Architecture Of A Real Time Operating Systems
ARCHITECTUREOF A REALTIME OPERATINGSYSTEMSJ . L. P r u i t tMr. W. C a s eM&S Computing, Inc.Huntsville, A l a b a m aArchitecture is receiving increasing recognition as a major design factor foro p e r a t i n g s y s t e m s d e v e l o p m e n t w h i c h c o n t r i b u t e s to the c l a r i t y , a n d m o d i f i a b i l i t y ofthe c o m p l e t e d s y s t e m .T h e MOSS O p e r a t i n g S y s t e m u s e s a n a r c h i t e c t u r e b a s e d onhierarchical levels of system functions overlayed dynamically b F asynchronous coopera t i n g p r o c e s s e s c a r r y i n g o u t the s y s t e m a c t i v i t i e s .S i n c e e f f i c i e n t o p e r a t i o n in a r e a l t i m e e n v i r o n m e n t r e q u i r e s t h a t the n u m b e r ofp r o c e s s e s a n d p r o c e s s s w i t c h e s b e k e p t to a m i n i m u m , t h e MOSS s y s t e m u s e s p r o c e s s e s o n l y w h e r e a t r u l y a s y n c h r o n o u s a c t i v i t y i s i d e n t i f i e d . T h e l a y e r s of the MOSSO p e r a t i n g S y s t e m do n o t r e p r e s e n t a h i e r a r c h i c a l s t r u c t u r e of v i r t u a l m a c h i n e p r o c e s s e s , b u t r a t h e r a h i e r a r c h y of f u n c t i o n s u s e d to c r e a t e the p r o c e s s e s .T h i s p a p e r d e s c r i b e s the l a y e r i n g c o n c e p t s a n d p r o c e s s c o n c e p t s d e f i n i n g thesystem architecture.It a l s o p r e s e n t s a n o v e r v i e w of the s p e c i f i c f u n c t i o n s a h d p r o c e s s e s o f the M O S S O p e r a t i n g S y s t e m .Key words and phrases:operating system design, real time operating system,layered operating system, software architecture,and process communication.C R Categories:i.3.80, 3.83, 4.35.h a s b e e n s e l e c t e d to p r o v i d e a b a s i c f r a m e w o r kinto which relatively independent subsystems fita n d i n t e r a c t in a w e l l - d e f i n e d m a n n e r .Theses u b s y s t e m s c a n b e m o d i f i e d o r e x c h a n g e d inresponse to changing user requirements.INTRODUCTIONThe Modular Operating System for SUMC(MOSS) i s a g e n e r a l p u r p o s e r e a l t i m e o p e r a t i n gs y s t e m f o r the R C A d e v e l o p e d S p a c e U l t r a r e l i a b l e M o d u l a r C o m p u t e r ( S U M C ) . MOSS is c u r r e n t l y b e i n g i m p l e m e n t e d a t M&S C o m p u t i n g ,I n c . , in H u n t s v i l l e , A l a b a m a , a n d i s s c h e d u l e dto b e f u l l y o p e r a t i o n a l in the f i r s t q u a r t e r o f1977.S i n c e MOSS i s a r e a l t i m e o p e r a t i n gsystem, performance has been consideredt h r o u g h o u t the s y s t e m d e f i n i t i o n a n d d e s i g n .C o n t i n u a l t r a d e o f f s h a v e b e e n m a d e b e t w e e n theconflicting performance and modifiability objectives.Amajor architectural decision wasthe s e p a r a t i o n of the s t a t i c a n d d y n a m i c s t r u c t u r e s o f the s y s t e m .The static structure, cons i s t i n g of a h i e r a r c h y of f u n c t i o n s , d e f i n e s theb a s i c f r a m e w o r k of the s u b s y s t e m s a n d t h e i rinteraction.The dynamic structure, consistingof cooperating processes representing userapplications and asynchronous system activities,i s s u p e r p o s e d on the s t a t i c f r a m e w o r k .Thus,the f r e q u e n t p r o c e s s s w i t c h i n g a n d a s s o c i a t e do v e r h e a d of d e e p l y l a y e r e d p r o c e s s e s h a s b e e na v o i d e d . I n e f f i c i e n c i e s d u e to f r e q u e n t h a n d s h a k i n g b e t w e e n hhe s t a t i c l a y e r s o f f u n c t i o n sMOSS a t t e m p t s to m e r g e r e c e n t d e v e l o p m e n t s in o p e r a t i n g s y s t e m d e s i g n to c r e a t e areal time operating system which is more modifiable than ad hoc systems developed with previous design policies.S i n c e MOSS is to b e u s e da s a n e x p e r i m e n t a l b a s e to t e s t v a r i o u s r e a ltime processing concepts, modifiability has beena major design consideration.The a r c h i t e c t u r e T h i s w o r k w a s s u p p o r t e d b y N A S A C o n t r a c t No.N A S 8 - 3 1 2 2 2 , m a n a g e d b y the M a r s h a l l S p a c eFlight Center.51
