1Computer System OverviewChapter 12Operating SystemExploits the
hardware resources of one or more processorsProvides a set of
services to system usersManages secondary memory and !O
devices"#asic ElementsProcessorMain Memory$volatile$referred to as
real memory or primary memory!O modules$secondary memory
devices$communications e%uipment$terminalsSystem
&us$communication among processors' memory' and !O
modules(Processor)wo internal registers$Memory address register
*M+,-Specifies the address for the next read or write$Memory
&uffer register *M#,-Contains data written into memory or
receives data read from memory$!O address register$!O &uffer
register.)op/0evel ComponentsPC MARIR MBRI/O ARI/O BRCPU Main
MemorySystemBusI/O
ModuleBufersInstruction012n2n1!ata!ata!ata!ataInstructionInstruction"i#ure
1$1Com%uter Com%onents& 'o%(e)el *ie+PC , Pro#ram counterIR ,
Instruction re#isterMAR , Memory address re#isterMBR , Memory -ufer
re#isterI/O AR, In%ut/out%ut address re#isterI/O BR, In%ut/out%ut
-ufer re#ister./ecutionunit1Processor ,egisters2ser/visi&le
registers$Ena&le programmer to minimi3e main/memory references
&y optimi3ing register useControl and status registers$2sed
&y processor to control operating of the processor$2sed &y
privileged operating/system routines to control the execution of
programs42ser/5isi&le ,egistersMay &e referenced &y
machine language+vaila&le to all programs / application
programs and system programs)ypes of registers$6ata
$+ddressndexSegment pointerStac7 pointer82ser/5isi&le
,egisters+ddress ,egisters$ndexnvolves adding an index to a
&ase value to get an address$Segment pointer9hen memory is
divided into segments' memory is referenced &y a segment and an
offset$Stac7 pointerPoints to top of stac7:Control and Status
,egistersProgram Counter *PC-$Contains the address of an
instruction to &e fetchednstruction ,egister *,-$Contains the
instruction most recently fetchedProgram Status 9ord
*PS9-$Condition codes$nterrupt
ena&le!disa&le$Supervisor!user mode1;Control and Status
,egistersCondition Codes or eroOverflow11nstruction Execution)wo
steps$Processor reads instructions from memory
