From 899e65b942bef4dec7fec6383a7f45e270fbc463 Mon Sep 17 00:00:00 2001
From: Tariq Kurd <tariq.kurd@codasip.com>
Date: Tue, 28 May 2024 11:22:45 +0200
Subject: [PATCH] remove the M-bit from AP

---
 src/cap-description.adoc        | 14 ++++++++------
 src/img/cap-encoding-xlen32.edn |  2 +-
 src/img/cap-encoding-xlen64.edn |  7 ++++---
 3 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/src/cap-description.adoc b/src/cap-description.adoc
index 7147e0f0..8fb57f89 100644
--- a/src/cap-description.adoc
+++ b/src/cap-description.adoc
@@ -136,8 +136,8 @@ Therefore, it is only possible to encode a subset of all combinations.
 [#perms_bit_width,options=header,align="center",width="80%",cols="^13%,^27%,<60%"]
 |==============================================================================
 ^| MXLEN | Permissions width      | Comment
-^| 32    | {cap_rv32_perms_width} | Encodes some combinations of 5 permission bits and the <<m_bit>>
-^| 64    | {cap_rv64_perms_width} | Separate bits for each permission and the <<m_bit>>
+^| 32    | {cap_rv32_perms_width} | Encodes some combinations of 5 permission bits and _includes_ the <<m_bit>> in quadrant 1 (see <<cap_perms_encoding32>>)
+^| 64    | {cap_rv32_perms_width} | Separate bits for each permission, _excluding_ the <<m_bit>>
 |==============================================================================
 
 For MXLEN=32, the permissions encoding is split into four quadrants.
@@ -206,12 +206,14 @@ permission.
 | 2   | <<r_perm>>
 | 3   | <<x_perm>>
 | 4   | <<asr_perm>>
-| 5   | <<m_bit>>
+//| 5   | <<m_bit>>
 |==============================================================================
 
-For both MXLEN=32 and MXLEN=64, the <<m_bit>> is only assigned meaning when the
-implementation supports {cheri_default_ext_name}, otherwise the bit is reserved
-and must be 0 in valid capabilities.
+The <<m_bit>> is only assigned meaning when the
+implementation supports {cheri_default_ext_name} _and_ <<x_perm>> is set.
+
+. For MXLEN=64, the bit assigned to the <<m_bit>> must be zero if <<x_perm>> isn't set.
+. For MXLEN=32, the <<m_bit>> is only encoded in quadrant 1 and does _not_ exist in the other quadrants.
 
 ===== Permission Transitions
 
diff --git a/src/img/cap-encoding-xlen32.edn b/src/img/cap-encoding-xlen32.edn
index 83bc6594..1478b500 100644
--- a/src/img/cap-encoding-xlen32.edn
+++ b/src/img/cap-encoding-xlen32.edn
@@ -9,7 +9,7 @@
 (draw-column-headers {:height 50 :font-size 26 :labels (reverse ["0" "1" "2" "" "" "" "" "" "" "9" "10" "11" "12" "" "" "" "" "17" "18" "19" "20" "21" "" "" "24" "25" "" "" "" "29" "30" "31"])})
 
 (draw-box "SDP"         {:span 2})
-(draw-box "AP"          {:span 5})
+(draw-box "AP, M"       {:span 5})
 (draw-box "Reserved"    {:span 4})
 (draw-box "S"           {:span 1})
 (draw-box "EF"          {:span 1})
diff --git a/src/img/cap-encoding-xlen64.edn b/src/img/cap-encoding-xlen64.edn
index 67319396..d9e3af04 100644
--- a/src/img/cap-encoding-xlen64.edn
+++ b/src/img/cap-encoding-xlen64.edn
@@ -6,12 +6,13 @@
 (def left-margin 100)
 (def right-margin 100)
 (def boxes-per-row 32)
-(draw-column-headers {:height 50 :font-size 26 :labels (reverse ["0" "2" "3" "" "" "" "13" "14" "16" "17" "" "" "25" "26" "27" "28" "" "" "" "" "" "" "" "46" "47" "52" "53" "56" "57" "" "" "63"])})
+(draw-column-headers {:height 50 :font-size 26 :labels (reverse ["0" "2" "3" "" "" "" "13" "14" "16" "17" "" "" "25" "26" "27" "28" "" "" "" "" "" "46" "" "47" "51" "52" "53" "56" "57" "" "" "63"])})
 
 (draw-box "Reserved"    {:span 4})
 (draw-box "SDP"         {:span 2})
-(draw-box "AP"          {:span 2})
-(draw-box "Reserved"    {:span 9})
+(draw-box "M"           {:span 1})
+(draw-box "AP"          {:span 3})
+(draw-box "Reserved"    {:span 7})
 (draw-box "S"           {:span 1})
 (draw-box "EF"          {:span 1})
 (draw-box "T[11:3]"     {:span 4})