h a v e b e e n t o l e r a t e d to a i d m o d i f i a b i l i t y .individual layers m u s t conform to the followingrules:The remainder of this paper is devotedto a description of the actual M O S S architectureand the concepts on which it is based. A briefdescription of the S U M C hardware is also included to aid reader understanding of the M O S Sarchitecture.2.Z.E a c h l a y e r in the h i e r a r c h y is n o ta w a r e of the e x i s t e n c e of a n yl a y e r s above it and cannot invokea n y p a r t i t i o n of a h i g h e r l a y e r .P a r t i t i o n s of h i g h e r l a y e r s m a y i n v o k e the e x t e r n a l f u n c t i o n s of a p a r t i t i o n of a l o w e r l a y e r o rthe s a m e l a y e r to u t i l i z e the r e s o u r c e s of t h a tpartition.T h e i n v o c a t i o n h i e r a r c h y of l a y e r s isn o t r e s t r i c t e d to r e f e r e n c e s o n l y o n e l e v e l b e l o wthe requesting layer. Rather, layers m a y directly reference any lower layer in the hierarchy.The enforcement of the layer interaction rulesand the knowledge of external functions availableto higher layers has been maintained as a designm a n a g e m e n t responsibility, not a function of theexecuting software.A multiple level interrupt structure with a separate set of regis ters in scratchpad m e m o r y foreach interrupt level.Addre s s translation hardwarea n d m i c r o c o d e to s u p p o r t v i r t u a lmemory.Memory protection microcodewhich provides access protection( r e a d , w r i t e , e x e c u t e ) on a p a g ebasis.3. ZMemory units which provide a d a t a l o c k c a p a b i l i t y to p r e v e n ts i m u l t a n e o u s m u l t i p l e a c c e s s tolocked data [6].MOSSLAYERCONCEPTSP a r t i t i o n s a r e p l a c e d on l a y e r s b a s e d onthe n a t u r e of t h e i r a b s t r a c t i o n .The lowestl a y e r s c o n t a i n p a r t i t i o n s w h i c h a r e c l o s e s t tothe a c t u a l m a c h i n e a n d p r o v i d e b a s i c s e r v i c e s .Succeedingly higher layers are selected basedon s e r v i c e s r e q u i r e d f r o m l o w e r l a y e r s a ndservices provided for higher layers.Section 5i d e n t i f i e s a n d d e s c r i b e s the p a r t i t i o n s s e l e c t e dfor MOSS.Within each partition there are internaland external functions. External functions mayb e i n v o k e d d i r e c t l y b y f u n c t i o n s of a n o t h e r p a r t i t i o n . T h e e x t e r n a l f u n c t i o n s of a p a r t i t i o n p r o v i d e the p r i m i t i v e o p e r a t i o n s of the p a r t i t i o n toother partitions in the system. Internal functionsare used only within a partition and cannot bereferenced f r o m other partitions. The internalfunctions are derived f r o m the decomposition ofthe partition into modules which support "information hiding [5]".PartitionPartition Selection CriteriaPartitions are selected to supportabstractions of system capabilities. E a c h abstraction is supported by a set of software functionswhich m a p the characteristics of hardwarecapabilities and/or elementary logical structures into higher level logical structures whichpresent a simple, consistent view of the hardw a r e or software represented. Partitions areprovided to hide the actual implementation ofc o m p l e x m e c h a n i