Editing IBM System/4 Pi (section)

== Early models ==
In 1967, the System/4 Pi family consisted of these basic models:{{sfn|IBM|1967}}<ref>{{cite book |last1=Bedford |first1=D. P. |last2=Markarian |first2=H. |last3=Pleszkoch |first3=N. L. |title=Study of control computers for control moment gyro stability and control systems. Volume I - Engineering |date=Mar 1967 |pages=E-1 - E-21 (126-147) |chapter-url=https://ntrs.nasa.gov/search.jsp?print=yes&R=19670020826 |chapter=Appendix E: SYSTEM 4 Pi COMPUTER CHARACTERISTICS|others=Model TC and CP-2}}</ref>

{| class="wikitable" style="text-align: right; float:right; margin-left: 10px;"
|+ Specifications{{sfn|IBM Overview}}
|-
! Model !! [[Instruction set architecture|ISA]] <br>(instructions) !! [[Instructions per second|Performance]] <br>(IPS) || Weight <br>(pounds)
|-
| style="text-align: left | TC || 54 || 48,500 || {{convert|17.3|lb|kg}}
|-
| style="text-align: left | CP || 36 || 91,000 || {{convert|80|lb|kg}}
|-
| style="text-align: left | CP-2 || 36 || 125,000 || {{convert|47|lb|kg}}
|-
| style="text-align: left | EP || 70 || 190,000 || {{convert|75|lb|kg}}
|}

* Model TC (Tactical Computer){{sfn|IBM|1967|loc=Section 2: Model TC, pp. 2-1 - 2-13/2-14 (20-32)}}{{sfn|IBM Overview|loc=Model TC, pp. -2-13 (1-16)}} - A briefcase-size computer for applications such as missile guidance, helicopters, satellites and submarines.
* Model CP (Customized Processor/Cost Performance){{sfn|IBM|1967|loc=Section 3: Model CP, pp. 3-1 - 3-9/3-10 (33-41)}}{{sfn|IBM Overview|loc=Model CP, pp. -3-15 (17-35)}} - An intermediate-range processor for applications such as aircraft navigation, weapons delivery, radar correlation and mobile battlefield systems.{{sfn|IBM Overview|loc=Model CP, p. 3 (23)}}
** Model CP-2 (Cost Performance - Model 2){{sfn|IBM Overview|loc=Model CP-2, pp. -2-13 (36-51)}}
* Model EP (Extended Performance){{sfn|IBM|1967|loc=Section 4: Model EP, pp. 4-1 - 4-13/4-14 (42-54)}}{{sfn|IBM Overview|loc=Model EP, pp. -2-18 (52-72)}} - A large-scale data processor for applications requiring real-time processing of large volumes of data, such as crewed spacecraft, airborne warning and control systems and command and control systems. Model EP used an instruction subset of IBM System/360<ref>{{cite book|chapter-url=https://archive.org/details/bitsavers_ibm4pi4PIEcationNov1966_12400348|title=System/4 Pi Engineering Description: Model EP|author=<!--Staff writer(s); no by-line.-->|date=1966|publisher=Federal Systems Division of IBM|location=Owego, NY|chapter=''1.1 System/360 Compatibility'' and ''2.2 System/360 Compatibility''|pages=1, 4-5 (6, 9-10)}}</ref> ([[IBM System/360 Model 44|Model 44]]){{sfn|IBM Overview|loc=Model EP: Summary, p. 2 (56)}} - user programs could be checked on System/360

The Skylab space station employed the model TC-1,<ref>{{cite web |url=https://history.nasa.gov/sts1/pages/computer.html |title=Advanced Vehicle Automation and Computers Aboard the Shuttle |last1=Jenkins |first1=Dennis |date=April 5, 2001  |website=NASA History Homepage |publisher=NASA |access-date=27 October 2013}}</ref> which had a [[16-bit]] [[Word (data type)|word length]] and 16,384 words of memory with a custom input/output assembly. Skylab had two, redundant, TC-1 computers: a prime (energized) and a backup (non energized.) There would be an automatic switchover (taking on the order of one second) to the backup in the event of a critical failure of the prime.<ref>{{cite journal |doi=10.1147/rd.201.0005 |title=Development of On-board Space Computer Systems |date=1976 |last1=Cooper |first1=A. E. |last2=Chow |first2=W. T. |journal=IBM Journal of Research and Development |volume=20 |pages=5–19 }}</ref> A total of twelve were delivered to NASA by 1972. Two were flown on Skylab in 1973-1974; the others were used for testing and mission simulators.<ref name=Tomayko1988 /> The software management effort was led by [[Harlan Mills]] and [[Fred Brooks]]. The Skylab flight software development process incorporated many lessons learned during the [[IBM System/360 Operating System]] project, as described in Brooks' 1975 book ''[[The Mythical Man-Month]]''.<ref name=Tomayko1988 />