Add Zimop/Zcmop draft

src/images/wavedrom/c-mop.adoc

@@ -0,0 +1,12 @@
+[wavedrom, ,svg]
+....
+{reg:[
+    { bits:  2, name: 0x1, type: 8 },
+    { bits:  5, name: 0x0 },
+    { bits:  1, name: 0x1, type: 4 },
+    { bits:  3, name: 'n[2:0]', type: 4 },
+    { bits:  1, name: 0x0, type: 4 },
+    { bits:  1, name: 0x0 },
+    { bits:  3, name: 0x3 },
+]}
+....

src/images/wavedrom/mop-r.adoc

@@ -0,0 +1,15 @@
+[wavedrom, ,svg]
+....
+{reg:[
+    { bits:  7, name: 0x73, attr: ['SYSTEM'], type: 8 },
+    { bits:  5, name: 'rd', type: 2 },
+    { bits:  3, name: 0x4 },
+    { bits:  5, name: 'rs1', type: 4 },
+    { bits:  2, name: 'n[1:0]' },
+    { bits:  4, name: 0x7 },
+    { bits:  2, name: 'n[3:2]' },
+    { bits:  2, name: 0x0 },
+    { bits:  1, name: 'n[4]' },
+    { bits:  1, name: 0x1 },
+], config: {fontsize: 11}}
+....

src/images/wavedrom/mop-rr.adoc

@@ -0,0 +1,15 @@
+[wavedrom, ,svg]
+....
+{reg:[
+    { bits:  7, name: 0x73, attr: ['SYSTEM'], type: 8 },
+    { bits:  5, name: 'rd', type: 2 },
+    { bits:  3, name: 0x4 },
+    { bits:  5, name: 'rs1', type: 4 },
+    { bits:  5, name: 'rs2', type: 4 },
+    { bits:  1, name: 0x1 },
+    { bits:  2, name: 'n[1:0]' },
+    { bits:  2, name: 0x0 },
+    { bits:  1, name: 'n[2]' },
+    { bits:  1, name: 0x1 },
+], config: {fontsize: 11}}
+....

src/riscv-unprivileged.adoc

@@ -111,6 +111,7 @@ include::rvwmo.adoc[]
 //rvwmo.tex
 include::c-st-ext.adoc[]
 //c.tex
+include::zimop.adoc[]
 include::b-st-ext.adoc[]
 //b.tex
 include::j-st-ext.adoc[]

src/zimop.adoc

@@ -0,0 +1,72 @@
+[[zimop]]
+== "Zimop" May-Be-Operations Extension, Version 0.1
+This chapter defines the "Zimop" extension, which introduces the concept of
+instructions that _may be operations_ (MOPs).  MOPs are initially defined to
+perform no useful operation and raise no exceptions, but might be redefined by
+later extensions to perform some other action.  The Zimop extension defines an
+encoding space for 40 MOPs.
+
+[NOTE]
+====
+It is sometimes desirable to define instruction-set extensions whose
+instructions do not raise illegal-instruction exceptions when the extension is
+not implemented.  For example, programs with control-flow integrity checks can
+execute correctly on implementations without the corresponding extension,
+provided the checks are simply ignored.  Implementing these checks as MOPs
+allows the same programs to run on implementations with or without the
+corresponding extension.
+
+Although similar in some respects to HINTs, most MOPs cannot be encoded as
+HINTs, because MOPs might sometimes alter architectural state, which HINTs are
+forbidden from doing.
+====
+
+The Zimop extension defines 32 MOP instructions named `mop.r.__n__`, where
+__n__ is an integer between 0 and 31, inclusive.
+Unless redefined by another extension, these instructions simply write 0 to
+`x[rd]`.  Their encoding allows future extensions to define them to read `x[rs1]`,
+as well as write `x[rd]`.
+
+include::images/wavedrom/mop-r.adoc[]
+[[mop-r]]
+
+The Zimop extension additionally defines 8 MOP instructions named
+`mop.rr.__n__`, where __n__ is an integer between 0 and 7, inclusive.
+Unless redefined by another extension, these instructions simply
+write 0 to `x[rd]`.  Their encoding allows future extensions to define them to
+read `x[rs1]` and `x[rs2]`, as well as write `x[rd]`.
+
+include::images/wavedrom/mop-rr.adoc[]
+[[mop-rr]]
+
+NOTE: These MOPs are encoded in the SYSTEM major opcode in part because it is
+expected their behavior will be modulated by privileged CSR state.
+
+NOTE: These MOPs are defined to write `x[rd]`, rather than performing no
+operation, to simplify instruction decoding and to allow testing the presence
+of features by branching on the zeroness of the result.
+
+The MOPs defined in the Zimop extension are not guaranteed to carry
+a syntactic dependency from `x[rs1]` or `x[rs2]` to `x[rd]`.
+
+NOTE: Not carrying a syntactic dependence relieves straightforward
+implementations of reading `x[rs1]` and `x[rs2]`.
+
+<<<
+
+=== "Zcmop" Compressed May-Be-Operations Extension, Version 0.1
+
+This section defines the "Zcmop" extension, which defines eight 16-bit MOP
+instructions named `c.mop.__n__`, where __n__ is an integer between 0 and 7,
+inclusive.  `c.mop.__n__` is encoded in the reserved encoding space
+corresponding to `c.lui x__m__, 0`, where __m__=2__n__+1.  These instructions
+are defined to not write any register. Their encoding allows future extensions
+to define them to read register `x[__m__]`.
+
+include::images/wavedrom/c-mop.adoc[]
+[[c-mop]]
+
+
+NOTE: The expectation is that each Zcmop instruction is equivalent to some
+Zimop instruction, but the choice of expansion (if any) is left to the
+extension that redefines the MOP.