1 files changed, 52 insertions, 0 deletions
diff --git a/src/zihintpause.tex b/src/zihintpause.tex
new file mode 100644
index 0000000..fd652a2
--- /dev/null
+++ b/src/zihintpause.tex
@@ -0,0 +1,52 @@
+\chapter{``Zihintpause'' Pause Hint, Version 1.0}
+\label{chap:zihintpause}
+
+The PAUSE instruction is a HINT that indicates the current hart's rate of
+instruction retirement should be temporarily reduced or paused.  The duration of its
+effect must be bounded and may be zero.  No architectural state is changed.
+
+\begin{commentary}
+Software can use the PAUSE instruction to reduce energy consumption while
+executing spin-wait code sequences.  Multithreaded cores might temporarily
+relinquish execution resources to other harts when PAUSE is executed.
+It is recommended that a PAUSE instruction generally be included in the code
+sequence for a spin-wait loop.
+
+A future extension might add primitives similar to the x86 MONITOR/MWAIT
+instructions, which provide a more efficient mechanism to wait on writes to
+a specific memory location.
+However, these instructions would not supplant PAUSE.
+PAUSE is more appropriate when polling for non-memory events, when polling for
+multiple events, or when software does not know precisely what events it is
+polling for.
+
+The duration of a PAUSE instruction's effect may vary significantly within and
+among implementations.
+In typical implementations this duration should be much less than the time to
+perform a context switch, probably more on the rough order of an on-chip cache
+miss latency or a cacheless access to main memory.
+
+A series of PAUSE instructions can be used to create a cumulative delay loosely
+proportional to the number of PAUSE instructions.
+In spin-wait loops in portable code, however, only one PAUSE instruction should
+be used before re-evaluating loop conditions, else the hart might stall longer
+than optimal on some implementations, degrading system performance.
+\end{commentary}
+
+PAUSE is encoded as a FENCE instruction with {\em pred}=W, {\em succ}=0,
+{\em fm}=0, {\em rd}={\tt x0}, and {\em rs1}={\tt x0}.
+
+\begin{commentary}
+PAUSE is encoded as a hint within the FENCE opcode because some
+implementations are expected to deliberately stall the PAUSE instruction until outstanding
+memory transactions have completed.
+Because the successor set is null, however, PAUSE does not {\em mandate} any
+particular memory ordering---hence, it truly is a HINT.
+
+Like other FENCE instructions, PAUSE cannot be used within LR/SC sequences
+without voiding the forward-progress guarantee.
+
+The choice of a predecessor set of W is arbitrary, since the successor set is
+null.
+Other HINTs similar to PAUSE might be encoded with other predecessor sets.
+\end{commentary}