s m s f r o m higher layers of thesystem.The M O S S architecture is based onhierarchical layers of partitions correspondingto l e v e l s of a b s t r a c t i o n s [Z, 3 , 4 ] . A l a y e r d e f i n e s a l e v e l in the h i e r a r c h i c a l s t r u c t u r e of thes y s t e m and c o n t a i n s one or m o r e p a r t i t i o n sw h o s e i n t e r a c t i o n is l i m i t e d and w e l l u n d e r s t o o d .A p a r t i t i o n is a g r o u p of f u n c t i o n s w h i c h a r e r e l a t e d in t h e i r e f f e c t s a n d s h a r e c o m m o n r e sources (data structures and/or hardware features).3. 1Each partition owns certain resources which other partitionsa r e n o t p e r m i t t e d to a c c e s s .Ther e s o u r c e s owned by a p a r t i t i o nc o n s i s t of the s y s t e m f u n c t i o n sa n d d a t a s t r u c t u r e s u s e d to s u p p o r t the p a r t i t i o n .SUMC HARDWAREThe S U M C is a microcoded processingunit organized around the basic S y s t e m 360 instruction set. The major features of the hard ware twareThe S U M C hardware and M O S S softwareare being developed concurrently, allowinghardware/software tradeoffs to be m a d e duringthe design phases. The hardware interruptstructure has been m a t c h e d to the softwarestructure, thus, eliminating the restrictions onthe M O S S design which would have resulted ifInteraction RulesI n t e r a c t i o n b e t w e e n partitions on the52
the design had to m a t c h an existing interrupts truc turedepends on the distinction between external andinternal interrupts. A n external interrupt is aninterrupt which signals an occurrence which isnot directly related to the currently executing instruction in the processor -- for example, anIIO completion interrupt. An internal interrupt,on the o t h e r h a n d , r e s u l t s d i r e c t l y f r o m the e x e c u t i o n of the c u r r e n t i n s t r u c t i o n - - f o r e x a m p l e ,a s u p e r v i s o r , c a l l (SVC) o r a p a g e t r a p i n t e r r u p t .A s u b p r o c e s s is u s e d to d e n o t e the p r o c e s s i n g ofan i n t e r n a l i n t e r r u p t .The i n t e r n a l i n t e r r u p tp e r f o r m s a s e r v i c e d i r e c t l y s u p p o r t i n g the a c t i v ep r o c e s s a n d , a s s u c h , is t r e a t e d a s a c o n t i n u a t i o n of the p r o c e s s .T h e s u b p r o c e s s is s i m i l a rto a s u b r o u t i n e i n v o k e d v i a a d i r e c t t r a n s f e r a ndu s e s the p r o c e s s s t a t e i n f o r m a t i o n a s p a r a m e t e r s .T h e h i e r a r c h y of the l a y e r s r e f l e c t s theh i e r a r c h y of the h a r d w a r e i n t e r r u p t s t r u c t u r e .S i n c e a n i n t e r r u p t r e s u l t s in a c h a n g e of e x e c u t i o n s e q u e n c e , m u c h the s a m e a s a d i r e c t c a l l ins o f t w a r e , a n i n t e r r u p t g e n e r a t e d d u r i n g the e x e c u t i o n of a l a y e r f u n c t i o n w h i c h is p r o c e s s e db y a h i g h e r l a y e r r e p r e s e n t s a v i o l a t i o n o f thelayer interaction rules.For example, sincethe MOSS partition responsible for handlingI / O i n t e r r u p t s is in l a y e r Z, the p a r t i t i o n s ofl a y e r 1 w o u l d n o t be a l l o w e d to r e c e i v e I / O i n t e r r u p t s d u r i n g their e x e c u t i o n and could not usethe I / O f a c i l i t i e s .H o w e v e r , p a r t i t i o n s in l a y e r3 a n d a b o v e w o u l d b e a l l o w e d to r e c e i v e I / O i n t e r r u p t s a n d to u s e the I / O f a c i l i t i e s of l a y e r Z.4.MOSSPROCESSThe other extension of the process concept is the task process. A task is the implementation of a user p r o g r a m on M O S S .The taskprocess represents the execution of the userp r o g r a m and all