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Feature: Two-Dimensional BASIC Scrolling
Posted by Nick on 8 March 2000, 01:23 GMT
Brandon Green of SiCoDe Software is the first person to take the mic in this feature concerning two-dimensional scrolling in a BASIC program. Hopefully it can teach you a thing or two about BASIC programming and how strings work.
SiCoDe Software has recently discovered a method to scroll the entire screen at four frames per second. One example of this technology is the TI-83 BASIC game Frogger. The method envolves using a seldom exploited function called a string. [Strings are frequently used in BASIC programs, just not in this specific way. -ed.]
A tutorial of how we did it follows:
ADVANCeD.BASiC Tutorial:
Scrolling the Entire Screen From Left To Right
Part 1: Strings Make it Possible
String?
Suprisingly enough, perhaps one of the most advanced functions of the TI-83 is also the least known. For those of you who dont know a string is used to store a, well, string of charachter data. It is very similar to lists, and with the many advanced string operations you can mainpulate it just as you would a list.
A string can be any length, it can be infinitely long, or when you run out of memory. As every one knows, when you use the Disp command and you try to disp a message that is wider than the screen you get those annoying little dots(...) and it cuts it off. However, when you use the Output( command to output a string, it automatically wraps the message in the string to the next line. This will come in very handy down below.
String Operations
Since string operations arent well known, I will list them here. All these operations can be found in the catalog on your calculator: [note that all these operations can be found in your manual:), if you have any syntax questions, consult your manual, believe it or not, you can learn something from reading a book]
->-(storing into a string)
In order to put a message into a string you put what you want to input in quotes, like so: "STRINGS ARE FUN!"->Str1. You could then do a Disp Str1, and STRINGS ARE FUN! would be put on the homescreen. You can also do Input Str1 and then whatever the user typed in would be stored into the string.
+-(concatenation)
Suppose you had , "HELLO_"->Str1, and , "BOB"->Str2. If you wanted to add these together to create Str3, "HELLO_BOB", you would just type in: Str1+Str2->Str3.
Length(string)
If you had a string and you wanted to know how many charachters were in "HELLO BOB", then you would type Length(Str3) and it would return a 9. You could store this number into a variable for later use by Length(Str3)->S.
Sub(string,begin,length)- This is used if you want to take just a part of a string and store it into another string.
For example:
"ABCDEFGHIJKLM"->Str1
Sub(Str1,4,3)->Str2
Disp Str2
[displays]
DEF This is a way to substract parts from a string. You'll understand more after you read the rest of the tutorial.
Part 2: Moving the String
Scrolling
The basic idea of scrolling is that you have a 'map' that is bigger than the screen. You can then move the point that the screen is centered on to see different parts of the 'map'. By moving the point the screen is centered on over and over 1, you generate the illusions that the 'map' is scrolling by on the screen. Here is an example program that will scroll a string across the screen:
This is written with the TI-82/83/83+ in mind, if your screen is not 16 charachters wide, you will have to change the code.
1:ClrHome
2:"THIS_STRING_IS_MORE_THAN_16_CHARACTERS_LONG."->Str1
3:"________________"+Str1-
>Str1 //16
spaces long
4:For(a,1,45)
5:Output(1,1,sub(str1,a,16)
6:End 2: Sets the 'map'(in this case its just a one dimensional 'map') into string 1;
3: Since we want to scroll the entire string ALL the way across the string, we have to add this. If we didn't when the for loop got to 30, it would try to take the next 16 charachters to output, but it isn't 46 characters long, so we would get an error. This prevents that.
4: This is the loop that keeps track of the point at which the screen is looking at the 'map'.
5: What makes this fast and possible is that it outputs everything in one output statement, it takes the position of the screen on the 'map', gets the next 16 characters, and outputs them.
6: Ends for loop.
As you can see, there is no need to clear the first line because the output statement overwrites it , with a space, every time. By using this same principle you can scroll the entire screen. Here is some sample code I came up with. I didn't provide documentation so you'll have to figure it out on your own. ClrHome
"*_+_*++*_+_*++*_+_*++*_+_*++*_+_*++*_+_*++*"->Str1
"________________"->Str2 //16 spaces long
6->A
While 1
0->K
While not(getKey
End
(Ans=26)-(Ans=24)+A->A
If A>27:A-1->A
If A<1:A+1->A
sub(Str1,A,16)+Str2->Str3
Output(1,1,Str3+Str3+Str3+Str3
End As you can see, with this you can move the entire screen at about 3-4 FPS! This is an incredible speed in a basic program.
Don't forget that it auto-wraps the string to the next line when you use strings, this is what is makes it so fast, you output the entire screen in one statement!
