On the following web-page, you can download a game called Phoenix for
the TI-89 graphing calculator.

http://www.ocf.berkeley.edu/~pad/game-ti89-phoenix.html

The latest version is 7.6.  According to the very same webpage, (at
the bottom...) it says that there is a cheat for the TI-89, but you
have to read the source code to figure out what it is.  I've tried
reading the source code (which is automatically downloaded when you
download the game file), but can't find what the cheat code is.  I've
e-mailed the creator of the game, but it appears that the e-mail
address is no longer valid.  It's driving me crazy.  Please help! 
Thanks

a quick check of the source code shows that the cheat is in titles.asm
which is the code for the title screen.  Here is the relevant snippet:

SSL:    bsr     Get_Key 
        cmp.w   #264,d0 
        beq     Game_Over       
        cmp.w   #268,d0 
        beq.s   Game_Normal     
        cmp.w   #269,d0 
        beq.s   Game_Fast
        cmp.w   #"^",d0
        beq.s   Game_Cheat
        bra.s   SSL     

Game_Normal:
        bsr     Status_Prepare  
        st      Game_Started(a5)        
        rts     

Game_Cheat:
        st      cheated(a5)
        st      weapons_available(a5)
        move.w  #-3,fire_delay(a5)
------------------------

I don't know ti-89 asm but it seems to be that you have to hit the
carrot/exponent key to activate the cheat. Note that it does record
the fact that you cheated and checks later in a different asm file. 
Simply remove the call to cheated(a5) and you should be able to post
scores.

I found this and I wonder if this is wht you are looking for

cheater:   
        clr.w   cash(a5)        
        move.w  #15,num_b(a5)   
        move.l  #144000,time_bonus(a5) 

found in PHOENIX.ASM