subprocesses associated with it.The subprocesses provide the synchronous operating s y s t e m services invoked to support the userp r o g r a m including explicit supervisor calls,implicit paging requests, and implicit error handllng. The state information for a task processincludes user supplied resource and real timecontrol p a r a m e t e r s in addition to the h a r d w a r estate information used for all processes.CONCEPTST h e s t a t i c s t r u c t u r e of MOSS is o v e r l a y e d b y a d y n a m i c s t r u c t u r e of a s y n c h r o n o u s ,c o o p e r a t i n g p r o c e s s e s c a r r y i n g o u t the s y s t e mactivities.P r o c e s s e s do n o t h a v e h i e r a r c h i c a lrelationships.I n s t e a d , a p r o c e s s is the i n d e p e n d e n t , a s y n c h r o n o u s e x e c u t i o n of o n e o r m o r eof the s y s t e m p a r t i t i o n s u n d e r c o n t r o l of thestatic structure.Several processes may executea p a r t i t i o n s i m u l t a n e o u s l y s i n c e the f u n c t i o n s ofthe p a r t i t i o n s c o n t a i n the f a c i l i t i e s w h i c h c o o r d i n a t e the m u l t i p l e r e q u e s t s f o r t h e i r r e s o u r c e s .The MOSS p r o c e s sthe f o l l o w i n g d e f i n i t i o n s :c o n c e p t is b a s e d onI.A processor is an entity whichp e r f o r m s transformations ofdata. In the M O S S environment,a processor is the CentralProcessing Unit (CPU).Z.A p r o g r a m is an o r d e r e d s e q u e n c e of t r a n f o r m a t i o n s p r e s e n t e d f o r e x e c u t i o n to the p r o c essor.3.4. 1Process Selection CriteriaP r o c e s s e s are selected to control theasynchronous execution of independent sequencesof activities. Independent is used in the sensethat the activities do not directly c o m m u n i c a t e(i. e., via direct call) and their execution is notdependent on the state information of anotherprocess.The following criteria are used toidentify processes in M O S S :1.A s e p a r a t e p r o c e s s is u s e d torepresent each user task.Z.A s e p a r a t e p r o c e s s is u s e d tor e p r e s e n t e a c h MOSS f u n c t i o nwhich:A p r o c e s s is the a c t u a l o r p o t e n t i a l e x e c u t i o n of an o r d e r e ds e q u e n c e of t r a n s f o r m a t i o n s on aset of d a t a [7].Performs a service whichdoes not directly supportanother process or task; orA process is a logical entity which existsindependently f r o m a processor, but requires ap r o c e s s o r to a d v a n c e in s t a t e . M o r e than o n ep r o c e s s , e a c h r e p r e s e n t i n g s o m e c o l l e c t i o n ofp r o g r a m s an d r e s o u r c e s ,c a n e x e c u t e concurrently s h a r i n g a v a i l a b l e r e s o u r c e s , e s p e c i a l l ythe p r o c e s s o r .Performs a service whichdirectly supports anotherprocess or task, but whichis e x e c u t e d a s y n c h r o n o u s l y .3.There are two significant extensions tothe basic process concept which have been usedin M O S S .The first, the subprocess concept,A s e p a r a t e p r o c e s s is u s e d torepresent each external interrupt.S e c t i o n 6 i d e n t i f i e s and d e s c r i b e sthe p r o c e s s e s s e l e c t e d f o r M O S S .53e a c h of
