Integer BASIC and Applesoft Floating Point BASIC Support
The Apple II started out using a simple BASIC interpreter known originally as Apple BASIC and later as Integer BASIC. It could only handle integer numbers ranging from -32768 to 32767 and only had built-in support for low-resolution graphics. But it was the first version of BASIC available for the Apple II and is what the earliest BASIC programs used.
Only months after the Apple II's release, Apple commissioned Microsoft to develop a much more capable interpreter known as Applesoft BASIC, capable of handling floating-point real numbers with up to nine digits of precision and base 10 multipliers from -38 to +38, and with support for high-resolution graphics. While more capable, Applesoft could not run Integer BASIC programs, causing some users to resist upgrading to it.
DOS 3.3 was released when Applesoft BASIC was standard in ROM on the Apple II Plus, so Apple designed it to support switching back and forth between the two BASIC interpreters. The DOS 3.3 System Master disk contained Integer BASIC and Applesoft on disk as a binary files which could be loaded into RAM, allowing whichever language was not resident in ROM to be used on any machine with enough RAM. The user could switch between BASICs by typing either FP or INT at either BASIC prompt. DOS automatically chose the needed language when running a saved program. Applesoft programs were saved as type A files, while Integer BASIC programs were saved as type I files.
Read more about this topic: Apple DOS, Technical Details
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