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clarify top bound and how it feeds into rep range check #512

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merged 6 commits into from
Feb 3, 2025
Merged

clarify top bound and how it feeds into rep range check #512

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clarify top bound and how it feeds into rep range check
tariqkurd-repo Jan 30, 2025
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tariqkurd-repo Jan 31, 2025
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tariqkurd-repo Jan 31, 2025
133a4ca
Update src/cap-description.adoc
tariqkurd-repo Jan 31, 2025
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Update src/cap-description.adoc
tariqkurd-repo Jan 31, 2025
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include limit on CAP_MAX_E
tariqkurd-repo Jan 31, 2025
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10 changes: 7 additions & 3 deletions src/cap-description.adoc
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Original file line number Diff line number Diff line change
Expand Up @@ -448,8 +448,8 @@ T[MW - 1:MW - 2] = B[MW - 1:MW - 2] + LCout + LMSB
Decoding the bounds:

```
top: t = { a[MXLEN - 1:E + MW] + ct, T[MW - 1:0] , {E{1'b0}} }
base: b = { a[MXLEN - 1:E + MW] + cb, B[MW - 1:0] , {E{1'b0}} }
top: t = { {1'b0, a[MXLEN - 1:E + MW]} + ct, T[MW - 1:0], {E{1'b0}} }
base: b = { a[MXLEN - 1:E + MW] + cb, B[MW - 1:0], {E{1'b0}} }
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```
The corrections c~t~ and c~b~ are calculated as as shown below using the
definitions in xref:cap_encoding_ct[xrefstyle=short] and
Expand Down Expand Up @@ -691,7 +691,7 @@ bounds are formed of two or three sections:
| EF=1, i.e. E=0 | address[MXLEN:MW] + ct 2+| T[MW - 1:0]
|==============================================================================

The top described by xref:comp_addr_bounds[xrefstyle=short] is MXLEN+1 bits
The top bound described by xref:comp_addr_bounds[xrefstyle=short] is MXLEN+1 bits
wide to allow capabilities to span the whole address space. The address is
zero-extended by one bit. The malformed check (see
xref:section_cap_malformed[xrefstyle=short]) ensures that the top never
Expand All @@ -717,3 +717,7 @@ xref:section_cap_encoding[xrefstyle=short].
This gives useful guarantees, such that if an executed instruction is in
<<pcc>> bounds, then it is also guaranteed that the next linear instruction
is _representable_.

NOTE: Even though address[MXLEN] in xref:comp_addr_bounds[xrefstyle=short] affects
the value of the top bound, it does not have an affect on the _representable range_
calculation and so can be excluded from the check.
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