ASA-EMulatR Reference Guide > Introduction > Architecture Overview > Chapter 8 - PAL and Privileged Boundary > 8.9 HW_REI

8.9.1 Purpose

HW_REI (Hardware Return from Exception/Interrupt) is the only architecturally legal exit from PAL mode. It restores processor execution from a privileged PAL context back to the interrupted or faulted execution context while preserving precise architectural state. No other instruction may exit PAL mode, restore privilege level, or resume interrupted execution. Any attempt to bypass HW_REI is architecturally illegal and must fault.

8.9.2 Implementation

AlphaCPU::executeREI() performs a complete architectural state restore:

BoxResult executeREI(PipelineSlot& slot) {

// 1. Restore COMPLETE context (HWPCB + registers)

m_iprGlobalMaster->restoreContext();

// 2. Get return PC (now restored)

quint64 returnPC = m_iprGlobalMaster->h->pc;

// 3. Setup pipeline redirect

slot.reiTarget = returnPC;

slot.pcModified = true;

// 4. Flush pipeline

return BoxResult().flushPipeline();

}

restoreContext() performs a full vector copy from the saved register-context snapshot back into the active processor state. This includes integer registers, floating-point registers (if enabled), PS, IPL, PC, and relevant internal execution state.

8.9.3 Restored State

On HW_REI execution, the following state is restored atomically:

•Program Counter (PC) — instruction address at which execution resumes

•Processor Mode (CM) — returns to prior mode (User, Supervisor, Executive, or Kernel)

•Processor Status (PS) — condition codes, interrupt enable state, architectural flags

•Interrupt Priority Level (IPL) — restores prior IPL for correct interrupt arbitration

•Interrupt state — re-enables or maintains interrupt masking as defined by the restored PS

8.9.4 Serialization Guarantees

HW_REI is an implicit serialization point. Before execution resumes: PAL execution completes, write buffers are drained if required, LL/SC reservations are cleared, pipeline is flushed, and instruction fetch restarts at the restored PC. No speculative instruction may execute across an HW_REI boundary.

8.9.5 Design Rationale