"Zimop" Extension for May-Be-Operations, Version 1.0
This chapter defines the "Zimop" extension, which introduces the concept of
instructions that may be operations (MOPs). MOPs are initially defined to
simply write zero to x[rd]
, but are designed to be redefined by later
extensions to perform some other action.
The Zimop extension defines an encoding space for 40 MOPs.
It is sometimes desirable to define instruction-set extensions whose
instructions, rather than raising illegal-instruction exceptions when the extension is
not implemented, take no useful action (beyond writing Although similar in some respects to HINTs, MOPs cannot be encoded as HINTs, because unlike HINTs, MOPs are allowed to alter architectural state. Because MOPs may be redefined by later extensions, standard software should not execute a MOP unless it is deliberately targeting an extension that has redefined that MOP. |
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]
.
Unresolved include directive in modules/chapters/pages/zimop.adoc - include::images/wavedrom/mop-r.adoc[]
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]
.
Unresolved include directive in modules/chapters/pages/zimop.adoc - include::images/wavedrom/mop-rr.adoc[]
The recommended assembly syntax for MOP.R.n is MOP.R.n rd, rs1,
with any x -register specifier being valid for either argument. Similarly for
MOP.RR.n, the recommended syntax is MOP.RR.n rd, rs1, rs2.
The extension that redefines a MOP may define an alternate assembly mnemonic.
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These MOPs are encoded in the SYSTEM major opcode in part because it is expected their behavior will be modulated by privileged CSR state. |
These MOPs are defined to write zero to 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.
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The MOPs defined in the Zimop extension do not carry a syntactic dependency
from x[rs1]
or x[rs2]
to x[rd]
, though an extension that redefines the
MOP may impose such a requirement.
Not carrying a syntactic dependency relieves straightforward
implementations of reading x[rs1] and x[rs2] .
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"Zcmop" Compressed May-Be-Operations Extension, Version 1.0
This section defines the "Zcmop" extension, which defines eight 16-bit MOP
instructions named C.MOP.n, where n is an odd integer between 1 and
15, inclusive. C.MOP.n is encoded in the reserved encoding space
corresponding to C.LUI xn, 0, as shown in C.MOP.n instruction encoding..
Unlike the MOPs defined in the Zimop extension, the C.MOP.n instructions
are defined to not write any register.
Their encoding allows future extensions to define them to read register
x[n]
.
The Zcmop extension requires the Zca extension.
Unresolved include directive in modules/chapters/pages/zimop.adoc - include::images/wavedrom/c-mop.adoc[]
Very few suitable 16-bit encoding spaces exist. This space was chosen
because it already has unusual behavior with respect to the rd /rs1
field—it encodes c.addi16sp when the field contains x2 --and is
therefore of lower value for most purposes.
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Mnemonic | Encoding | Redefinable to read register |
---|---|---|
C.MOP.1 |
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C.MOP.3 |
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C.MOP.5 |
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C.MOP.7 |
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C.MOP.9 |
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C.MOP.11 |
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C.MOP.13 |
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C.MOP.15 |
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The recommended assembly syntax for C.MOP.n is simply the nullary
C.MOP.n. The possibly accessed register is implicitly xn .
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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.
Note, a Zcmop instruction that does not write a value can expand into a write
to x0 .
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