primitives could be executed in a single clock cycle. Disk input/output (I/O) was handled by multitasking at the microcode level. A 68000 processor (termed the front-end processor, (FEP)) started the main computer up, and handled the slower peripherals during normal operation. An Ethernet interface was standard equipment, replacing the Chaosnet interface of the LM-2. The 3600 was roughly the size of a household refrigerator. This was partly due to the size of the processor (the cards were widely spaced to allow wire-wrap prototype cards to fit without interference) and partly due to the size of disk drive technology in the early 1980s. At the 3600's introduction, the smallest disk that could support the ZetaLisp software was wide (most 3600s shipped with the 10½-inch Fujitsu Eagle). The 3670 and 3675 were slightly shorter in height, but were essentially the same machine packed a little tighter. The advent of , and later , disk drives that could hold hundreds of megabytes led to the introduction of the 3640 and 3645, which were roughly the size of a two-drawer file cabinet. Later versions of the 3600 architecture were implemented on custom integrated circuits, reducing the five cards of the original processor design to two, at a large manufacturing cost savings and with performance slightly better than the old design. The 3650, first of the G machines, as they were known within the company, was housed in a cabinet derived from the 3640s. Denser memory and smaller disk drives enabled the introduction of the 3620, about the size of a modern full-size tower PC. The 3630 was a fat 3620 with room for more memory and video interface cards. The 3610 was a lower priced variant of the 3620, essentially identical in every way except that it was licensed for application deployment rather than general development. The various models of the 3600 family were popular for artificial intelligence (AI) research and commercial applications throughout the 1980s. The AI commercialization boom of the 1980s led directly to Symbolics' success during the decade. Symbolics computers were widely believed to be the best platform available for developing AI software. The LM-2 used a Symbolics-branded version of the complex space-cadet keyboard, while later models used a simplified version (at right), known simply as the . The Symbolics keyboard featured the many modifier keys used in Zmacs, notably Control/Meta/Super/Hyper in a block, but did not feature the complex symbol