4. ZProcessCoordinationMOSS m e c h a n i s m to i n s u r e the p a i r i n g of l o c k sand unlocks within a function; this has b e e nm a i n t a i n e d as a d e s i g n m a n a g e m e n t r e s p o n s i b i l ity.S i n c e the p r o c e s s e s w i t h i n the MOSS e n vironment cannot communicate directly, processcoordination mechanisms are provided whicha l l o w p r o c e s s e s to e x c h a n g e d a t a a n d s y n c h r o n ize their activities.MOSS p r o v i d e s t h r e e d i s tinct mechanismsfor process coordinationw h i c h h a v e b e e n c h o s e n to r e s t r i c t the p r o c e s sinteractions to a m i n i m a l ,These mechanismsare:4. Z.ZSoftware InterruptsT h e s o f t w a r e i n t e r r u p t (SINT) m e c h a n i s mp r o v i d e s the c a p a b i l i t y f o r p r o c e s s to be q u e u e df o r s e r v i c e in m u c h the s a m e m a n n e r t h a t ah a r d w a r e i n t e r r u p t q u e u e s an i n t e r r u p t handlingprocess.Each process contains a SINT counta s p a r t of i t s s t a t e i n f o r m a t i o n .T h e v a l u e ofthe S I N T c o u n t , r a n g i n g f r o m z e r o to a n a r b i t r a r y p o s i t i v e i n t e g e r , i n d i c a t e s the n u m b e r ofthe q u e u e d r e q u e s t s f o r the p r o c e s s .A SINTd r i v e n p r o c e s s is a b l e to s e r v i c e a q u e u e o f r e q u e s t s s e r i a l l y and s u s p e n d i t s e l f when f i n i s h e d( w h e n the S I N T c o u n t b e c o m e s z e r o ) . O t h e rp r o c e s s e s in the s y s t e m m a y q u e u e the S I N Td r i v e n p r o c e s s by i n c r e m e n t i n g its SINT count.T h e r e a r e two b a s i c p r i m i t i v e s a s s o c i a t e d w i t hthe S I N T m e c h a n i s m :yet, sufficient set.The data lock; mutually exclusivea c c e s s to s h a r e d d a t a .T h e s o f t w a r e i n t e r r u p t ( S I N T) ;queuing requests for service.The b l o c k / u n b l o c k ; a w a i t i n g c o m p l e t i o n of a r e q u e s t e d s e r v i c e .These functions could have been implementedwith a g e n e r a l m e c h a n i s m such as D i j k s t r a ' ss e m a p h o r e s , b u t o n l y a t the r i s k o f i n c r e a s e dc o m p l e x i t y in p r o c e s s i n t e r a c t i o n s .1.W a i t a l l o w s a s e r v i c e p r o c e s s tod e c r e m e n t its SINT count i n d i c a t i n g c o m p l e t i o n of the c u r r e n tservice request.If the v a l u e o fthe S I N T c o u n t g o e s to z e r o , thep r o c e s s is s u s p e n d e d u n t i l t heSINT count is i n c r e m e n t e d byanother process.Await requesta p p l i e s o n l y to t h e p r o c e s s m a k i n gthe r e q u e s t .Z.S e t S I N T i n c r e m e n t s the S I N Tc o u n t of a p r o c e s s , q u e u i n g thespecified process for service,ifthe v a l u e of the S I N T w a s z e r ob e f o r e the s e t S I N T , the p r o c e s simmediately becomes eligible foractivation. A process may nots e t its own SINT.MOSS p r o c e s s e s a r e n o t h i e r a r c h i c a l l yr e l a t e d a n d t h u s , n o t l i m i t e d in t h e i r i n t e r a c t i o n sb y the s t a t i c s t r u c t u r e of the s y s t e m .T h e MOSSprocess coordination mechanisms limit processi n t e r a c t i o n s a i d i n g the r e l i a b i l i t y a n d t e s t a b i l i t yof not only this d e s i g n , but a l s o f u t u r e c h a n g e s .4. Z. 1Data LocksProcesses accessing shared data mustb e c o o r d i n a t e d to i n s u r e the i n t e g r i t y of thedata.T h i s c o o r d i n a t i o n is p r o v i d e d b y a h a r d ware assisted lock mechanism which guarant e e s m u t u a l l y e x c l u s i v e a c c e s s to s h a r e d d a t a .The h a r d w a r e p r o v i d e s l o c k and unlock i n s t r u c t i o n s w h i c h a d d r e s s a l o c k w o r d in m e m o r y . T h elock instruction stores a process identificationc o d e in the l o c k w o r d .The unlock instructionr e q u i r e s a m a t c h i n g c o d e to e f f e c t the u n l o c k( z e r o i n g the l o c k w o r d ) .These instructions provide protection against accidentally issued unlocks.I n f o r m a t i o n p a s s i n g to a S I N T d r i v e n p r o c e s s is t h r o u g h a k n o w n r e q u e s t q u e u e f o r theprocess.ASINT driven process performs as i n g l e f u n c t i o n a s s o c i a t e d w i t h a l a y e r in thehierarchical