The encoding space is not the number of bits used to encode an instruction, the encoding space is the ratio of values that encode a valid instructions over the

The encoding space is not the number of bits used to encode an instruction, the encoding space is the ratio of values that encode a valid instructions over the total number of instructions that could be encoded. As an example, in an ISA with 8-bits fixed instruction length and 8 registers (reg index encoded on 3 bit): If the last 2 bits are the opcode and we define 2 instructions (eg. AND, XOR) that manipulate 2 registers (3 bits + 3 bits), then instruction word values 0b00_000_000 to 0b01_111_111 might encode these two instructions (ignore "_" they are separators). Therefore, instruction word values from 0b10_000_000 to 0b11_111_111 remain free, which represents half of the encoding space. So half of the encoding space remains free. This means we still have room to put new instructions. Similarly, RISC-V valid instructions use almost all the available bits, but there is still room in the encoding space because some opcodes remain free.