from cpudefs import *
class CPUDecoder(Module):
+ """ decodes a 32-bit instruction into an immediate and other constituent
+ parts, including the opcode and funct3 and funct7, followed by
+ a further (hierarchical) breakdown of the action required to be taken.
+ unidentified actions are decoded as an illegal instruction trap.
+ """
def __init__(self):
self.instruction = Signal(32)
""" calculate immediate
"""
ci = {}
- no_imm = 0x0
+ no_imm = Constant(0x0, 32)
# R-type: no immediate
- for op in [opcode_amo, opcode_op, opcode_op_32, opcode_op_fp]:
+ for op in [OP.amo, OP.op, OP.op_32, OP.op_fp]:
ci[op] = self.immediate.eq(no_imm)
- # I-type
+ # I-type: sign-extended bits 20-31
im = Cat(self.instruction[20:], Replicate(self.instruction[31], 20))
- for op in [opcode_load, opcode_load_fp, opcode_misc_mem,
- opcode_op_imm, opcode_op_imm_32, opcode_jalr,
- opcode_system]:
+ for op in [OP.load, OP.load_fp, OP.misc_mem,
+ OP.op_imm, OP.op_imm_32, OP.jalr,
+ OP.system]:
ci[op] = self.immediate.eq(im)
# S-type
im = Cat(self.instruction[7:12], self.instruction[25:31],
Replicate(self.instruction[31], 21))
- for op in [opcode_store, opcode_store_fp]:
+ for op in [OP.store, OP.store_fp]:
ci[op] = self.immediate.eq(im)
# B-type
im = Cat(Constant(0, 1),
self.instruction[8:12], self.instruction[25:31],
self.instruction[7], Replicate(self.instruction[31], 20))
- for op in [opcode_branch, ]:
+ for op in [OP.branch, ]:
ci[op] = self.immediate.eq(im)
# U-type
im = Cat(Constant(0, 1), self.instruction[12:], )
- for op in [opcode_auipc, opcode_lui]:
+ for op in [OP.auipc, OP.lui]:
ci[op] = self.immediate.eq(im)
# J-type
self.instruction[21:25], self.instruction[25:31],
self.instruction[20], self.instruction[12:20],
Replicate(self.instruction[31], 12))
- for op in [opcode_jal, ]:
+ for op in [OP.jal, ]:
ci[op] = self.immediate.eq(im)
# R4-type: no immediate
- for op in [opcode_madd, opcode_msub, opcode_nmsub, opcode_nmadd]:
+ for op in [OP.madd, OP.msub, OP.nmsub, OP.nmadd]:
ci[op] = self.immediate.eq(no_imm)
# unknown
- for op in [ opcode_custom_0, opcode_48b_escape_0, opcode_custom_1,
- opcode_64b_escape, opcode_reserved_10101, opcode_rv128_0,
- opcode_48b_escape_1, opcode_reserved_11010,
- opcode_reserved_11101, opcode_rv128_1, opcode_80b_escape]:
+ for op in [ OP.custom_0, OP.op_48b_escape_0, OP.custom_1,
+ OP.op_64b_escape, OP.reserved_10101, OP.rv128_0,
+ OP.op_48b_escape_1, OP.reserved_11010,
+ OP.reserved_11101, OP.rv128_1, OP.op_80b_escape]:
ci[op] = self.immediate.eq(no_imm)
# default
return Case(self.opcode, ci)
- def _decode_funct3(self, options, action):
+ def _decode_funct3(self, action, options):
""" decode by list of cases
"""
c = {}
for op in options:
c[op] = self.decode_action.eq(action)
# default
- c["default"] = \
- self.decode_action.eq(decode_action_trap_illegal_instruction)
+ c["default"] = self.decode_action.eq(DA.trap_illegal_instruction)
return Case(self.funct3, c)
def calculate_store_action(self):
""" decode store action
"""
- return self._decode_funct3([ funct3_sb, funct3_sh, funct3_sw, ],
- decode_action_store)
+ return self._decode_funct3(DA.store, [F3.sb, F3.sh, F3.sw, ])
def calculate_load_action(self):
""" decode load action
"""
- return self._decode_funct3([ funct3_lb, funct3_lbu, funct3_lh,
- funct3_lhu, funct3_lw, ],
- decode_action_load)
+ return self._decode_funct3(DA.load, [F3.lb, F3.lbu, F3.lh,
+ F3.lhu, F3.lw, ])
def calculate_branch_action(self):
""" decode branch action
"""
- return self._decode_funct3([ funct3_beq, funct3_bne, funct3_blt,
- funct3_bge, funct3_bltu, funct3_bgeu ],
- decode_action_branch)
+ return self._decode_funct3(DA.branch, [F3.beq, F3.bne, F3.blt,
+ F3.bge, F3.bltu, F3.bgeu ])
def calculate_jalr_action(self):
""" decode jalr action
"""
- return self._decode_funct3([ funct3_jalr, ],
- decode_action_jalr)
+ return self._decode_funct3(DA.jalr, [F3.jalr, ])
def calculate_op_action(self):
- """ decode op action
+ """ decode op action: the arith ops, and, or, add, xor, sr/sl etc.
