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The IBM 1620 was announced by IBM on October 21, 1959, and marketed as an inexpensive "scientific computer". After a total production of about two thousand machines, it was withdrawn on November 19, 1970. Modified versions of the 1620 were used as the CPU of the IBM 1710 and IBM 1720 Industrial Process Control Systems (making it the first digital computer considered reliable enough for real-time process control of factory equipment). Being variable word length decimal, as opposed to fixed-word-length pure binary, made it an especially attractive first computer to learn on — and hundreds of thousands of students had their first experiences with a computer on the IBM 1620.〔 Oklahoma State University had an IBM 1620 for engineering students in the 1960s.〕 Core memory cycle times were 20 microseconds for the Model I, 10 microseconds for the Model II (about a thousand times slower than typical computer main memory in 2006). Many in the user community recall the 1620 being referred to as ''CADET'', jokingly meaning "Can't Add, Doesn't Even Try", referring to the use of addition tables in memory rather than dedicated addition circuitry.〔 〕 For an explanation of all three known interpretations of the machine's code name see the section on the machine's development history. == The 1620s architecture == It was a variable "word" length decimal (BCD) computer with a memory that could hold anything from 20,000 to 60,000 decimal digits increasing in 20,000 decimal digit increments. (While the 5-digit addresses could have addressed 100,000 decimal digits, no machine larger than 60,000 decimal digits was ever built.) Memory was accessed two decimal digits at the same time (even-odd digit pair for numeric data or one ''alphameric'' character for text data). Each decimal digit was 6 bits, composed of an odd parity Check bit, a Flag bit, and four BCD bits for the value of the digit in the following format: C F 8 4 2 1 The Flag bit had several uses: *In the least significant digit it was set to indicate a negative number (signed magnitude). *It was set to mark the most significant digit of a number (''wordmark''). *In the least significant digit of 5-digit addresses it was set for indirect addressing (an option on the 1620 I, standard on the 1620 II). Multi-level indirection could be used (you could even put the machine in an infinite indirect addressing loop). *In the middle 3 digits of 5-digit addresses (on the 1620 II) they were set to select one of 7 index registers. In addition to the valid BCD digit values there were three ''special'' digit values (these could NOT be used in calculations): C F 8 4 2 1 1 0 1 0 - Record Mark (right most end of record) 1 1 0 0 - Numeric Blank (blank for punched card output formatting) 1 1 1 1 - Group Mark (right most end of a group of records for disk I/O) Instructions were fixed length (12 decimal digits), consisting of a 2-digit "op code", a 5-digit "P Address" (usually the ''destination'' address), and a 5-digit "Q Address" (usually the ''source'' address or the ''source'' immediate value). Some instructions, such as the B (branch) instruction, only used the P Address, and later smart assemblers included a "B7" instruction that generated an 7-digit branch instruction (op code, P address, and one extra digit because the next instruction had to start on an even-numbered digit). Fixed-point data "words" could be any size from two decimal digits up to all of memory not used for other purposes. Floating-point data "words" (using the hardware floating point option) could be any size from 4 decimal digits up to 102 decimal digits (2 to 100 digits for the mantissa and 2 digits for the exponent). The machine had no programmer-accessible registers: all operations were memory to memory (including the index registers of the 1620 II). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IBM 1620」の詳細全文を読む スポンサード リンク
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