For some reason there seem to be no basic games out there that use this technology effectively. Perhaps no knew about it. But dont worry, SiCoDe is working on some very impressive side scrollers... Check the site in the next few months (2/2000 to 5/2000) to get the side scroller when it is released.
Part 3: Conditionals and Strings
Let's say you were working on a game - a frogger game - and you already know how to scroll the cars across the screen. So if you put in the loop where you could control a character, and you had one whole list outputed to the screeen. Then you check to see if the character hit something by: If (sub(Str1,(16(y-1))+x,1))!="_" //for those nonC++ programmers, != means does not equal
disp " you hit something!"The (16(y-1))+x converts the two coordinates of the charachter to a position along the screen, it follows this pattern:
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16 row 1
17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32 row 2
33,34,35,36,37,38,39,40,41,42,43,45,46,47,48,49 row 3
....
113,114,...,128 row 8 (Each number represents a character position along the screen.)
This formula would only work if your string was outputed at 1,1. But it would be easy to change the formula to whatever you wanted.
In this way you could have hit detection using strings. And here is a sample program so that you can understand this better. Note that this is not optimised or anything, I threw it together in 3 minutes, so yes, it could be made a heck of a lot better. "_____(---)__(++)_____(**)__(---)___(///////)__(----)_____(+++)"->Str1 //you could put
//any string here that is more than 20 characters long
"________________"->Str2 //16 spaces
5->X:8->Y
1->P:1->H
length(Str1)-16->L
While H
Output(Y,X,"*"
sub(Str1,A,16)->Str3
Str2+Str3+Str2+Str3+Str2+Str3+Str3+Str2->Str4
Output(1,1,Str4
getKey->K
If K:Then
(K=26)-(K=24)+X->X
(K=34)-(K=25)+Y->Y
End
If (sub(Str4,(16(Y-1))+X,1))!="_" //does not equal a space
0->H
A+1->A
If A=L
1->A
End
Output(Y,X,"X"
There is also a 2-D scrolling engine which SiCoDe has invented. You can expect more advanced scrolling games from SiCoDe in the future.
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The comments below are written by ticalc.org visitors. Their views are not necessarily those of ticalc.org, and ticalc.org takes no responsibility for their content.
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Re: Feature: Two-Dimensional BASIC Scrolling
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Hieu-Trung Le 
(Web Page)
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WOW. Great job Brandon. I guess I can learn a lot from you!!
=) Good luck on your future programs
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8 March 2000, 02:04 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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Horse_Power
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SiCoDe will be realesing War soon that gets 6fps and is great fun. Everyone with 83 should try it too when it comes out.
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8 March 2000, 02:54 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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amicek
(Web Page)
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Hey Nick! I was talking to you on ICQ but ran out of time; here is that idea that I had:
When a large amount of people respond to a news article, the page takes a long time to load. I have noticed you have set up the page to load the logo and main nav and then the rest of the page using <!-- these things --> - that is a good idea; so could you set it up so you can have <!-- first comment block --> so it loads the comments in fairly small blocks - I dont' know if this is a feasible idea using cgi, but it would allow people to read comments while the page was loading. Under preferences, the person could set their connection speed and you could set the length of the comment "blocks" accordingly - the only possible problem i could see would be - a.)it wouldn't work or b.)implementation using cgi would be difficult or impossible. For example, if someone had a 14400 connection, you could set the comment block length to 5 comments, etc. So it would load five comments, and the person could read the comments while the next five were loading. Tell me what you think!
amicek
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8 March 2000, 04:23 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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TheMaker
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When I try to run this program I get a "INVALID DIM" error. I checked the code over again and I still get a problem. Please Help
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8 March 2000, 22:32 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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TheMaker
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now it works I can't read
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8 March 2000, 22:50 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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CrazyBillyO
(Web Page)
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This is all great, but what we really need here is some asm 68k tutorials and techniques. Like how to open and read strings (and open and read other files for that matter) in assembly code. That would be just nifty skippy.
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9 March 2000, 03:49 GMT
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Re: Feature: Two-Dimensional BASIC Scrolling
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EvanMath
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Wow. That's great... Using strings takes up a lot more space than using lists, which is what I usually do...
Also, kind of on topic, I submitted a feature a while ago about programming on calc, but it wasn't that informative, it mainly just said that it was possible... if anybody from ticalc sees this (actually, even if they don't) I'll probably submit a slightly more informative (meaning about 5 times as long, and 6-7 times more info) in a week or two...
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11 March 2000, 19:06 GMT
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Re: Re: Feature: Two-Dimensional BASIC Scrolling
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EvanMath
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Okay... um, sorry everybody, I think I just kind of, um, stopped thinking when I wrote this... Here's what I meant:
Wow. That's great... Using strings takes up a lot *less* space than using lists, which is what I usually do...
