Ti asm in 28 days problem

[jammasterz]

Hi. I've been reading a very known tutorial how to program in assembler for TI 83, called asm in 28 days. My problem is that i cant understand lesson 13.
For example:

IX_Offset .EQU $+2
INC (IX + 0) ; $DD $34 $00
LD A, (IX_Offset)
ADD A, 3
LD (IX_Offset), A

What does the "$+2" mean? And how does it affect IX offset?

Second example:
"It can also be used to save a register faster than the stack:"

LD (save_a), A
LD (save_hl), HL

; Do some math on A and HL ...

save_a .EQU $+1
LD A, $00

save_hl .EQU $+1
LD HL, $0000

again what does the $+1 mean?

PS. Is there any way to run asm programs without asm( on the calculator without the need to change OS?

[Xeda112358]

I like these questions! Okay, so I am pretty sure you understand how addresses work if you are up to lesson 13, so it should be easier to explain
When you have a label called HL_Times_A and you do call HL_Times_A+2, you will call the address 2 bytes into the routine (because of the +2). Likewise, $+2 will reference 2 bytes from that address.

So the first question:

[jammasterz]

Thank you so much! I could go on without this, and you made it so simple!
As for the hook- this is the soution i was most hoping for , so it would be very nice to see your solution to this problem.

[jammasterz]

I encountered another problem. It is a little difficult to me to understand what some of the instructions are doing, so im going to add a comment to every line.

LD H, 0 ;load 0 to high byte
LD L, A ;and A to low byte
LD HL, HL ;what is point of doind this?
LD DE, VectTbl ;load value of ClearScreen into DE
ADD HL, DE ; Add hl and de, no idea why

LD A, (HL) ;Is it really possible to load a 2 byte adress into a 1 byte reg?
INC HL ;increase value of(HL)+ value of ClearScreen(DE) by one. Why?
LD H, (HL) ;again, can you do this?
LD L, A ; totally lost right now
JP (HL)

VectTbl:
.DW ClearScreen
.DW PutSprite
.DW DrawLine
.DW EndPrgm
.DW BlackScreen

[Xeda112358]

EDIT: Also, so that folks don't think there was a double post, a bot had posted between the two posts and I had to delete it. It may look like a double post, but it is not :P

Hehe, okay, I think I have this. In case you have ever programmed in TI-BASIC, I will give an analogy, too.
Think of the memory as a giant list where each byte is a list element:

(Also, the ld hl,hl is a non-existent instruction and is supposed to be add hl,hl

[jammasterz]

You are amazing.

[Xeda112358]

Okay, so I haven't had time to comment, but if you see questionable content, please ask! This code is a bit complicated and uses some undocumented routines, but what it does is:
When you press enter on the homescreen, this checks if the first byte is the "prgm" token. If it is, it then checks the contents of the program named. If the first two bytes are BB6D or BB6C, it runs the program as an assembly program. if the program is in archive, it gets copied to RAM in a temporary var called @ (so it won't delete any normal pre-existing programs).

Also, this has room for improvement as I pretty much copied and pasted this and then modified it.

For those who want to just use this, it pretty much lets you run assembly programs without the Asm( token. If you leave off the Asm( token, you can run it from Archive, too.

[jammasterz]

So i have some additional questions, but this time about the calculator itself, and a little bit hardware theory.

1. The calculator uses so called ROM-calls. An example is BCALL(_ClrLCDFull), am i right?
2. Can i edit ROM-calls? Does the calculator use EEPROM or just ROM?
3. When i want to get mashine code of my program i run it on simulator(TI 83 Plus Flash Debugger), then search for adress 9D95h
on a disassembly window and check the code. What am i looking at? Is it RAM? Its a little strange, because there are holes in
the adresses.
4. Where is TI-OS? Can its edit the code?

PS. Thanks for the solution. I will try to understand when im done with the tutorial because if i'd ask about something i dont udnerstand i'd ask about every single line .