structure.T h i s l a y e r c o n t a i n s ther e q u e s t q u e u e a n d the q u e u e s e r v i c i n g r o u t i n e s .I n v o k i n g the p r o c e s s r e q u i r e s p l a c i n g the r e q u e s t in the q u e u e a n d t h e n p e r f o r m i n g the s e tSINT operation.A process requesting a data lock for ac u r r e n t l y l o c k e d l o c k w o r d is s u s p e n d e d u n t i l thelockwordhas been unlocked. Deadlocks are prev e n t e d b y a n o r d e r i n g s c h e m e t h a t is a n a t u r a lr e s u l t o f the h i e r a r c h i c a l s t r u c t u r e .All funct i o n s a c c e s s i n g a s p e c i f i c d a t a s t r u c t u r e a n d thed a t a s t r u c t u r e i t s e l f a r e on the s a m e l a y e r in thehierarchy.T h u s , e a c h f u n c t i o n is c o m p l e t e w i t hr e s p e c t to the d a t a s t r u c t u r e , i . e . , the l o c k a n du n l o c k a r e c o n t a i n e d w i t h i n the f u n c t i o n . D o w n w a r d c a l l s can be m a d e with a data s t r u c t u r el o c k e d , b u t u p w a r d r e t u r n s a l w a y s l e a v e the d a t astructure unlocked, thus, preventing circularlock requests among processes.T h e r e is no4. Z. 3B l o c k i n g and U n b l o c k i n g P r o c e s s e sT h e b l o c k / u n b l o c k m e c h a n i s m is i n d e p e n d e n t of the S I N T m e c h a n i s m .A blocked proc e s s is n o t r e l e a s e d w h e n i t s S I N T c o u n t e r isincremented.Th e two o p e r a t i o n s d e s c r i b e d b e low are provided.I.54B l o c k allows a p r o c e s sto s u s p e n d
Z.5.itself unconditionally.5.1.4U n b l o c k a l l o w s a p r o c e s s to r e a c tivate a blocked process.The u n conditional block/unblock mechani s m a l l o w s a p r o c e s s to s u s p e n di t s e l f p e n d i n g a r e q u e s t to be c o m pleted by another process.Whilethe p r o c e s s is s u s p e n d e d , it c a ns t i l l h a v e i t s SINT c o u n t e r i n c r e mented by other processes.Itsq u e u e of r e q u e s t s is t h e n h a n d l e ds e r i a l l y w h e n the p r o c e s s is u n blocked.This partition analyzes all detected err o r s i n MOSS. H a r d w a r e / f i r m w a r edetectede r r o r s a r e r e p o r t e d v i a the SUMC i n t e r r u p tstructure while software detected errors are reported via normal procedure invocation.MOSS L A Y E R5. 1.5SPECIFICATIONS5. ZLayer 1The p a r t i t i o n is p l a c e d at this h i e r a r c h i c a l l e v e l to e n a b l e the m e m o r y m a n a g e m e n t p a r t i t i o n to r e q u e s t p a g i n g I / O o p e r a t i o n s .5.3ProcessorManagement PartitionT h i s p a r t i t i o n m a i n t a i n s the s t a t u s of thec e n t r a l p r o c e s s i n g u n i t a n d a l l o c a t e s the p r o c e s s o r to the h i g h e s t p r i o r i t y r e a d y p r o c e s s .Times l i c i n g of the p r o c e s s o r is n o t u t i l i z e d .5. 1 . 3ProcessLayer 3L a y e r 3 c o n t a i n s the m e m o r y m a n a g e m e n t p a r t i t i o n . T h i s p a r t i t i o n s u p p o r t s the a b s t r a c t i o n o f v i r t u a l m e m o r y ; i. e . , the a b i l i t y tor e f e r e n c e an a d d r e s s space which is r e l a t i v e l yi n d e p e n d e n t of p h y s i c a l m e m o r y a n d w h o s e c o n tents m a y a c t u a l l y be in m a i n m e m o r y or in abacking store called external paging memory( E P M ) . T h i s a l l o w s m a i n m e m o r y to be s h a r e da m o n g p r o g r a m s whose i n d i v i d u a l or total m e m o r y r e q u i r e m e n t s e x c e e d the m a i n m e m o r y s i z e .Timer Management PartitionThis partition provides timing servicesf o r the MOSS p r o c e s s e s .The p a r t i t i o n c o n t r o l sthe s e t t i n g of a n i n t e r n a l t i m e r a n d the p r o c e s s i n g of the e x t e r n a l i n t e r r u p t g e n e r a t e d w h e nthis t i m e r e x p i r e s ( s e e S e c t i o n 6. Z).5. 