Non-standard extensions
Risc-V: an open source instruction set

A number of extensions are listed for possible standardization.

While RVG instructions are all of fixed 32-bit length, other extensions may require instructions of different lengths, for example :

• 16 bits for compact instruction coding, reducing code size ;

• more than 32 bits for special extensions.

Figure 3 shows how different instruction lengths are coded in RISC-V. The five least significant bits of the operation code bbb11 with bbb≠111 define 32-bit instructions. The two least significant bits other than 11 code 16-bit instructions. Then the five least significant bits 11111 are used to code sizes greater than 32 bits, in 16-bit increments (figure...