[Xeda112358]

I do not have any formal training so the meaning of terms often escapes me, but here is what I can tell you:
1) The flash pages can be written to-- if you know the secret protocol (not really secret, but it is advanced)
2) Certain pages of flash and are protected so writing does nothing (and I believe some pages cannot be read, either).
3) Certain RAM pages are also protected that prevent executing code
4) I believe the flash chip is a NAND chip or something like that. Whatever it is called, what it means is that you can change a set bit to reset, but not vice-versa. So 1→0, but not 0→1.
5) As for editing ROM calls, if you have the power of editing the flash, you technically could edit these. I personally think creating an OS is easier, though.
6) If you want the machine code, you can look at the address 9D95h. Assembly programs are executed from RAM (which is what allows for self modifying code).
7) Adresses are stored little endian (all words, in fact) on the calc. So pretty much, call 9D95h looks like CD959D in RAM.

Also, if you can figure out how to get a ROM image of your calc (the process is a bit complicated), I would suggest WabbitEmu for debugging. Plus, if you use WabbitEmu, you can edit flash as well from the debugger.


If at any point you want a breakdown of what happens at bcall(), I do happen to know a little. I have class soonish, though, so it might not be an immediate response.


Finally, my answers gave me an idea for one of my programs. It involves making Apps use less power... (reading from flash uses more power than reading from RAM and the TI-84+ has an extra RAM page that is largely unused)

[jammasterz]

Thank you for your answers. As allways very helpful.
Here come another questions that i have :)

Is there any easy way to avoid memory leak when pressing [2nd][ON] When GetKey is running?
I heard of call called _GetKeyRetOff which will return kOff when [2nd][ON] is pressed, but
i have to press [2nd][Mode](quit) to be able to turn off my calculator.

1.Is there any way of getting the calc to return to its normal key-input state so that i dont have to press enything else
to turn it off when program ends?
2.How do i harmlessly turn off calculator? With asm of course.
3.Is it possible to output an integer with _DispHL that is smaller than 256 and not get 2 spaces from left corner?
4.How to connect 2 decimal numbers to make a new number? For example i have 2 in HL and 5 in A, what do i do to make it
25? Im aware that i can multiply HL with 10 and then add A, but is there any other way?

[Xeda112358]

To turn off the calc with [2nd][ON]:

[jammasterz]

What i ment is that when you use _GetKeyRetOff and quit the program you cant turn off your calculator
because "all subsequent _GetKey's will behave similarly". This means that i cant turn it off till i press [2nd][Quit] after
i quit the program. I think this is really annoying if users have to know that they have to do that after they execute my programs.

Thank you again

[NanoWar]

I never use _GetKey because of that interrupting behavior. Use _GetCSC (4018h) in a loop instead (as used here).

[jammasterz]

[Xeda112358]

Well i have a solution to your _GetKey issue that you wouldn't otherwise learn. Before exiting, run this code:

[jammasterz]

Okay so i made a few changes(the paranthesis bug was actually a type mistake ) and the program works right.
Untill i overflow HL by typing a number that has more than 5 digits. What happens is when i press [2nd][Quit] the calc crashes.
So my first question is if overflow of HL in some way affects the top value of the stack?
EDIT: Solved- i forgot to pop 2 values from the stack . I dont even know why i pushed those...

Another strange thing that happens is that the program is not allways loaded into 9D95h. And if i want to write the program in HEX directly on the calc i get a problem.

For example:

[Xeda112358]

Hmm, well when you run an assembly program on the calculator, the code is copied to the address 9D95h. So even if the program itself isn't at 9D95h, the code is run from there. However, if you are making your own shell that loads the program to another address or just runs it where it is, that could pose a problem . In that case, you will want to find some way to use relative addresses. (so using jr instead of jp and not using call).

[jammasterz]

Thank you again. I hope it doesnt bother you that i ask so many questions.

[Xeda112358]

No, it does not bother me at all I like being able to help out and it gives me something to do! Plus, it means a bit more activity here >.>

[jammasterz]

Im very happy to hear that . Im not shure if i will be able to continue learning asm untill christmass, because i have 5 tests comming. Thank you for your help!