[cesarb]

> Yeah, it's a little harder than for pure fixed-width Arm64, but it is massively massively easier than for amd64.

For those who haven't read the details of the RISC-V ISA: the first two bits of every instruction tell the decoder whether it's a 16-bit or a 32-bit instruction. It's always in that same fixed place, there's no need to look at any other bit in the instruction. Decoding the length of a x86-64 instruction is much more complicated.

[Narishma]

Why do they use two bits for it? Do they plan to support other instruction lengths in the future?

[brucehoult]

So that there are 48k combinations available for 2-byte instructions and 1 billion for 4-byte (or longer) instructions. Using just 1 bit to choose would mean 32k 2-byte instructions and 2 billion 4-byte instructions.

Note that ARMv7 uses a similar scheme with two instruction lengths, but using The first 4 bits from each 2-byte parcel to determine the instruction length. It's quite complex, but the end result is 7/8 (56k) 2-byte instructions are possible and 1/8 (512 million) 4-byte instructions.

IBM 360 in 1964 thru Z-System today also uses a 2-bit scheme to choose between 2-byte instructions with 00 meaning 2-bytes (16k instructions available), 01 or 10 meaning 4-bytes (2 billion instructions available), and 11 meaning 6-bytes (64 terra instructions available).

[cesarb]

> Why do they use two bits for it?

To increase the number of 16-bit instructions. Of the four possible combinations of these two bits, one indicates a 32-bit or longer instruction, while the other three are used for 16-bit instructions.

> Do they plan to support other instruction lengths in the future?

They do. Of the eight possible combinations for the next three bits after these two, one of them indicates that the instruction is longer than 32 bits. But processors which do not know any instruction longer than 32 bits do not need to care about that; these longer instructions can be naturally treated as if they were an unknown 32-bit instruction.