main memory with an indication of which process is assigned to each regionSimilar references will appear in I/O and file tablesWhen processes communicate with one another, the process identifier informs the OS of the destination of a particular communicationWhen processes are allowed to create other processes, identifiers indicate the parent and descendents of each process3633343536
2/12/202010• Contains condition codes plus other status information•EFLAGS register is an example of a PSW used by any OS running on an x86 processorProgram status word (PSW)• User-visible registers•Control and status registers• Stack pointersConsists of the contents of processor registers 37Figure 3.12 x86 EFLAGS RegisterX ID = Identification flagX VIP = Virtual interrupt pendingX VIF = Virtual interrupt flagX AC = Alignment checkX VM = Virtual 8086 modeX RF = Resume flagX NT = Nested task flagX IOPL = I/O privilege levelS OF = Overflow flagC DF = Direction flagX IF = Interrupt enable flagX TF = Trap flagS SF = Sign flagS ZF = Zero flagS AF = Auxiliary carry flagS PF = Parity flagS CF = Carry flag31302928272625242322212019181716151413 1211109876543210000000000IDVIPVIFACVMRF0NTIOPLOFDFIFTFSFZF0AF0PF1CF0S Indicates a Status FlagC Indicates a Control FlagX Indicates a System FlagShaded bits are reserved38Table 3.6x86EFLAGS Register BitsStatus Flags (condition codes) AF (Auxiliary carry flag) Represents carrying or borrowing between half-bytes of an 8-bit arithmetic or logic operation using the AL register.CF (Carry flag) Indicates carrying out or borrowing into the leftmost bit position following an arithmetic operation. Also modified by some of the shift and rotate operations.OF (Overflow flag) Indicates an arithmetic overflow after an addition or subtraction.PF (Parity flag) Parity of the result of an arithmetic or logic operation. 1 indicates even parity; 0 indicates odd parity.SF (Sign flag) Indicates the sign of the result of an arithmetic or logic operation.ZF (Zero flag) Indicates that the result of an arithmetic or logic operation is 0. Control Flag DF (Direction flag) Determines whether string processing instructions increment or decrement the 16-bit half-registers SI and DI (for 16-bit operations) or the 32-bit registers ESI and EDI (for 32-bit operations). System Flags (should not be modified by application programs) AC (Alignment check) Set if a word or doubleword is addressed on a nonword or nondoubleword boundary.ID (Identification flag) If this bit can be set and cleared, this processor supports the CPUID instruction. This instruction provides information about the vendor, family, and model.RF (Resume flag) Allows the programmer to disable debug exceptions so that the instruction can be restarted after a debug exception without immediately causing another debug exception.