Integer BASIC
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Integer BASIC, written by Steve Wozniak, was the BASIC interpreter included in ROM on the original Apple II computer at release in 1977, and as such was the first version of BASIC used by many early home computer owners. Thousands of programs were written in Integer BASIC. Apple originally called it "Apple BASIC", the name "Integer BASIC" was used for it after Applesoft BASIC became available.
The most frequently cited flaw of Integer BASIC was, as one might expect from the name, that its variables were all 16-bit integers and it was very difficult to write a program that could do calculations using floating point numbers. It was therefore very difficult to write financial or math programs. Apple Computer licensed a more full-featured (but also much slower) version of BASIC from Microsoft, introduced some tweaks, named it Applesoft BASIC, and included it in the ROMs of the Apple II Plus, which was released in 1979. Integer BASIC was relegated to a file on a floppy disk that Apple II Plus users could load into a RAM card for backward compatibility, if needed. Applesoft BASIC was included in the ROMs of all subsequent Apple II models, and became the foundation of probably hundreds of thousands of programs.
The Integer BASIC ROMs also included a "mini-assembler" that let programmers type assembly language programs, line by line, which were entered into memory. This was of course far easier than looking up the corresponding opcodes in machine language and typing those in. These ROMs also included an interpreter for a simple 16-bit assembly language, called Sweet16, which was very simple, compact and worthy of study. These two features, some cassette tape I/O routines, and a few seldom-used floating point math routines were removed in the transition from the Integer BASIC ROMs to the Apple II Plus ROMs, in order to accommodate the larger size of the Applesoft BASIC interpreter.