When dealing with text output, you have your choice of two different fonts: the large font you see when you hit keys in the home screen and all the menus, and the small text you get with the
Text( draw command.
The large and small text fonts use two different systems for locating where the text should be drawn. The large font uses cursor coordinates, the small font uses pen coordinates.
The cursor coordinates are referenced by row and column, which may be thought of as y and x. There are two variables that hold the current coordinates at CurRow and CurCol. Valid ranges are 0 to 7 for (CurRow), 0 to 15 for (CurCol). The top-left part of the screen is (0, 0).
Pen coordinates are also referenced by row and column at the bytes (PenCol) and (PenRow). The row may range from 0 to 63 and the column may range from 0 to 95.
Large (6x8) Text
To begin with, we'll look at displaying single characters, then strings.
Displays a character at the current cursor position.
Displays a character at the current cursor position, and advances the cursor.
Display 'I' at location (3, 4).
LD A, 3 LD (CurRow), A ; Set row 3 LD A, 4 LD (CurCol), A ; Set column 4 LD A, 'I' ; Use a character constant instead of b_call(_PutC) ; giving a cryptic ASCII code. RET
At the end of this program, the cursor is at (3, 5) because
PutC will increases the cursor column. In memory, CurCol is the byte immediately after CurRow, so you can save time and space by loading the coordinates with HL (just keep in mind how HL is loaded into RAM).
Display 'I' at location (3, 4) with different coordinate setup.
LD HL, 4*256+3 ; Could also be $0403 LD (CurRow), HL ; Set coordinates LD A, 'I' b_call(_PutC) RET
Other ways to affect the cursor position:
- Moves cursor to start of next line. (CurCol) = 0, (CurRow) is incremented (provided the display didn't scroll).
- (CurCol) = 0, (CurRow) = 0.
That wasn't so bad was it? Now for strings; a little more complex.
Display a null-terminated string starting at the current cursor position.
- Pointer to start of string.
If the string is longer than the current row, will wrap to next row. Will scroll display if necessary. Cursor postion set to postion after the last character in the string.
- Address of byte after the null.
A pointer is a variable or register that holds the address of another variable. When
PutS says it needs a pointer to the start of the string, it means that we just need to find the memory address of whichever byte holds the first character of the string, and put it into HL. Of course, we can't load a static value in, since it would be too damn difficult to figure out, and making any alteration to the program would render it invalid.
This is the general procedure to follow when displaying text.
; Set up the cursor coordinates here LD HL, text ;This loads our pointer to the string. b_call(_PutS) . . . RET text: .DB "A message to display", 0
So you see that to get a pointer to the string you use a label. Remember that a label is assigned the value of the location counter, therefore it is the address of where the next byte of data will be put. In this case, text is equal to whatever byte the character 'A' will wind up in.
The zero at the end of the string is how
PutS knows when to stop displaying characters. This is what is meant by a "null-terminated string" ("null" is just a fancy (or German) way to say "zero"). If you don't supply a zero,
PutS will keep on displaying characters until it comes across a byte containing zero (try it and see for yourself, it won't cause a crash if you're wondering).
Small Variable-Width Text
Again, single characters first, then strings.
Displays a character at the current pen location.All but
Display the character 'q' in small font at (26, 31):
LD HL, $1F1A LD (PenCol), HL ; PenCol comes before PenRow LD A, 'q' b_call(_VPutMap) RET
Displays a null-terminated string starting at the current pen location.
- Pointer to start of string.
- Address of byte after the null.
Notice how the code is almost identical to that for large-font strings.
; Set up the pen coordinates here LD HL, text ; This loads our pointer to the string. b_call(_VPutS) . . . RET text: .DB "A message to display.", 0
Simple, just display the value of HL in the large font.
Displays the contents of the HL register right-justified in a field of five characters. For example, if HL
= 125, output will be <space> <space> '
- Number to display.
HLString is cut-off at the screen's edge.
TextShadow is a 128-byte block of RAM that stores a copy of every character written in the large font. The primary purpose of Text Shadow is so that the home screen is preserved when you enter any of the TI-OS's manifold menus or get your graph on. You can see what a potential annoyance this can be by pressing 2nd, MODE after running this:
b_call(_HomeUp) b_call(_ClrLCDFull) LD A, 'K' b_call(_PutC) RET
You should see the homescreen from before running has returned. To prevent this, you need to wipe out Text Shadow.
Clears the screen and sets text shadow to all spaces.All
Sets text shadow to all spaces.
You'll probably want to stop
PutCfrom writing to text shadow. This is done by resetting the system flag
(IY + AppFlags)This will, as an added bonus, free up text shadow for variable storage. You will then have to clear it when you exit, or you'll see junk on the screen. Anyone who's ever played ZTetris knows what I'm talking about.
There are three flags in particular that are useful for modifying text display.
Inverted Text — The Pinnacle of Monochrome Graphics
This is probably the most widely used flag. If
TextInverse, (IY + TextFlags)is set, text will appear in reverse video (white on black). This gives the effect of a highlight, and can also give a psychedelic, seizure-inducing flash effect.
Large Text — Where You Want It, When You Want It
FracDrawLFont, (IY + FontFlags)is set, then any routine that normally uses the small font will instead use the large. The point here is to display large text that isn't confined to a 16×8 grid.
Scrolling — I Can't Think of a Clever Subtitle
If you reset
AppAutoScroll, (IY + AppFlags), the display will not scroll when
(CurRow)is greater than 7. The problem is that you have to make sure to set
(CurRow)to under 8 when you want to display text again, or it'll suck to be you.