and 924 instruction sets, some backward compatibility also existed. However the systems did diverge from each other in areas such as relocation and the BDP instructions. Memory All 3000 series computers used magnetic-core memory. The CDC 3500 machine used the same core memory modules as used in the CDC 6000 / Cyber 70 series computers. Architecture The lower 3000 CPU was a 24-bit architecture: instructions were 24 bits in length, as were the two operand registers A and Q. There were four index registers of 15 bits, B0 through B3, though B0 is always zero (zero when read; writes do not affect the value). There was no status (flags or condition code) register. Up to 32,768 words of core memory, 24 bits per word, could be directly addressed, and multiple banks could be switched in. Two or three memory bank configurations were the most common. Each instruction contained six bits of opcode, one bit specifying whether indirect addressing used, two bits of index register address and fifteen bits of address. Arithmetic was ones' complement, so there were two forms of zero: positive zero and negative zero. The A and Q register could function as a combined 48-bit register for certain arithmetic instructions. The E register had 48 bits. The 3600 CPU could execute around one million instructions per second (1 MIPS), giving it supercomputer status in 1965. Much of the basic architecture design of the 3000 series was done by Seymour Cray, then passed on to others to complete as he moved on to designing the CDC 6000 series. Several of the innovative features that made the 6600 'the first supercomputer' can be seen in prototype in the 3000 series. Software The operating systems for the upper 3000 were called SCOPE (Supervisory Control Of Program Execution). Tape SCOPE was a serial batch OS with no buffering for card reading or print spooling. Drum SCOPE upgraded performance and featured print spooling. CDC developed a OS for the 3800 called SUMMIT (Simultaneous Usage of Multiprogramming, Multiprocessing with Interactive Timesharing) to take advantage of hardware's advanced features. Because CDC focused all its resources in advancing the 6600 system, SUMMIT was never released. The earliest operating system for the lower 3000 series was called RTS OS. However it was quickly replaced with MSOS (Mass Storage OS). The premier operating system for the CDC 3300 and CDC 3500 was called MASTER (Multi Access, Shared Time