processor flags

Processor flags store information about specific processor functions. The processor flags are usually kept in a flag register or a general status register. This can include result flags that record the results of certain kinds of testing, information about data that is moved, certain kinds of information about the results of compations or transformations, and information about some processor states. Closely related and often stored in the same processor word or status register (although often in a privileged portion) are control flags that control processor actions or processor states or the actions of certain instructions.

• IBM 360/370: program status word (PSW)
• Intel 8086/80286: 16-bit flag register (FLAGS); system flags, control flag, and status flags)
• Intel 80386: 32-bit flag register (EFLAGS); system flags, control flag, and status flags)
• MIX: an overflow toggle and a comparison indicator
• Motorola 680x0, 68300: 16-bit status register (SR); high byte is system byte and requires privileged access, low byte is user byte or condition code register (CCR)

A few typical result flags (with processors that include them):

• auxilary carry Set if a carry out of the most significant bit of a BCD operand occurs (binary coded decimal addition). Also commonly set if a borrow occurs in a BCD subtract. Used in Intel 80x86 [AF].
• carry Set if a carry out of the most significant bit of an operand occurs (addition). Also commonly set if a borrow occurs in a subtract. Used in Digital VAX [C], Intel 80x86 [CF], Motorola 680x0 [C], Motorola 68300 [C], Motorola M68HC16 [C].
• comparison indicator contains one of three values: less, equal, or greater. Used in MIX.
• extend Set to the value of the carry bit for arithmetic operations (used to support implementation of multi-byte arithmetic larger than that implemented directly by the hardware. Used in Motorola 680x0 [X], Motorola 68300 [X].
• half carry Set if a carry out of bit 3 of an operand occurs during BCD addition. Used in Motorola M68HC16 [H].
• negative Set if the most significant bit of a result is set. Used in Digital VAX [N], Motorola 680x0 [N], Motorola 68300 [N], Motorola M68HC16 [N].
• overflow Set if arithmetic overflow occurs. Used in Digital VAX [V], Intel 80x86 [OF], Motorola 680x0 [V], Motorola 68300 [V], Motorola M68HC16 [V].
• overflow toggle a single bit that is either on or off. Used in MIX.
• parity For odd parity machines, set to make an odd number of one bits; for an even parity machine, set to make an even number of one bits. Used in Intel 80x86 [PF]. The IBM 360/370 has odd parity on memory.
• sign Set for negative sign. Used in Intel 80x86 [SF].
• trap Set for traps. Used in Intel 80x86 [TF].
• zero Set if a result equals zero. Used in Digital VAX [Z], Intel 80x86 [ZF], Motorola 680x0 [Z], Motorola 68300 [Z], Motorola M68HC16 [Z].

Some conditions are determined by combining multiple flags. For example, if a processor has a negative flag and a zero flag, the equivalent of a positive flag is the case of both the negative and zero flags both simultaneously being cleared.

A few typical control flags (with processors that include them):

• decimal overflow trap enable Set if decimal overflow occurs (or conversion error on a VAX). Used in Digital VAX [DV].
• direction flag Determines the direction of string operations (set for autoincrement, cleared for autodecrement). Used in Intel 80x86 [DF].
• floating underflow trap enable Set if floating underflow occurs. Used in Digital VAX [FU].
• integer overflow trap enable Set if integer overflow occurs (or conversion error on a VAX). Used in Digital VAX [IV].
• interupt enable Set if interrupts enabled. Used in Intel 80x86 [IF].
• i/o privilege level Used to control access to I/O instructions and hardware (thereby seperating control over I/O from other supervisor/user states). Two bits. Used in Intel 80x86 [IO PL].
• nested task flag Used in Intel 80x86 [NF].
• resume flag Used in Intel 80x86 [RF].
• virtual 8086 mode Used to switch to virtual 8086 emulation. Used in Intel 80x86 [VM].