TI BASIC (TI 99/4A)

Unlike most BASICs of the era, TI BASIC did not provide a full-screen editor. Instead, a line editor was provided, which allowed the user to add or edit one line at a time. Explicit line numbers were used to order each statement. It used a > prompt to indicate the current new line in immediate mode, as opposed to the more common READY.[6] Line numbers ranged from 1 to 32767, inclusive, and entering a line outside that range resulted in the "BAD LINE NUMBER" error.[7] Line entry was aided by the NUMBER command, available only in immediate mode, which entered ascending line numbers,[8] and RESEQUENCE, which renumbered an existing program.[9]

TI BASIC also included a number of debugging commands. BREAK worked something like STOP, stopping execution on certain lines. Unlike STOP, the exit to immediate mode did not occur on the line where BREAK appeared, but on the lines BREAK referred to. For instance, BREAK 130 would cause the program to exit to immediate mode whenever it moved to line 130. This could be used, for example, by inserting a single BREAK at the top of the program to control execution, rather than having to insert it in the middle of the code. UNBREAK turned off existing breakpoints.[10] Additionally, TRACE printed out the line number of the currently executing line in angle-brackets: <100><110> etc, and UNTRACE turned it off.[11]

The ANSI-compatible statements of TI BASIC are DATA, DEF, DIM, END, FOR..TO..STEP..NEXT, GOSUB, GOTO, IF..THEN..ELSE, INPUT, LET, NEXT, ON..GOSUB, ON..GOTO, OPTION BASE, PRINT, RANDOMIZE, READ, REM, RESTORE, RETURN, STOP. Most of these operate in the same fashion as their MS counterparts with two additions; RANDOMIZE restarts the random number generator at a given "seed" value, and OPTION BASE sets the first entry in arrays to either 0 or 1, whereas MS is always zero-based. To this standard set it added CALL, CLOSE, DISPLAY and OPEN.[12]

In keeping with the Minimal BASIC standard,[13] IF statements could only perform branches, they could not perform arbitrary statements as was common in almost every other BASIC of the era. For instance, code such as:

100 IF X>5 THEN PRINT"IT IS LARGE"

is not valid in TI BASIC. Instead, this would have to be performed using multiple lines:

100 IF X<=5 THEN 300
200 PRINT"IT IS LARGE"

This can easily lead to off-by-one errors if the conversion is not careful about changing the sense of the boolean comparison. TI BASIC did, however, support the ELSE clause:[14]

100 IF X>5 THEN 200 ELSE 300

The PRINT statement used colons to separate items on different lines, in addition to the more common comma or semicolon. This precluded its use as a statement separator, a concept that TI BASIC did not have.[15] This means a line can have only a single statement. Due to the way BASIC interpreters work, GOTO-based loops can be sped up significantly by combining code onto a single line, which avoids having to search through the program for line numbers. This seemingly minor missing feature may result in much slower code, and adding this feature was part of Extended BASIC.

Extensions to the Minimal BASIC system were often not represented directly in BASIC, but were instead accessed via the CALL command and a series of named GPL-based subroutines. For instance, CALL CLEAR clears the screen, and CALL KEY returns the keycode of the currently pressed key on the keyboard. The language lacked PEEK and POKE so there was no official way[16] to create new CALLable code within BASIC, to do this one would require the TI Editor/Assembler, the TI Mini Memory cartridge which included a small assembler,[17] or by using Extended BASIC.

Unlike Microsoft BASICs, which used LEFT$, MID$, RIGHT$, and INSTR for manipulating strings, TI BASIC used the ANSI-compliant SEG$ and POS.

ABS

Absolute value

ASC

ASCII numeric value of the first character of a string

ATN

Arctangent

CHR$

Convert a number into a string with an ASCII character

COS

Cosine

EOF

Test whether the end of a file has been reached

EXP

Exponentiation

INT

greatest integer less than or equal to the parameter

LEN

Length of a string

LOG

Natural logarithm

POS

First occurrence of a string in another string

RND

Pseudorandom number generator

SEG$

Return a substring of a string

SGN

Sign function

SIN

Sine

SQR

Square root

STR$

Convert a number to a string

TAN

Tangent

VAL

Convert a string to a number

Subprograms are called with CALL statement (e.g. CALL CLEAR).

• CHAR Definition of graphical characters
• CLEAR Clears the screen
• COLOR Defines foreground- and background color for 8 characters
• GCHAR Reads one character at a specified position from the screen
• HCHAR Writes a character to a screen position and repeats it horizontally
• JOYST Returns the position of the joystick
• KEY Reads from the keyboard without echo on the screen
• SCREEN Changes the color of the screen
• SOUND Creates sounds (using a frequency) and noise
• VCHAR Writes a character to a screen position and repeats it vertically

When equipped with the TI Speech Synthesizer, TI Extended BASIC users could also generate speech from a predefined vocabulary as easily as writing text on-screen. For example, the following line of text would cause the speech synthesizer to identify the computer:[18]

 CALL SAY("HELLO I AM A #TEXAS INSTRUMENTS# T I NINETY NINE FOUR A HOME COMPUTER") 

Multi-word phrases are delimited with the # symbol, as #TEXAS INSTRUMENTS# in this example. Using a word not included in the speech synthesizer's built-in vocabulary of 338 words and phrases would cause it to slowly spell out the word. TI's Terminal Emulator II cartridge provided text-to-speech functionality.[18]

• Shaw, Stephen (1983). "Extended Basic". Getting Started With the Texas Instruments TI99/4A. Phoenix Publishing. ISBN 978-0-946576-04-3.
• User's Reference Guide (PDF). Texas Instruments. 1981.