This instruction reads the contents of the source register snd then writes it into the Special Register area. The index of this Special Register is specified by the other operand, a register or an immediate value (si4, Imm6 or Imm16).
This instruction is the reverse of the GET instruction.
Because of the rule that "the immediate value follows the opcode", PUT's last register argument is the value to write when an immediate address is provided, which might look a bit counter-intuitive.
PUT 123h R2 ; Write R2's contents to SR#123h. PUT R3 R4 ; Write R3's contents to SR at address R4