Texas Instruments TI89 Developer Guide - Page 185

Chapter 13: Memory Management 143 (includes files), pictures, and text variables. Only variables of the same type can be copied to a system-protected variable. If the variable name already exists, the lock flag is checked - a variable that is locked or archived cannot be overwritten. VarStore throws an error in this case. System variables cannot be locked and some system variables cannot be changed by the user. VarStore also checks the in-use flag. Any variable being used by an application cannot be overwritten except by that application. It cannot be deleted, renamed, or linked either. The in-use count is verified to be 0 for functions and programs. If the variable is not a system variable, locked, system protected, or in use, a value can be stored to it no matter what it contained before. However, there are type restrictions on individual elements of lists or matrices. Usually, VarStore makes sure there is enough memory left to store the new contents to the variable before deleting the current contents of the variable. When storing to an existing variable, both graph reference flags are tested. If one or both is set, the corresponding dirty graph and dirty table flag(s) are set to indicate the graph and table are no longer valid. Storing to an individual element or submatrix of a matrix or list does the same checks as for an existing variable and updates the length. In addition, the data type of the element needs to be correct (expression, relation, string). 13.3.2.7. System Functions Only functions with the correct function argument can be stored in the variables reserved as system functions. These are y1(x) - y99(x), xt1(t) - xt99(t), yt1(t) - yt99(t), r1(θ) - r99(θ), u1(n) - u99(n), y1'(t) - y99'(t), and z1(x, y) - z99(x, y). They can be single line or multiline functions but an error is returned if the user tries to create any other data type with those names or if the number of arguments is incorrect. Since they are system variables, they cannot be locked and must be in the main folder. Empty functions are not valid and should not be added to the symbol table. This is true for user functions also. 13.3.3. Low-Level Routines Low-level routines allow direct access to the symbol table with little data type, status checking, or regard to reserved names. There are general purpose utility routines, routines to directly manipulate folders (including temporary folders) and variables. As stated earlier, they do NOT use tokenized names but deal with names in C string format (the pointer to the first letter in the name is passed, not to the zero byte terminator as with tokenized names). TI-89 / TI-92 Plus Developer Guide Not for Distribution Beta Version January 26, 2001