Magazine - 1000 BiT

fidelity, tlic tcst of \\~liicli is that no piccc of soft\\.nrcrunning on the simulntor sl~o~~lci bcli;i\,c dil'li.rcntl\than it \\,auld on the targct llarti\\rnrc. In practice, s ~~chperfect mimicr!. is diffic~~lt to ,~cliic\.c, as it ~.ccl~~ircs ,Ipainstalting re-crention of tim~ng tictnil (fiw csaniplc,tllc ncrunl '~ccclcration c~~l-\,c of a I)EC:tal>c stomgcs!,stcln) and access to implcmcntatio~i docu~ncnr~tiolithat has often vanished. Noncthclcss, some si~nulatorslia\.c acliie\.ecl results vcr!. close to this goal: iMIMIEplicd l),ltnResearch, \\.as able to run C:l'LT- ,lnd dc\ficc-spccifcdiagnostics. (As testimony to the \,~~lncl-nbility ofcomp~~ting's past, dl ~machinc-rcndnl>lcopies of tllcICIIh4I(: sources nppcnr to lia\.c been lost.)r\n inst~.~~ctio~i sim~~l,~tor steps back from the 1ClI.Ic\.c.I alld trics to si~llill~tc ,I[ the f~nctionnl or tllcbclin\~ior-al Ic\.cl. S!rstcm clc~licnts 'Ire treated as functionstlint tr;lnsf(>r~n st,ltc nccor-ding to tlic a[>st~.actdcti nitions of tlic s!,stcm architccturc, rnthcr thanas logic blocks that transhr~n state h~scd on implc-~iientation equations. Instruction sim~~lators sacrifccabsolutc tidelit\. to the idiosqcr,lsics of a p;~rtici~I.~rimplsmcntntion and focus on tlic intentions of thearchitccti~rc specification. As a result, i~lstruction simulatorscan us~~~lly run s\.stemr soft\\,arc and npplic'ltionsbut can rarcl!. fool iiiag~lostics.Finall!; a sott\\.,~~.e-spccitic sinu~lntio~l fi~rthcrabstracts the f~~nctions oftl~c tnrgct s!.stcm to onl! tl~oscnccdcd L?!, n pnrticular piccc of tnrgct systcm soh\rnrc.For csamplc, the OS/S opcr-nring s!.stcm on the I'l)l'-Sco111puter docs not use program interrupts; a sim~~lntoraillied at running onl! tlic OS/S opcmting s!*stcm\vould not need to implement interrupts or c\.cnqi~ci~ed c\7ents. A recent 1'131'-1 1 sinlulator designed torun the 2.9 RSD UNIS operating system abstr.lcrcdparts oftlic 1'1)l'-11 s\.stcrn1s interrupt ~iiodcl and couldnot run other 1'131'- 11 operating s\.stcms. ''Simulating Minicomputers: A Case StudySIh4 is n portable instr~~ction-lc\cl rni~licornpi~tc~. siniulatori~nplcn~c~ltcd ill C. Its objccti\,cs arc to thcilitatctlic study ;uid use of historic computer arcl~itccturcs I)!'making simulated i~i~plcmcntatio~~s and historic soft-\\,arc a\,ailablc to anyone \\-lio has a 32-bit computer. Itsi~pports the follo\ving target architcct~~rcsand lias been succcssti~ll\, ported to tlic \/AS VMS, tllcAlpha OpcnVhilS, the 1)igital UNIS, and tllc L,inusarchitcct~~res. Ports to the Windo\\.s NT and theWindo\\.s 95 architccturcs and to an IRM 1401 simulatorarc ~~ndcr \\.:I?General Design Considerations The design of aninsrl.~~ction-lc\.cl sim~~lato~. is 11ot tcchnicall!. complic~tcii;indeed, sim~~lnting 3 1'1>1'-8 s\,stcrn is a commonprol3lc1ii ill ~~~icic~.g~.nd~~atc cornputel- scicncc cou~.scs.Slh'l hllo\\.s the proccsso~--mc~~~c)r~~s\\-itch (l'i\/lS)structure pro1x)scd by l\cll and Nc\\.cll and ilnplcmcntcdin MIMIC: and countless otllcr sin~i~latorssincc.~~~.~.: The simulnted ~!~stcrn is a collcctio~i ofdc\.iccs, one of \\~liicli hns spccinl properties (theCI'U). l.;,~cIi dc\.icc 11.1s state (rcsistcrs) and one ormore c~nirs. Each unit hns srntc and tiscd- or vari,lblcsizedstolagc. 111 the (:PU cie\.icc, tlic stolxgc is m~innlcrnol.\.. 111 211 I/() cic\.icc, the storngc is the dc\.iccnlccii.1. Tlic (:I'L: is ciisring~~ishcii from ott~c~ ric\.iccsh\. h,l\ ing tllc Jnnstcl 1.o11tinc fix instr~~ction cscc~~tion.-I-llis ro~~tinc is ~.cspo~isiblc for rllc sciluentinl c\.;il-~~ntiori ofi~lsr~.uctio~is and for the stfltc tl.ansformatio~isth~t rcpl-cxnt simulntcd csccution. The