-R-type opcodes xcmd0, .. xcmd7. Each takes in rs1 a 12bit "logical unit" (lun)
-identifying a (on or off chip) (sub)device with at most N commands that can execute the command, together with xlen - 12 bits of additional data. Based on the logical unit bits in rs1 the CPU routes each of the 8 commands to a specific interface on a specific (sub)device on the CPU. Effectively, the xcmd0, ... xcmd7 instructions become "virtual method" opcodes, that can be overloaded for different extension (sub)devices
+R-type opcodes xcmd0 rd, rs1, rs2, .. , xcmd7 rd, rs1, rs2.
+The input value in rs2 is arbitrary.
+The content of the first input rs1, however, is divided in a 12bit "logical unit" (lun) together with xlen - 12 bits of additional data.
+The lun bits in rs1, determines a specific (sub)device, and the CPU routes the command to this device with rs1 and rs2 as input, and rd as output. Effectively, the xcmd0, ... xcmd7 instructions are "virtual method" opcodes, overloaded for different extension (sub)devices.