1. ZLayer ZL a y e r Z c o n t a i n s the c h a n n e l m a n a g e m e n tp a r t i t i o n . T h i s p a r t i t i o n c e n t r a l i z e s the c o n t r o lof the SUMC c h a n n e l h a r d w a r e .It is r e s p o n s i b l ef o r s c h e d u l i n g the c h a n n e l s , p r o c e s s i n g I / O i n t e r r u p t s , a n d m a i n t a i n i n g the c h a n n e l s ' l o g i c a lstatus.L a y e r 1 c o n s i s t s of the t i m e r m a n a g e ment, processor management, process managem e n t , e x c e p t i o n h a n d l e r , a n d log q u e u e m a n a g e ment partitions.These partitions provide basics y s t e m s e r v i c e s which can be r e q u e s t e d by allsystem partitions.5.1.1Log Q u e u e M a n a g e m e n t P a r t i t i o nT h i s p a r t i t i o n m a n a g e s the i n - c o r e b u f f e r s of the s y s t e m log. A l l e n t r i e s i n t o the s y s t e m log a r e m a d e v i a t h i s p a r t i t i o n . T h e p a r t i t i o n a c t i v a t e s the log m a n a g e m e n t p r o c e s s w h e n e v e r a log b u f f e r b e c o m e s f u l l i n o r d e r to h a v ethe f u l l log b u f f e r w r i t t e n to the s y s t e m log d a t aset.MOSS is o r g a n i z e d i n t o e l e v e n h i e r a r c h i c a l l a y e r s , e a c h c o n t a i n i n g one o r m o r e p a r t i tions. This section describes each layer startingw i t h the l o w e s t h i e r a r c h i c a l l a y e r a n d p r o c e e d i n gto the h i g h e s t ( s e e F i g u r e 1). R e c a l l that m o d ules within a given layer may not invoke modulesof h i g h e r l a y e r s .5. 1Exception Handler PartitionU n d e r MOSS e a c h t a s k is a s s i g n e d al i n e a r a d d r e s s s p a c e of Zz4 b y t e s . E a c h a d d r e s ss p a c e is d i v i d e d i n t o f o u r s e g m e n t s ; t a s k p r i v a t e ,j o b c o m m o n , s y s t e m c o m m o n , a n d MOSS p r i v a t e .T h e r e a r e n o t s e p a r a t e c o p i e s i n m e m o r y of thel a s t t h r e e s e g m e n t s f o r e a c h t a s k . The h a r d w a r e p e r m i t s the s h a r i n g of s i n g l e c o p i e s of e a c hj o b ' s c o m m o n a r e a , the s y s t e m c o m m o n a r e a ,a n d the MOSS p r i v a t e a r e a .Management PartitionT h i s p a r t i t i o n c o n t r o l s the p r o g r e s s ofp r o c e s s e s i n the MOSS e n v i r o n m e n t .The p a r tition creates processes, coordinates their activities, and deletes them. Process coordinat i o n i s a c c o m p l i s h e d v i a the SINT a n d u n c o n d i tional blocking/unblocking mechanisms.M a i n m e m o r y is a l l o c a t e d on a job b a s i sand varies between a user-defined minimum ando p t i m u m a m o u n t d e p e n d i n g on the r e q u i r e m e n t sof o t h e r j o b s . P a g i n g f o r a l l t a s k s of a j o b isd o n e in the m a i n m e m o r y a l l o c a t e d to the j o b .T h i s s c h e m e c o n t r i b u t e s to the r e p e a t a b i l i t y ofs u c c e s s i v e e x e c u t i o n s of a j o b ; a n i m p o r t a n tT h e f u n c t i o n s of the p r o c e s s o r m a n a g e ment and process management partitions producea multiprograrnming environment.55
MOSS S T R U C T U R E O V E R V I E WHAR DWAREINTERRUPTLE VE LUserPAR TITIONSLAYERIIi01ZUser tasksSystems tasks1Z3SVC h a n d l e rExternal controlLog m a n a g e m e n tSampling performancemonitoringsvc4Page TrapI/OZ1Programmanagement1ZLogical I/OConsole management1I/O resource management1Access management1Event management1Memory management1Channel management1 Timer managementZ Processor management3 . Process management4 Exception handler5 Log q u e u e m a n a g e m e n tTir2 e rExceptionsFigure 156