Also, kind of on topic, I submitted a feature a while ago about programming *ASM* on calc, but it wasn't that informative, it mainly just said that it was possible... if anybody from ticalc sees this (actually, even if they don't) I'll probably submit a slightly more informative (meaning about 5 times as long, and 6-7 times more info) in a week or two...
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12 March 2000, 16:43 GMT
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Re: Re: Re: Feature: Two-Dimensional BASIC Scrolling
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Jeff Meister
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I never thought about writing an article about this, but...
From here on I'm assuming you were talking about a calculator that doesn't have an on-calc assembly compiler that some programming genius created for it. I'll use the 83, cuz I was messing with it one day and I learned a little about it.
You can't _really_ type in the assembly language we all know on-calc. But, you can, if you're crazy enough, type in hexadecimal.
A couple things I learned when messing with hex code:
CD means call.
C9 means ret.
When calling something, you put it after the CD, with the bytes swapped. If you ever looked at ASMGuru, in one of the first tutorials there was a program that did absolutely nothing but clear the screen. The author showed you a view of the program as if it were opened on-calc. It looked like this:
CD5547C9
End
0000
End
The End 0000 End stuff just means that it's an asm program. The CD means call, and C9 means ret, like I said earlier. But one day I was looking through ti83asm.inc, and I saw:
_clrLCDFull equ 4755h
So the 5547 in the program is the romcall _clrLCDFull, with the bytes swapped!
Of course no one is crazy enough to program an actual, useful asm program, on-calc, in hex code. But using this info, and exploring through it a bit more than I did, couldn't someone create a program that does all the hex dirty work for you, and you program with pre-defined romcalls? It would be just like an include file, except on-calc with a user-friendly program attached. Either that, or someone could just create a really really really huge printable include file-type document that pretty much tells you the hex code for every romcall and command you would ever use.
Ok, I think it's about time I shut up.
- Jeff
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12 March 2000, 20:14 GMT
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Re: Re: Re: Re: Feature: Two-Dimensional BASIC Scrolling
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EvanMath
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>But, you can, if you're crazy enough, type in hexadecimal.
<snip>
>Of course no one is crazy enough to program an
>actual, useful asm program, on-calc, in hex code.
<manianical laughing> You're WRONG! You're all WRONG! I'll show you!
:F3ED562100841101840100013682EDB0214E93
:118282010001EDB03E84ED47ED5E3E01326682
:C908D9
:FDCB1246207C
:3A0A80FE6A30103E74320A803E08D303
:3E01D303E1FB76
:3A6682B72850
:3EFFD3013EFDD301DB01FEFD200BFD
:CB125E28053E05326582
:3EFFD3013EF7D301DB01
:FEBF20053E01326582
:3A6582B7280A3D326582
:20053E0F320080
:3EFFD3013EFDD301DB01
:FEAB20143E00326682
:3EFDD301DB01FED520053E01326682
:C33A00
:End:0000:End
</manianical laughing>
I was sitting in front of the TV one night (yes, I admit it, I do watch TV occasionally), holding my calc (don't ask), and I remembered my friends had been bugging me to make a program to keep jerks at school from clearing their calc's memory. So I made this little program. Instructions:
1.Get the program to your calc
2.Type "send(9prgmNAME", the way to run ASM programs written like this.
3.Press [ln(].
4.Press 2nd-mem.
5.Press [Clear]
6.Press [log(]
7.Press 2nd-mem.
8.Repeat if desired.
>...someone could just create a really really really >huge printable include file-type document that pretty
>much tells you the hex code for every romcall and
>command you would ever use.
Actually, it wouldn't be that long.... I took ti83inc, TI's include file, dropped some of the OP calls, like OP5EXOP6 (I don't see the use in swapping OP5 and OP6, but I kept OP1EXOP2, and some of the others with the lower OP's), and it was only about a page long in size 5 courier new with .7 size line spacing in three columns, and it's a lot more legible than you might think.
And the hex codes for the commands themselves are longer, but there are several patterns that make it much easier. For example, if you assign the values
b = 0 or %000
c = 1 or %001
d = 2 or %010
e = 3 or %011
h = 4 or %100
l = 5 or %101
(hl) = 6 or %110
a = 7 or %111
"ld r,s" where r and s are any two registers becomes
$40+8r+s or %01rrrsss
Right there, you have 64 commands, all in one simple, easy to remember package. And that numbering systym for variables is used in almost every command that requires a register to operate on.
And, after writing several small programs, you will learn to remember many of the more common commands, so now I can generally write a program without any sort of table or anything.
Wow! That was long. Maybe I should just modify this a little, and send it to ticalc.org as that feature I was talking about.
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12 March 2000, 23:24 GMT
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