R32V2020 Adding an Instruction to the ISA

Example: Add LIX instruction

For this example, we will add a new instruction, LIX, which loads a register with an extended immediate value.

Add opcode to ops.csv file

• Duplicate LIL and shift opcode table values
  • This may break all past code since the .hex values will be changed for opcodes after the inserted opcode in that category
  • If past code is "broken" then it will need to be recompiled since the opcode decode values will have changed
  • In this case the LIX will be added after LIU so it won't break past code

Immed IMM_DEST Load reg immediate extend LIX 0 1 0 0 0 0 1 0

• Save ops.csv file (as CSV file)

Create Opcode Constants File

• In assembler folder, run (per this procedure):

python assembler.py --gen-constants

Result will be:

Constants written to OpCodeConstants.vhd

Edit R32V2020_Pkg.vhd

• Opcodes are packaged in R32V2020_Pkg.vhd
• Open OpCodeConstants.vhd in editor
• Copy all OpCodeConstants.vhd
• Replace in R32V2020_Pkg.vhd

Edit OpCodeConstants.vhd

• Edit OpCodeDecoder.vhd
  • Add opcode to entity
  • If opcode affects writes to register, add to o_WrRegFile
    • This example does, so added: Op_LIX = '1' or
• Create Op_ function

Edit R32V2020.vhd

• Find similar group opcode
• Copy and change name to new opcode
• Take note of all VHDL files which use the similar opcode
  • In this case, the following VHDL files changed:
    • OpCodeDecoder.vhd (already fixed above)
    • CCRControl.vhd
    • RegisterFile.vhd