[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).