real time performanceconsideration.l o g i c a l s t a t u s of the S U M C I / O d e v i c e s ( e x c e p tf o r the p a g i n g d e v i c e c o n t r o l l e d by the m e m o r ymanagement partition).W h e n a p a g e is r e f e r e n c e d w h i c h is n o t inm a i n m e m o r y , a p a g e t r a p i n t e r r u p t is g e n e r a t e d . T h i s i n t e r n a l i n t e r r u p t is p r o c e s s e d b y them e m o r y m a n a g e m e n t p a r t i t i o n w h i c h b r i n g s ther e q u i r e d page into m e m o r y .H o w e v e r , the l a y e r i n g of M O S S p r e v e n t s t h i s i n t e r r u p t f r o m b e i n gh o n o r e d if i t i s i n c u r r e d b y m o d u l e s a t o r b e l o wthe m e m o r y m a n a g e m e n t p a r t i t i o n .Therefore,s u c h m o d u l e s m u s t a l w a y s b e l o c k e d in m a i nmemory.The p a r t i t i o n t r a n s l a t e s I / O r e q u e s t s intoc h a n n e l p r o g r a m s a n d q u e u e s the r e q u e s t s u n t i lthe r e q u i r e d d e v i c e b e c o m e s a v a i l a b l e .T hec h a n n e l m a n a g e m e n t p a r t i t i o n is s u b s e q u e n t l y inv o k e d to s c h e d u l e the a s s o c i a t e d c h a n n e l f o re x e c u t i o n of the c h a n n e l p r o g r a m .The partitiona l s o p e r f o r m s I / O e r r o r r e c o v e r y if r e q u i r e d .An a c t u a l s i t u a t i o n i n v o l v i n g this p a r t i t i o np r o v i d e s a n e x a m p l e of the m o d i f i a b i l i t y ofM O S S : B e c a u s e d e v i c e c h a r a c t e r i s t i c s a nd c o n f i g u r a t i o n s a r e m a i n t a i n e d b y t h i s p a r t i t i o n andh i d d e n f r o m the o t h e r MOSS p a r t i t i o n s , the o t h e rp a r t i t i o n s could be d e s i g n e d and coded e v e nt h o u g h the d e v i c e m o d e l s an d c o n f i g u r a t i o n s w e r es till undefined.P a g e s w a p p i n g is p e r f o r m e d on a p a g eg r o u p b a s i s , i . e . , one to f o u r p a g e s of 1K b y t e seach.The SUMC v i r t u a l m e m o r y h a r d w a r es u p p o r t s a d d r e s s t r a n s l a t i o n and page traps atthe p a g e g r o u p l e v e l .5.4Layer 4L a y e r 4 c o n t a i n s the e v e n t m a n a g e m e n tpartition.T h i s p a r t i t i o n p r o v i d e s the s e r v i c e sf o r c o m m u n i c a t i n g the o c c u r r e n c e of s i g n i f i c a n tconditions (called events) between processes.T h e p a r t i t i o n a l s o p r o v i d e s the c a p a b i l i t y to s u s p e n d a p r o c e s s p e n d i n g the o c c u r r e n c e of al o g i c a l c o m b i n a t i o n of e v e n t s .5.7Layer 7 contains the logical I/O and console m a n a g e m e n t partitions.5.7. I5.7. ZConsole ManagementPartitionLayer 5T h i s p a r t i t i o n c o o r d i n a t e s the u s e of thes y s t e m c o n s o l e a m o n g the MOSS p r o c e s s e s .Thepartition supports the abstraction of a virtualconsole for each process.This enables a p r o c e s sto u s e i t s o w n v i r t u a l c o n s o l e w i t h o u t the n e c e s s i t y of c o
Key words and phrases: operating system design, real time operating system, layered operating system, software architecture, and process communication. CR Categories: 3.80, 3.83, 4.35. i. INTRODUCTION The Modular Operating System for SUMC (MOSS) is a general purpose real time operating
1.1 Hard Real Time vs. Soft Real Time Hard real time systems and soft real time systems are both used in industry for different tasks [15]. The primary difference between hard real time systems and soft real time systems is that their consequences of missing a deadline dif-fer from each other. For instance, performance (e.g. stability) of a hard real time system such as an avionic control .
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