Microsoft BASIC - Altair BASIC and Early Microcomputers

Altair BASIC and Early Microcomputers

The Altair BASIC interpreter was developed by Microsoft founders Paul Allen and Bill Gates with help from Monte Davidoff, using a self made Intel 8080 software simulator running on a PDP-10 minicomputer. The dialect of BASIC was similar to Digital Equipment Corporation interpreters, especially in string operations, which varied between BASIC implementations. BASIC used dynamically allocated strings which stored their size. Early BASIC only supported single letter and digit names, but Microsoft BASIC supported long variable names. Only two characters were significant though; AD, ADDRESS1, and ADDRESS2 would all point to the same value. The runtime symbol table used a linear search so that a program which used many distinct variables would run much slower than a program which used a single array for all its variables.

It was delivered on paper tape and in its original version took 4 KB of memory. The extended 8 KB version was then generalized into BASIC-80 (8080/85, Z80), and ported into BASIC-68 (6800), BASIC-69 (6809), and MOS Technology 6502-BASIC (unfortunately spilling over to 9 KB, in an era when 8 KB ROM chips were standard), as well as the 16-bit BASIC-86 (8086/88). It was ideal for ROM-based computers since it did not require an editor (each line requires a number), nor a disk drive to store object code or linked executable. It was less sophisticated than software for industrial desktop computers, which had dedicated keys to load, store, and keys for editing within a line and debugging, but personal computer pricing, in contrast, started at $1,565, not $7,000.