Z80:Application Code

Now that we have the header of the application, it's time to discuss the code for applications. Intro to Flash Applications discussed a few general things, but now we're ready to get into the nitty-gritty details. Note that this page will focus on things specific to single page flash applications. Multiple page applications have an added level of complexity, and are discussed here.

Hello world

Let's get started! We'll create a simple hello world application to demonstrate a few of the novelties of flash application programming. So, here's the code (with out header):

   .org $4000
   
   .include ti83plus.inc
   
   ;app header is here. Omitted for now
   
   main:
    ld hl,0
    ld (curRow),hl
    ld hl,txtHello
    call myPutS
    bcall(_GetKey)
    bjump(_JForceCmdNoChar)
   
   myPutS:
    ld a,(hl)
    or a
    ret z
    bcall(_PutC)
    jr myPutS
    
   txtHello:
   .db "Hello world!",0

It looks really similar to a regular assembly program at first glance. However, look carefully and the differences can be quite interesting. First of all, .org is set to $4000. This is because flash addresses are addressed starting at $4000, quite different from assembly programs which address from $9D93.

Also notice that we wrote our own PutS routine. What's up with that? The reason why we wrote our own PutS routine is because PutS can only display strings residing in RAM. Technically, we're not in RAM (actually, we've just swapped our flash app into "RAM" so it can be executed, but it's not what the OS thinks is "RAM). So, we have two solutions: copy our text to saferam (such as appbackupscreen, textShadow, etc.) or somewhere in actual RAM, then bcall PutS, or write our own PutS routine and just call it. Places that are actually in "RAM" are addresses outside of $4000-$8000.

   ;alternative way copying to saferam:
   .org $4000
   
   .include ti83plus.inc
   
   ;app header is here. Omitted for now
   
   main:
    ld hl,0
    ld (curRow),hl
    ld hl,txtHello
    ld de,appbackupscreen
    ld bc,12
    ldir
    ld hl,appbackupscreen
    bcall(_PutS)
    bcall(_GetKey)
    bjump(_JForceCmdNoChar)
   
   txtHello:
   .db "Hello world!",0

The last difference you can note here is that to quit an flash application, you must use bjump(_JForceCmdNoChar).

Self Modifying Code (SMC)

In flash application coding, you can not create code that will modify anything between the addresses of $4000-$8000 (where the actual application is). To create SMC, you must first copy your code to somewhere else (saferam works best) and then that code is free to be modified.

   ;error
    ld (putix+1),hl
   putix:
    ld ix,$0000
   
   ;ok
    ld hl,putix
    ld de,appbackupscreen
    ld bc,4
    ldir
    ld (appbackupscreen+1),hl
    call putix
   ;...
   putix:
    ld ix,$0000
    ret

Technically, I believe instructions that try to write to memory between $4000-$8000 are ignored, but their behavior is not really documented. It could result in a crash that could completely mess up your calculator.

Emulators Note

It is always a good idea to test on an emulator before testing on a real calculator. However, it is wise to note that emulators are not exactly accurate, especially when it comes to Flash memory. It is the least documented, and has the worst possible consequences if you mess with something you're not suppose to.