DAY 11. Displaying Text

novice intermediate advanced expert

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.

Text Position

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.

Displaying Characters

_PutMap Displays a character at the current cursor position.
INPUT A ASCII code of character to display.
_PutC Displays a character at the current cursor position, and advances the cursor.
INPUT A ASCII code of character to display.

Program 11-1

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.
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).

Program 11-2

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'
Other ways to affect the cursor position:
_NewLine Moves cursor to start of next line. (CurCol) = 0, (CurRow) is incremented (provided the display didn't scroll).
_HomeUp (CurCol) = 0, (CurRow) = 0.
That wasn't so bad was it? Now for strings; a little more complex.

Displaying Strings

_PutS Display a null-terminated string starting at the current cursor position.
INPUT HL Pointer to start of string.
OUTPUT HL Address of byte after the null.
Remarks 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.
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.

The Procedure

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.
    .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.

Displaying Characters

_VPutMap Displays a character at the current pen location.
INPUT A ASCII code of character to display.
DESTROYS All but BC and HL

Program 11-3

Display the character 'q' in small font at (26, 31):
    LD     HL, $1F1A
    LD     (PenCol), HL      ; PenCol comes before PenRow
    LD     A, 'q'

Displaying Strings

_VPutS Displays a null-terminated string starting at the current pen location.
INPUT HL Pointer to start of string.
OUTPUT HL Address of byte after the null.

The Procedure

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.
    .DB    "A message to display.", 0

Displaying Numbers

Simple, just display the value of HL in the large font.
_DispHL 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> '1' '2' '5'.
INPUT HL Number to display.
Remarks String is cut-off at the screen's edge.

Text Shadow

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:

Program 11-4

    LD     A, 'K'
You should see the homescreen from before running has returned. To prevent this, you need to wipe out Text Shadow.
_ClrScrnFull Clears the screen and sets text shadow to all spaces.
_ClrTxtShad Sets text shadow to all spaces.
You'll probably want to stop PutS and PutC from writing to text shadow. This is done by resetting the system flag AppTextSave at (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.

Formatting Text

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

When 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.

This is part of Learn TI-83 Plus Assembly In 28 Days
Copyright (c) 2002, 2003, 2004 Sean McLaughlin
See the file gfdl.html for copying conditions