Microsoft BASICA
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Microsoft BASICA (short for "Advanced BASIC") is a simple disk-based BASIC interpreter written by Microsoft for PC-DOS. BASICA allows use of the ROM-resident BASIC included with early models of IBM's PC while DOS is loaded (the ROM BASIC itself runs when nothing is loaded when booting) and adds functionality such as file access and storage of programs on disk. It does not run on non-IBM PCs and even on later IBM models, since those lack the requisite ROM BASIC.
Lineage
BASICA's development environment is very similar to that of the Dartmouth Time Sharing System associated with Dartmouth BASIC. In both, a user enters instructions at a prompt. If an instruction begins with a line number (used to sequence the instructions and as a label), it is stored as part of the current program. If not, it is executed immediately.
BASICA's successor was Microsoft GW-BASIC, which was very similar but didn't use any ROM-based BASIC routines and thus can run on pretty much any IBM compatible system.
Example session
The IBM Personal Computer Basic Version A2.00 Copyright IBM Corp. 1981, 1982, 1983 Ready. > 10 PRINT "Name: "; Ready. > 20 INPUT "", A$ Ready. > 30 PRINT A$; ", there are"; LEN(A$); " letters in your name !" Ready. > RUN Name: John John, there are 4 letters in your name ! Ready. >