There were two different 3000 series computers from Control Data Corporation, upper and lower.

The upper 3000 series were 48 bit word size, and consisted of the CDC 3600 and the CDC 3800. Even though they had higher numbers, they preceded the lower series.

The lower 3000 series used 24 bit word size. There was the smaller CDC 3100, the larger CDC 3300 and later the CDC 3500, which used integrated circuits instead of discrete components.

The assembly language for the lower 3000 series was called COMPASS. The premier operating system for the CDC 3300 and CDC 3500 was called Master. All three computers used core memory.

Architecture

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The CPU was a 24-bit architecture: instructions were 24 bits in length, as were the two operand registers A and Q. There were 4 index registers of 15 bits, R0 through R3, though R0 is always zero (zero when read; writes don't affect the value). There was no status (flags or condition code) register. Up to 32,768 (24-bit) words of core memory could be directly addressed, and multiple banks could be switched in. Two or three memory bank configurations were the most common.

Each instruction contained 6 bits of opcode, 1 bit specifying whether indirect addressing used, 2 bits of index register address and 15 bits of address.

Arithmetic was ones complement, so the complexities of zero and negative zero existed. The A and Q register could function as a combined 48-bit register for certain arithmetic instructions.

The CPU could execute around 1 million instructions per second (1 MIP), giving it supercomputer status in 1965.

Character set

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The operating systems for the CDC 3xxx typically used 6 bit characters, so the 64 character set did not contain lowercase letters. The character set contained (in display code order) : A-Z 0-9 + - * / ( ) $ = (space) , . # % " _ ! & ' ? < > @ \ ^ ; Depending on the device, some characters are rendered differently, especially the lineprinter and some terminals:

character	sometimes displays as
#	≡
"	≠
_	→ or {
!	∨
&	∧
'	↑
?	↓ or }
@	≤
\	≥
^	¬

Note the absence of control characters, especially carriage return and line feed. These were encoded by the record structure.

Trivia

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Oregon State University offered a CDC 3300 for use from the mid to late 1960s up until about 1980. It used a home grown operating system known as OS3 (for Oregon State Open Shop Operating System).

External references

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• "Bizarre Architecture" lecture series at Univ. of Mass. (pdf)

CDC hardware