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APSR, Application Program Status Register

The APSR characteristics are:

Purpose

Hold program status and control information.

Configuration

This register is present only when AArch32 is supported at any Exception level. Otherwise, direct accesses to APSR are UNKNOWN.

Attributes

APSR is a 32-bit register.

Field descriptions

The APSR bit assignments are:

313029282726252423222120191817161514131211109876543210
NZCVQRES0GERES0RES1RES0

N, bit [31]

Negative condition flag. Set to bit[31] of the result of the last flag-setting instruction. If the result is regarded as a two's complement signed integer, then N is set to 1 if the result was negative, and N is set to 0 if the result was positive or zero.

Z, bit [30]

Zero condition flag. Set to 1 if the result of the last flag-setting instruction was zero, and to 0 otherwise. A result of zero often indicates an equal result from a comparison.

C, bit [29]

Carry condition flag. Set to 1 if the last flag-setting instruction resulted in a carry condition, for example an unsigned overflow on an addition.

V, bit [28]

Overflow condition flag. Set to 1 if the last flag-setting instruction resulted in an overflow condition, for example a signed overflow on an addition.

Q, bit [27]

Cumulative saturation bit. Set to 1 to indicate that overflow or saturation occurred in some instructions.

Bits [26:20]

Reserved, RES0.

GE, bits [19:16]

Greater than or Equal flags, for parallel addition and subtraction.

Bits [15:5]

Reserved, RES0.

Bit [4]

Reserved, RES1.

Bits [3:0]

Reserved, RES0.

It is permitted that, on a read of APSR:

  • Bit[22] returns the value of PSTATE.PAN

  • Bit[9] returns the value of PSTATE.E.

  • Bits[8:6] return the value of PSTATE.{A, I, F}, the mask bits.

  • Bit[4:0] returns the value of PSTATE.M[4:0]

Note

This is an exception to the general rule that an UNKNOWN field must not return information that cannot be obtained, at the current Privilege level, by an architected mechanism.

For more information see 'The Application Program Status Register, APSR'.

Accessing the APSR

APSR can be read using the MRS instruction and written using the MSR (register) or MSR (immediate) instructions.



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