Encoding:

Format:

EHB 

Purpose:

Execution Hazard Barrier

To stop instruction execution until all execution hazards have been cleared.

Description:

EHB is used to denote execution hazard barrier. The actual instruction is interpreted by the hardware as SLL r0, r0, 3.

This instruction alters the instruction issue behavior on a pipelined processor by stopping execution until all execution hazards have been cleared. Other than those that might be created as a consequence of setting StatusCU0, there are no execution hazards visible to an unprivileged program running in User Mode. All execution hazards created by previous instructions are cleared for instructions executed immediately following the EHB. The EHB instruction does not clear instruction hazards-such hazards are cleared by the JALR.HB, JR.HB, and ERET instructions.

Restrictions:

None

Operation:

ClearExecutionHazards()

Exceptions:

None

Programming Notes:

In Release 2 implementations, this instruction resolves all execution hazards. On a superscalar processor, EHB alters the instruction issue behavior in a manner identical to SSNOP. For backward compatibility with Release 1 implementations, the last of a sequence of SSNOPs can be replaced by an EHB. In Release 1 implementations, the EHB will be treated as an SSNOP, thereby preserving the semantics of the sequence. In Release 2 implementations, replacing the final SSNOP with an EHB should have no performance effect because a properly sized sequence of SSNOPs will have already cleared the hazard. As EHB becomes the standard in MIPS implementations, the previous SSNOPs can be removed, leaving only the EHB.