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Introduction

Interactive C (IC for short) is a C language consisting of a compiler (with interactive command-line compilation and debugging) and a run-time machine language module. IC implements a subset of C including control structures (for, while, if, else), local and global variables, arrays, pointers, structures, 16-bit and 32-bit integers, and 32-bit floating point numbers.

IC works by compiling into pseudo-code for a custom stack machine, rather than compiling directly into native code for a particular processor. This pseudo-code (or p-code) is then interpreted by the run-time machine language program. This unusual approach to compiler design allows IC to offer the following design tradeoffs:

• Interpreted execution that allows run-time error checking. For example, IC does array bounds checking at run-time to protect against some programming errors.
• Ease of design. Writing a compiler for a stack machine is significantly easier than writing one for a typical processor. Since IC's p-code is machine-independent, porting IC to another processor entails rewriting the p-code interpreter, rather than changing the compiler.
• Small object code. Stack machine code tends to be smaller than a native code representation.
• Multi-tasking. Because the pseudo-code is fully stack-based, a process's state is defined solely by its stack and its program counter. It is thus easy to task-switch simply by loading a new stack pointer and program counter. This task-switching is handled by the run-time module, not by the compiler.

Since IC's ultimate performance is limited by the fact that its output p-code is interpreted, these advantages are taken at the expense of raw execution speed.

The current version of IC was designed and implemented by Randy Sargent, Anne Wright, and Carl Witty, with the assistance of Fred Martin. As of this writing, there are many related 6811 systems in use: the 6.270 LEGO Robot Controller Board (Rev 2.21: 1994, Rev 2.2: 1993, Rev 2.1: 1992, Rev 2.0: 1991), the Handy Board (1995), the Rug Warrior (1993), and the Sensor Robot (1991). This document currently specifically covers the Rev. 2.21 6.270 LEGO Robot Controller Board, but much of this manual is applicable to all platforms.

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