Field in ACPI

How Field definitions map bits and bytes inside an OperationRegion to names used by AML methods.

June 10, 2026 2 min read

OperationRegion says where an address space exists. Field says how AML names map onto bits and bytes inside that region.

Without Field, an EC region is just a byte range. With Field, AML can say LIDS, BSTA, or TMP0 and the OS interpreter knows exactly which bits to access.

OperationRegion (ERAM, EmbeddedControl, 0x00, 0xFF)
Field (ERAM, ByteAcc, NoLock, Preserve)
{
    Offset (0x10),
    BSTA, 8,
    Offset (0x20),
    LIDS, 1,
    Offset (0x30),
    TMP0, 16
}

Item	Value	Note
Region name	ERAM	The OperationRegion that backs this field.
Access type	ByteAcc / WordAcc / DWordAcc	How the interpreter accesses the region.
Lock rule	NoLock / Lock	Whether the global lock is used.
Update rule	Preserve / WriteAsZeros / WriteAsOnes	How untouched bits are handled during writes.
Offset	Byte offset	Moves the field cursor to a specific byte position.
Named field	Name + bit length	Maps a readable AML name to a bit range.

Real World Example: one wrong bit breaks lid wake

If LIDS is defined at the wrong offset, the OS may think the lid is always open or always closed.

Offset (0x20),
LIDS, 1

If the EC firmware moved lid state to 0x21[0] but ACPI still reads 0x20[0], the AML method is syntactically correct but semantically wrong.

Common Pitfall: Preserve is not always harmless

When writing a small bit field, the update rule matters. Preserve means the interpreter should preserve other bits in the same access unit. But if the hardware register is write-one-to-clear or has side effects, a naive field write can still be dangerous.

Field review checklist

Verify offset and bit width against EC RAM map, register map, or GPIO documentation.
Check access width. Byte access and word access may behave differently on side-effect registers.
Check update rule before writing to hardware-backed fields.
Check whether fields are read-only in hardware but written by AML.
Trace all methods that read or write the field.

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Real World Example: one wrong bit breaks lid wake

Common Pitfall: Preserve is not always harmless

Field review checklist

Related notes

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Embedded Controller OperationRegion

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