Applesoft does not have commands for file or disk handling, other than to save and load programs via cassette tape. The Apple II disk operating system, known simply as DOS, augments the language to provide such abilities. Only the first two letters of variables names are significant. For example, "LOW" and "LOSS" are treated as the same variable, and attempting to assign a value to "LOSS" overwrites any value assigned to "LOW". A programmer also has to avoid consecutive letters that are Applesoft commands or operations. The name "SCORE" for a variable is interpreted as containing the OR Boolean operator, rendered as SC OR E. "BACKGROUND" contains GR, the command to invoke the low-resolution graphics mode, and results in a syntax error. Sound and graphics The only built-in sound support is the option to PRINT an ASCII bell character to sound the system alert beep. Applesoft supports drawing in the Apple II's low resolution and high resolution modes. There are commands to plot pixels and draw horizontal and vertical lines in low resolution. High resolution allows arbitrary lines and vector-based shape tables for drawing scaled and rotated objects. The only provision for mixing text and graphics is the four lines of text at the bottom of a graphic display. Beginning with the Apple IIe, a "double-high resolution" mode became available on machines with 128k of memory. This mode essentially duplicates the resolution of the original high resolution mode, but including all 16 colors of the low resolution palette. Applesoft does not provide direct support for this mode. Apple IIGS-specific modes are likewise not supported. Extensions Applesoft BASIC can be extended by two means: the ampersand () command and the function. These are two features that call low-level machine-language routines stored in memory, which is useful for routines that need to be fast or require direct access to arbitrary functions or data in memory. The function takes one numerical argument, and can be programmed to derive and return a calculated function value, to be used in a numerical expression. is effectively a shorthand for , with an address that is predefined. By calling routines in the Applesoft ROM, it is possible for ampersand routines to parse values that follow the ampersand. Numerous third-party commercial packages were available to extend Applesoft using ampersand routines. Bugs A deficiency with error-trapping via ONERR means that the system stack is not reset if an error-handling