BPOKE, BPEEK,›BCLEAR if you want to make sure. ›They can even be compiled, with one›catch:  under the TURBO BASIC›compiler, the screen jitters when the›banks are accessed.››From a technical standpoint, the›130XE has only twice the addressable›memory of the older 800XL.  But as a›programmer, you actually have over›three times more usable RAM.  An›Atari BASIC program which manipulates›the extra 64K creates quite a›powerful computing package.  And you›can take that to the bank.››-------------------------------------›--››This article was downloaded from the›ACEC BBS in November, 1989›