Discuss DDC-base vs DDC-address offsetting (#87)

CTSRD-CHERI · Aug 4, 2023 · 8ae8a62 · 8ae8a62
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diff --git a/chap-cheri-riscv.tex b/chap-cheri-riscv.tex
@@ -1199,8 +1199,12 @@ \subsection{\DDC{} and \PCC{} Relocation}
 CHERI-RISC-V originally specified that legacy memory accesses using
 integer pointers were relocated by \DDC{} and \PCC{} in addition to
 being constrained.  In this model, integer pointers were treated as
-offsets relative to the base of \DDC{} and \PCC{} rather than
-addresses.
+offsets relative to the address of \DDC{} or base of \PCC{}\footnote{%
+While offsetting with respect to \DDC{}'s address avoids unnecessary
+alignment requirements, the base of \PCC{} must be used since its
+address must change as the program runs, so cannot be used for
+offsetting.}
+rather than addresses.
 
 The current version of CHERI-RISC-V does not relocate integer
 pointers.  However, it may be desirable to provide optional relocation

diff --git a/chap-rationale.tex b/chap-rationale.tex
@@ -1470,7 +1470,9 @@ \section{\DDC{} and \PCC{} Offsetting}
 Originally, CHERI always treated integer pointers used for legacy
 memory accesses as offsets.  For example, loads and stores using an
 integer pointer treated integer address as an offset relative to the
-base of \DDC{}.  Similarly, branch instructions which targeted an
+base of \DDC{}\footnote{Some CHERI instantiations performed offsetting
+with respect to the address of DDC, rather than the base.}.
+Similarly, branch instructions which targeted an
 absolute integer pointer set the offset of \PCC{} to the value of the
 integer pointer.