<< Click to Display Table of Contents >> Navigation:  ASA-EMulatR Reference Guide > Introduction > Architecture Overview > Chapter 20 – Boot Sequence, PAL, and SRM Integration > 20.4 PAL Mode Entry and Exit

20.4.1 enterPal() Entry Sequence

 

PalBoxBase::enterPal() performs the complete PAL mode transition in a fixed sequence:

 

BoxResult enterPal(PalEntryReason reason, quint64 vectorOrSelector, quint64 faultPC) {

 // 1. Record metadata (entry reason, vector, fault PC)

 m_entryReason = reason;

 m_entryVector = vectorOrSelector;

 m_faultPC = faultPC;

 

 // 2. Save complete context (UNIFIED — same for all entry types)

 m_iprGlobalMaster->saveContext();

 

 // 3. Compute entry PC

 if (reason == PalEntryReason::CALL_PAL_INSTRUCTION)

 entryPC = computeCallPalEntry(vectorOrSelector);

 else

 entryPC = vectorOrSelector; // Direct vector for faults/interrupts

 

 // 4. Set EXC_ADDR to faulting/return PC

 m_iprGlobalMaster->h->exc_addr = faultPC;

 

 // 5. Enter PAL mode: PC = vector | 0x1, IPL = 7, CM = KERNEL

 m_iprGlobalMaster->h->pc = entryPC | 0x1ULL;

 m_iprGlobalMaster->h->setIPL_Unsynced(7);

 m_iprGlobalMaster->h->setCM(CM_KERNEL);

 

 // 6. Activate shadow registers

 m_shadowRegsActive = true;

 

 // 7. Return flush request

 return BoxResult().flushPipeline();

}

 

The low bit of PC (| 0x1) is the architectural PAL mode indicator checked by the execution engine on every instruction. saveContext() captures the full register-context snapshot including integer registers, floating-point registers, PS, IPL, PC, and relevant execution state for later restoration by HW_REI.

 

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20.4.2 PalEntryReason

 

PalEntryReason classifies why PAL mode was entered, which determines how the entry vector is computed:

 

 

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20.4.3 HW_REI Exit Sequence

 

HW_REI (Hardware Return from Exception/Interrupt) is the only architecturally legal exit from PAL mode. 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: integer registers, floating-point registers (if enabled), PS, IPL, PC, and relevant execution state. HW_REI is an implicit serialization point — before execution resumes, write buffers are drained if required, LL/SC reservations are cleared, the pipeline is flushed, and instruction fetch restarts at the restored PC. No speculative instruction may execute across an HW_REI boundary.

 

The design choice to restore the full register context (rather than selectively restoring fields) provides stronger correctness guarantees and ensures no residual PAL state leaks into non-PAL execution.

 

See Also: PalBoxLib/PalBoxBase.h – enterPal(); cpuCoreLib/AlphaCPU.h – executeREI(); 8.4 CALL_PAL - Entering the Privileged Boundary; 8.9 HW_REI - Exiting PAL Mode .