Dependency Expression

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A DEPendency EXpression describes which PPIs/protocols must be installed before a PEIM or DXE driver can be launched by the dispatcher. The dependency expression guarantees that needed services are present before initialization.

The dependency expressions are encoded as sections within the file's image, as specified in the Firmware File System. There are two different section types, one for PEIMs and another for DXE drivers. UEFI drivers do not have dependency expressions.

The dependency expressions are encoded in prefix notation, where the operands precede the operator. Operands are pushed on an internal expression stack. Operators pop operands from the expression stack, perform the operation and push the result back on the expression stack. If entire expression has been processed and the result is logical TRUE, then the driver can be launched. If the result is logcal FALSE, then the driver will not be launched.

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