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Pipeline Stall

Concept WIKI v1 · 5/26/2026

A pipeline stall is a temporary halt of instruction progress in a pipelined processor, used to handle hazard conditions. In the Y86-64 PIPE model described in the evidence, a STALL variant avoids data forwarding and instead holds an instruction in the decode stage for up to three cycles when a later pipeline stage creates a data hazard.

Definition

A pipeline stall is the behavior in which a pipelined processor temporarily prevents an instruction from advancing in order to handle a hazard condition. In the cited Y86-64 PIPE verification work, stalls are described as occurring when the pipeline deals with a hazard condition, and the STALL design variant specifically makes an instruction stall in the decode stage for up to three cycles when an instruction farther down the pipeline imposes a data hazard.

Role in the STALL pipeline variant

The STALL variant of the PIPE processor uses no data forwarding. Instead of forwarding values from later stages to earlier dependent instructions, it stalls the dependent instruction in the decode stage for up to three cycles whenever a data hazard is imposed by an instruction farther down the pipeline.

This establishes a design tradeoff within the provided PIPE variants: the STALL model resolves data hazards by waiting, while the LF variant adds a forwarding path from the data-memory output to the pipeline register feeding the data-memory input, allowing some load/use hazards to be handled by forwarding rather than by stalling.

Distinction from branch cancellation

The evidence distinguishes stalls from instruction cancellation caused by branch misprediction. For example, the NT and BTFNT branch-prediction variants may cancel up to two instructions when a branch is mispredicted, whereas the STALL variant describes holding an instruction in decode because of a data hazard.

Verification concern: indefinite stalling

Stalls introduce a liveness concern. The verification discussion notes that a pipeline may stall to handle hazards, but safety-style verification alone can still be satisfied by a processor that deadlocks or even by a device that does nothing. Therefore, the report states that liveness must also be verified to show that the processor cannot reach a state where it never makes forward progress, and specifically to show that the pipeline does not stall indefinitely.

Related pipeline-state constraints

The same verification work also discusses pipeline-state restrictions and invariants. Some possible pipeline states are described as unreachable in real operation, such as intra-instruction inconsistencies involving a nop with a regular register identifier, or inter-instruction inconsistencies involving a ret instruction followed by other instructions rather than by at least three bubbles. Such restrictions can be excluded by a verifier only if they are proved invariant: they must hold from reset and be preserved by each processor operation.

LINKED ENTITIES

1 links
Data Forwarding alternative_to

CITATIONS

6 sources
6 citations
[1] In the STALL variant, no data forwarding is used; an instruction stalls in the decode stage for up to three cycles when a later pipeline instruction imposes a data hazard. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[2] The LF variant adds a forwarding path that allows some load/use hazards to be resolved by data forwarding rather than stalling. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[3] Branch-prediction variants such as NT and BTFNT may cancel up to two instructions when a branch is mispredicted, which is distinct from the STALL variant's data-hazard stalling behavior. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[4] A pipeline may stall to deal with hazards, but safety verification alone can allow deadlocking or non-progressing processors to pass; liveness verification is needed to show the pipeline does not stall indefinitely. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[5] The PIPE verification work treats restrictions on possible pipeline states as predicates over pipeline states and requires such restrictions to be proved invariant from reset and preserved by each processor operation. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5
[6] The evidence gives examples of unreachable or inconsistent pipeline states, including a nop with a regular program register identifier and a ret instruction followed by other instructions rather than by at least three bubbles. Formal Verification of Pipelined Y86-64 Microprocessors with UCLID5