The overloadable opcodes proposal defines 8 standardised R-type instructions xcmd0, xcmd1, ...xcmd7 preferably in the brownfield opcode space.
Each xcmd takes in rs1 a 12 bit "logical unit" (lun) identifying a device on the cpu that implements some "extension interface" (xintf) together with some additional data. An xintf is a set of up to 8 commands with 2 input and 1 output port (i.e. like an R-type instruction), together with a description of the semantics of the commands. Calling e.g. xcmd3 routes its two inputs and one output ports to command 3 on the device determined by the lun bits in rs1. Thus, the 8 standard xcmd instructions are standard-designated overloadable opcodes, with the non standard semantics of the opcode determined by the lun.
The overloadable opcodes proposal defines 8 standardised R-type instructions xcmd0, xcmd1, ...xcmd7 preferably in the brownfield opcode space.
Each xcmd takes in rs1 a 12 bit "logical unit" (lun) identifying a device on the cpu that implements some "extension interface" (xintf) together with some additional data. An xintf is a set of up to 8 commands with 2 input and 1 output port (i.e. like an R-type instruction), together with a description of the semantics of the commands. Calling e.g. xcmd3 routes its two inputs and one output ports to command 3 on the device determined by the lun bits in rs1. Thus, the 8 standard xcmd instructions are standard-designated overloadable opcodes, with the non standard semantics of the opcode determined by the lun.