Encoding
Encoding of instructions.
Instructions are encoded as specified in [ISA] and [ISAP].
Here we define encoding as a mapping from the instruction fixtypes to their encodings.
Without the C extension [ISA:27],
instructions are encoded in 32 bits per instruction.
So we define our mapping from instructions to 32-bit unsigned integers.
We plan to model the C extension in the future:
we will define a second encoding mapping,
which maps certain instructions to 16 bits per instruction;
the features will be also extended with
an indication of whether the C extension is present.
Even without modeling the C extension yet,
our encoding mapping depends on the features,
because it is only defined on instructions
that are valid according to the features.
We also provide a characterization of the valid encodings
as the image of the encoding function.
See encoding-decoding-illustration
for an illustration of encoding and decoding.
Subtopics
- Encode
- Encode an instruction in the normal way (i.e. in 32 bits).
- Encoding-decoding-illustration
- Illustration of instruction encoding and decoding.
- Encoding-validp
- Check if a 32-bit word is a valid instruction encoding.
- Encode-load-funct
- Encode the name of
an instruction with the LOAD opcode
into the func3 field
[ISA:2.6] [ISA:4.3] [ISA:35].
- Encode-store-funct
- Encode the name of
an instruction with the STORE opcode
into the funct3 field
[ISA:2.6] [ISA:4.3] [ISA:35].
- Encode-op-imms-32-funct
- Encode the name of
a shift instruction with the OP-IMM opcode
into the funct3 field and the high 6 immediate bits
[ISA:4.2.1] [ISA:35].
- Encode-op-funct
- Encode the name of
an instruction with the OP opcode
into the funct3 and funct7 fields
[ISA:2.4.2] [ISA:12.1] [ISA:12.2] [ISA:35].
- Encode-op-32-funct
- Encode the name of
an instruction with the OP-32 opcode
into the funct3 and funct7 fields
[ISA:4.2.2] [ISA:12.1] [ISA:12.2] [ISA:35].
- Encode-op-imms64-funct
- Encode the name of
a 64-bit shift instruction with the OP-IMM opcode
into the funct3 field and the high 6 immediate bits
[ISA:4.2.1] [ISA:35].
- Encode-op-imms32-funct
- Encode the name of
a 32-bit shift instruction with the OP-IMM opcode
into the funct3 field and the high 7 immediate bits
[ISA:2.4.1] [ISA:35].
- Encode-op-imm-funct
- Encode the name of
a non-shift instruction with the OP-IMM opcode
into the funct3 field
[ISA:2.4.1] [ISA:35].
- Encode-op-imm-32-funct
- Encode the name of
a non-shift instruction with the OP-IMM-32 opcode
into the funct3 field
[ISA:4.2.1] [ISA:35].
- Encode-branch-funct
- Encode the name of
an instruction with the BRANCH opcode
into the funct3 field
[ISA:2.5.2] [ISA:35].
