Why aren’t opcode and funct7 and funct3 a single 17-bit field?
In RISC-V, I want to know why opcode and funct7 and funct3 don't get together to be a 17-bits field?
RISC V has 6 different instruction formats for 32-bit instructions. All formats share the opcode field, and the opcode field is used to determine the instruction size and instruction format.
While some of the other fields are shared between two or more formats, no other field is present in all formats.
For example, the funct7 field is only present in one of the six formats for 32-bit instructions, namely the register/register instruction format.
The funct3 field is present in 4 of the 6 formats for 32-bit instructions, but not all formats.
I want to know why opcode and funct7 and funct3 don't get together to be a 17-bits field?
Since these two other fields are essentially specific only to a subset of instructions / instruction formats, they really cannot be considered part of the opcode in the general case. When the format is register/register, then all three of those fields together determine the operation to be performed.
We can extract all three fields simultaneously, though; however, we would have to be aware that such an extraction would only be applicable when the instruction is of the right format.
In fact, this is how the hardware generally works: it starts decoding fields even before it knows which fields are applicable to this particular instruction. After this initial, overly broad decoding takes place, other hardware selects which of the fields to use and how, ignoring other computations that were done just in case the instruction was actually different.
For example, the hardware will likely be extracting the 5-bit fields rs2 and rs1, using those as indexes into the register file to obtain values from those registers and make them ready for computation, regardless of the actual instruction format. This, then, is wasted work when the instruction doesn't have these fields, but doing that work before knowing the opcode speeds things up when these fields are applicable. Other parts of the hardware are extracting immediate values, and all this is begin done in parallel for performance.
Here are all the 32-bit formats in one image:
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