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Family Range, Compatibility, and Evolution
The relentless pace of technology has fostered the concept of computer systems that are related through some architectural similarity. Such sets of related computer systems form a family. At least three types of computer families have been historically identified.
- Family by evolution. The most primitive relationship is that of cultural compatibility. A computer architecture slowly changes over a number of technological generations. The basic architectural concepts remain (e.g., stack versus general-register) while enhancing the architectural functionality. The PMS organization and even the ISP may change. Machine language programs written for one family member will not execute on another family member. Programs written in high-level languages, with little or no changes, are usually transportable between machines. The Burroughs computers (Part 1, Sec. 3), the CDC 6600/7600, the CRAY-1 (Part 3, Sec. 4), and the Hewlett-Packard calculators (Part 4, Sec. 3) are all examples of families by evolution.
- Family for compatibility. The next strongest family tie is that of program compatibility. Successive generations of family members attempt to capture as much existing software as possible. Assembly language level programs usually execute directly, since each ISP is a proper superset of its predecessor. The machine languages of family members may differ, thus necessitating a reassembly or recompilation of transported programs. Frequently compatibility is also sought at the PMS level so that peripherals, whose technology changes less rapidly than the Pc technology, can be transported between family members. The Intel 8080/8086 (Part 3, Sec. 2), the PDP-8 (Part 4, Sec. 2), and the IBM System/360-System/370 (Part 4, Sec. 5) are examples of families for compatibility.
- Family for range. A family can be planned so that the various members span a wide cost and performance range. Total compatibility is sought so that software (including machine language) written on any family member can execute unmodified on other family members. Peripherals are also interchangeable. Thus the cost of developmental efforts in software, peripherals, documentation, training, and maintenance is shared among all the family members. The PDP-11 (Part 4, Sec. 2), the IBM System/360 (Part 3, Sec. 4), and the IBM System/370 (Part 4, Sec. 5) are examples of families designed for range.
Part 4 focuses on six families ranging from microcomputers to maxicomputers. Each family grew up under a different set of constraints. The evolution of these families and the decisions on how to utilize technology are traced. The range, compatibility, cost, and cost/performance will be analyzed for each family.
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