add formulat for logical to physical entry translation

body.adoc

@@ -238,7 +238,7 @@ and software should ignore but preserve any fields that it does not recognize.
 [NOTE]
 [%unbreakable]
 ====
-_Logical entry 0, accessed via `sireg*` when `siselect`=0x200, is always the physical entry preceding the WRPTR entry ((WRPTR-1) % depth), where depth = 2^(DEPTH+4)^._
+_Logical entry 0, accessed via `sireg*` when `siselect`=0x200, is always the physical buffer entry preceding the WRPTR entry.  More generally, the physical buffer entry Y associated with logical entry X can be determined using the formula Y = (WRPTR - X - 1) % depth, where depth = 2^(DEPTH+4)^._
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 [NOTE]
 [%unbreakable]