only to Symbolics (and MIT but not to Symbolics competitors), and thereby choke off its competitor LMI, which at that time had insufficient resources to independently maintain or develop the OS and environment. Symbolics felt that they no longer had sufficient control over their product. At that point, Symbolics began using their own copy of the software, located on their company servers, while Stallman says that Symbolics did that to prevent its Lisp improvements from flowing to Lisp Machines, Inc. From that base, Symbolics made extensive improvements to every part of the software, and continued to deliver almost all the source code to their customers (including MIT). However, the policy prohibited MIT staff from distributing the Symbolics version of the software to others. With the end of open collaboration came the end of the MIT hacker community. As a reaction to this, Stallman initiated the GNU project to make a new community. Eventually, Copyleft and the GNU General Public License would ensure that a hacker's software could remain free software. In this way, Symbolics played a key, albeit adversarial, role in instigating the free software movement. The 3600 series In 1983, a year later than planned, Symbolics introduced the 3600 family of Lisp machines. Code-named the "L-machine" internally, the 3600 family was an innovative new design, inspired by the CADR architecture but sharing few of its implementation details. The main processor had a 36-bit word (divided up as 4 or 8 bits of tags, and 32 bits of data or 28 bits of memory address). Memory words were 44 bits, the additional 8 bits being used for error-correcting code (ECC). The instruction set was that of a stack machine. The 3600 architecture provided 4,096 hardware registers, of which half were used as a cache for the top of the control stack; the rest were used by the microcode and time-critical routines of the operating system and Lisp run-time environment. Hardware support was provided for virtual memory, which was common for machines in its class, and for garbage collection, which was unique. The original 3600 processor was a microprogrammed design like the CADR, and was built on several large circuit boards from standard TTL integrated circuits, both features being common for commercial computers in its class at the time. Central processing unit (CPU) clock speed varied depending on which instruction was being executed, but was typically around 5 MHz. Many Lisp