Instruction and Data Formats
There were five instruction formats, referred to as Types A, B,C, D and E. Most instructions were of type B.
Type A instructions had, in sequence, a three bit prefix (instruction code), a 15 bit decrement field, a 3 bit tag field, and a 15 bit address field. They were conditional jump operations based on the values in the decrement registers specified in the tag field. Some also subtracted the decrement field from the contents of the index registers. The implementation required that the second two bits of the instruction code be non-zero, giving a total of six possible type A instructions. One (STR, instruction code binary 101) was not implemented until the IBM 709.
Type B instructions had, in sequence, a 12 bit instruction code (with the second and third bits set to 0 to distinguish them from type A instructions), a two bit flag field, four unused bits, a 3 bit tag field, and a 15 bit address field.
Types C, D and E were used for specialized instructions.
- Fixed point numbers were stored in binary sign/magnitude format.
- Single precision floating point numbers had a magnitude sign, an 8-bit excess-128 exponent and a 29 bit mantissa
- Alphanumeric characters were 6-bit BCD, packed six to a word.
The instruction set implicitly subdivided the data format into the same fields as type A instructions: prefix, decrement, tag and address. Instructions existed to modify each of these fields in a data word without changing the remainder of the word.
Read more about this topic: IBM 709
Famous quotes containing the words instruction and/or data:
“A little instruction in the elements of chartography—a little practice in the use of the compass and the spirit level, a topographical map of the town common, an excursion with a road map—would have given me a fat round earth in place of my paper ghost.”
—Mary Antin (1881–1949)
“Mental health data from the 1950’s on middle-aged women showed them to be a particularly distressed group, vulnerable to depression and feelings of uselessness. This isn’t surprising. If society tells you that your main role is to be attractive to men and you are getting crow’s feet, and to be a mother to children and yours are leaving home, no wonder you are distressed.”
—Grace Baruch (20th century)