lec26_s09 - de <2 cm 3 03:0E0_QE_ 0.303220...

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M ‘ $30.30 2308292 Eta—coo m:_u:_uo.=£ \ , :fi\ll ,W k. $2 :5. :39. :23:me :oEEoo m 5.? 35.9.3 omflmé .msn. $2303.53 .mfioflwnsw =5. .E: .. 5.385 . muoom. v 5.3.3:. . . 5.83%... =5 :89 «E: 5:333. 5.5098 5.5365 6325:... 2m 3.? x020 m .mE: .0 96.35: a ma mmflm sumo DB 3.3m 9.... 3. 05.2.5 mmflmfi <. E an: :22 5.538... m. \r 6333.8 9:2. m. am.‘ 2.52, 3...th on. :3 5.33.3... EN a ram 3.39m 5.3098 :23:me we... mm 5505. om.< $2.52 28$ v.23 3...; 5.5098 20.525... Ainmmmuoc E E28... :8: mucfiano :03“. a . .. mac—Ego 330.. 38% 55025:. a :33 5.325... 22.6 :owzomxm 5.5.3.3.... .332... . m\: 6E .5539. =mEmd .5558 m_ .2200 umEEEmEnEoE .m . .mcosfiuno onEoo 2.5 335520 5.0th mcouoshmfi . $59.2 Enact? gm: wasp—:8 20:2.me . . .mwuoE mEmmE—uca >55. . .EoEmE mmwuua :8 95:03me 505.. .F . "www.320— SuoEmE 028:8 32m. mEEmEQ “02:58 33.59% mtowuahmE “mammamtfi mSEEEmEQ E 39...: mtoamumuo .3me :86 50 3 £00 HEW :O_u.03.-um:_ X0— EOU "Om—U NMV‘ .mw—Ewai ame .m 69:36.5: 2.833 m_ ucm 29.5» 29:29. E :c: .2200 . .mcowfiwno E5552m £3th ,mcozozbms . . . .mcowusbwfi 595. 69a". . .mfimto =me “3.55 9.58.38 26". .N 6533203 239—30. < ._‘ _ "mfltmgen— . 00V") . .2830on 33% BE uzqcmaogfi 3:9: $3.303 banat new 383532.. EQEE amen... a 38:2. uwosumm 69m g _ THERAc-zs CAUSES OF PROBLEMS Institutional (AECL) issues: ~Code was not independently reviewed _ ° Did not consider the design of the sfoftware during its reliability ' modeling ‘ I osystem documentation did not adequately explain error c'odes -Did not initially belieVe complaints . _\do ‘1 THERAcLzs (cont) . Engineering issues: a ' 0 Design did not have hardware interlocks to prevent the electron-beam from operating in its Shigh-energy mode without "the target in place - 4 ' Software from an earlier model was re-used; the earlier models did have hardware interlocks . ‘ . . 1 dc - Hardware provided no way for the software to verify that ' (X sensors were working correctly 3 I , ' * '- Race conditions occurred if the operator changed the setup too quickly 4 ' l ‘ ' -Software set a flag variable by increinenting. Occasionally an overflow occurred, causing the softWare to bypass safety checks ' v ‘ oSoftware was written in assembly language - hard to debug . Risks in Software vs. Risks in Hardware User-friendliness maybe at odds User-friendliness may be at odds with safety Problems harder to identify and eliminate Problems occur in clusters Skilled operators can maximize risks Problems often arise at the system_ level Attitudes toward software 0 Overconfidence ‘ o Ignoring it - first safety analysis of Therac-25 did not consider software with safety <> Child-proof caps on medicine bottles <> Safety features on power tools Problems are generally much more limited Easier to identify and eliminate risk factors Accidents are usually the result of _a single factor Skilled operators minimize risks oAutomobiles <> Rlfles Major risks are usually the result of one component breaking down . do a} ‘ Moore’s Law (1965): \_ The number of transistors on an integrated circuit doubles every 2 years. do v} The term “law” was-first used by Carver Mead (Caltech prof, VLS! pioneer) moon '5 1976 9 "19's Processor 4004 8008 8080 8086 206 386Wprooessor 486'" DX processor Pentium® processor Pentium II processor Pentium III mom Pentium 4 promor . 1980 1986 1990 Year of Introduction 197! 1972 1974 y 1978 1982 1985 1989 1993 1997 . I999 2000 1995 "9 do 3*“ Transistors -~ ' 2.250 2500 5,000 29,000 120,000 275,000 1,130,000 3,100,000 7,500,000 24,000,000 42060000 Eb: .320 a :0 :3— mn :3 35 905329: ho .6253: 2: 5 $3.05 RES». .x. co a 30.3.5 332 .me 5 .35. V . bo‘EoE 2m 95m: 9: E :33. £58. 35682:. F or mmmw owwr mam" Omar .mhaw aha? momp 8' v: 82. m. ‘88. w 888 m 8088 2% . 8882. . 2m: oooooooov $9.0". o_:.,_o:oum tam ._m.o_mo_o:;ow._. ...
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lec26_s09 - de &amp;lt;2 cm 3 03:0E0_QE_ 0.303220...

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