BASIC interpreter
[1] BASIC, as a streamlined language designed with integrated line editing in mind, was naturally suited to porting to the minicomputer market, which was emerging at the same time as the time-sharing services.
These machines had very small main memory, perhaps as little as 4 KB in modern terminology, and lacked the high-performance storage like hard drives that make compilers practical.
[5] In June 1974, Alfred Weaver, Michael Tindall, and Ronald Danielson of the University of Illinois at Urbana-Champaign proved it was possible to produce "A BASIC Language Interpreter for the Intel 8008 Microprocessor," in their paper of the same name, though their application was deployed to an 8008 simulator for the IBM 360/75 and required 16 KB.
This was released by MITS in punch tape format for the Altair 8800 shortly after the machine itself,[7] showcasing BASIC as the primary language for early microcomputers.
[9] Other members of the Homebrew Computer Club began circulating copies of Altair BASIC on paper tape, causing Gates to write his Open Letter to Hobbyists, complaining about this early example of software piracy.
This led to a wide variety of Tiny BASICs with added features or other improvements, with well-known versions from Tom Pittman and Li-Chen Wang, both members of the Homebrew Computer Club.
In 1976, SCELBI introduced SCELBAL for the 8008[11] and the University of Idaho and Lawrence Livermore Laboratory announced that they would be publishing to the public domain LLL BASIC, which included floating-point support.
[36] In 2000, Lee Bamber and Richard Vanner released DarkBASIC, a game creation system for Microsoft Windows, with accompanying IDE and development tools.
[42] The associated IDE provides a simplified programming environment with functionality such as syntax highlighting, intelligent code completion, and in-editor documentation access.
Initially, interpreters were either bundled with computer hardware or developed as a custom service, before an industry producing independently packaged software for organizations came about in the late 1960s.
Generally, compilers examine the entire program in a multi-step process and produce a second file that is directly executable in the host computer's underlying machine language without reference to the source code.
Splitting the task up in this fashion reduced the amount of memory needed by any one of the parts of the overall BASIC system; at any given time, only the editor, compiler, or runtime had to be loaded, the rest was on storage.
Interpreters ultimately perform the same basic tasks as compilers, reading the source code and converting that into executable instructions calling runtime functions.
Producing a language with all of these components that can fit into a small amount of memory and still has room for user's source code is a major challenge, but it eliminates the need for secondary storage and was the only practical solution for early minicomputers and most of the history of the home computer revolution.
[54] Allison was urged to create the standard by Bob Albrecht of the Homebrew Computer Club, who had seen BASIC on minicomputers and felt it would be the perfect match for new machines like the Altair.
[63] When Harvard stopped their use of this system, Gates and Allen bought computer time from a timesharing service in Boston to complete their BASIC program debugging.
While on final approach into the Albuquerque airport on a trip to demonstrate the interpreter, Allen realized he had forgotten to write a bootstrap program to read the tape into memory.
[72] Thus the choice of a virtual machine approach economized on memory space and implementation effort, although the BASIC programs run thereon were executed somewhat slowly.
SWEET16 is based on bytecodes that run within a simple 16-bit virtual machine, so memory could be addressed via indirect 16-bit pointers and 16-bit math functions calculated without the need to translate those to the underlying multi-instruction 8-bit 6502 code.
Shepardson referred to this early-tokenizing concept as a "pre-compiling interpreter"; statements with syntax errors could not actually be stored, and the user was immediately prompted to correct them.
With the exception of arrays and (in some implementations) strings, and unlike Pascal and other more structured programming languages, BASIC does not require a variable to be declared before it is referenced.
This was to permit a region of memory to be reserved for the installation of machine language subroutines that could be called by the interpreted program, for greater speed of execution.
[108] Consultants were typically brought into handle floating-point arithmetic, a specialist domain well studied and developed for the scientific and commercial applications that had characterized mainframes.
When Allen and Gates were developing Altair BASIC, fellow Harvard student Monte Davidoff convinced them to switch from integer arithmetic.
For instance, the GE-235 was chosen for implementing the first version of Dartmouth BASIC specifically because it featured an "Auxiliary Arithmetic Unit" for floating point and double-precision calculations.
Later adopted by ANSI BASIC, HP's notation can also be used on the destination side of a LET or INPUT statement to modify part of an existing string value, for example 100 A$[3,5]="XYZ" or 120 B$[3]="CHANGE ALL BUT FIRST TWO CHARS", which cannot be done with early implementations of LEFT$/MID$/RIGHT$.
On early microcomputers, with their limited memory and slow processors, BASIC garbage collection could often cause apparently random, inexplicable pauses in the midst of program operation.
Even complex mathematical operations are ready-to-run, with any numerical constants already converted to its internal 48-bit format, and variables values are looked up by address rather than having to be searched for.
In the Ahl benchmark, a single exponent operation, which internally loops over the slow multiplication function, was responsible for much of the machine's poor showing.
[156] Whenever one of these is encountered, for instance, in the line number in GOTO 100, the tokenized BCD value has to be converted to an integer, an operation that can take as long as 3500 microseconds.