"""
c = {}
immz = Constant(0, 12)
regz = Constant(0, 5)
- # fence
- c[funct3_slli] = \
+ # slli
+ c[F3.slli] = \
If((self.funct7 == Constant(0, 7)),
- self.decode_action.eq(decode_action_op_op_imm)).\
- Else(
- self.decode_action.eq(decode_action_trap_illegal_instruction))
- # fence.i
- c[funct3_srli_srai] = \
+ self.decode_action.eq(DA.op_op_imm)
+ ).Else(
+ self.decode_action.eq(DA.trap_illegal_instruction))
+ # srli/srai
+ c[F3.srli_srai] = \
If((self.funct7 == Constant(0, 7) | \
(self.funct7 == Constant(0x20, 7))),
- self.decode_action.eq(decode_action_op_op_imm)).\
- Else(
- self.decode_action.eq(decode_action_trap_illegal_instruction))
+ self.decode_action.eq(DA.op_op_imm)
+ ).Else(
+ self.decode_action.eq(DA.trap_illegal_instruction))
# default
- c["default"] = \
- self.decode_action.eq(decode_action_op_op_imm)
+ c["default"] = self.decode_action.eq(DA.op_op_imm)
return Case(self.funct3, c)
def calculate_misc_action(self):
- """ decode misc mem action
+ """ decode misc mem action: fence and fence_i
"""
c = {}
immz = Constant(0, 12)
regz = Constant(0, 5)
# fence
- c[funct3_fence] = \
+ c[F3.fence] = \
If((self.immediate[8:12] == immz) & (self.rs1 == regz) & \
- (self.rd == regz),
- self.decode_action.eq(decode_action_fence)).\
- Else(
- self.decode_action.eq(decode_action_trap_illegal_instruction))
+ (self.rd == regz),
+ self.decode_action.eq(DA.fence)
+ ).Else(
+ self.decode_action.eq(DA.trap_illegal_instruction))
# fence.i
- c[funct3_fence_i] = \
+ c[F3.fence_i] = \
If((self.immediate[0:12] == immz) & (self.rs1 == regz) & \
- (self.rd == regz),
- self.decode_action.eq(decode_action_fence_i)).\
- Else(
- self.decode_action.eq(decode_action_trap_illegal_instruction))
+ (self.rd == regz),
+ self.decode_action.eq(DA.fence_i)
+ ).Else(
+ self.decode_action.eq(DA.trap_illegal_instruction))
# default
- c["default"] = \
- self.decode_action.eq(decode_action_trap_illegal_instruction)
+ c["default"] = self.decode_action.eq(DA.trap_illegal_instruction)
return Case(self.funct3, c)
- def calculate_action(self):
- """ calculate action
+ def calculate_system_action(self):
+ """ decode opcode system: ebreak and csrs
"""
c = {}
- # load opcode
- c[opcode_load] = self.calculate_load_action()
- c[opcode_misc_mem] = self.calculate_misc_action()
- c[opcode_op_imm] = self.calculate_op_action()
- c[opcode_op] = self.calculate_op_action()
- c[opcode_lui] = self.decode_action.eq(decode_action_lui_auipc)
- c[opcode_auipc] = self.decode_action.eq(decode_action_lui_auipc)
- c[opcode_store] = self.calculate_store_action()
- c[opcode_branch] = self.calculate_branch_action()
- c[opcode_jalr] = self.calculate_jalr_action()
- c[opcode_jal] = self.decode_action.eq(decode_action_jal)
+ b1 = Constant(1, 32)
+ regz = Constant(0, 5)
+ # ebreak
+ c[F3.ecall_ebreak] = \
+ If((self.immediate == ~b1) & (self.rs1 == regz) & \
+ (self.rd == regz),
+ self.decode_action.eq(DA.trap_ecall_ebreak)
+ ).Else(
+ self.decode_action.eq(DA.trap_illegal_instruction))
+ # csrs
+ for op in [ F3.csrrw, F3.csrrs, F3.csrrc,
+ F3.csrrwi, F3.csrrsi, F3.csrrci]:
+ c[op] = self.decode_action.eq(DA.csr)
+ # default
+ c["default"] = self.decode_action.eq(DA.trap_illegal_instruction)
- return Case(self.opcode, c)
+ return Case(self.funct3, c)
-"""
+ def calculate_action(self):
+ """ calculate action based on opcode.
- function `decode_action calculate_action(
- input [6:0] funct7,
- input [2:0] funct3,
- input [4:0] rd,
- input [4:0] rs1,
- input [4:0] rs2,
- input [31:0] immediate,
- input [6:0] opcode);
- begin
- case(opcode)
- `opcode_system: begin
- case(funct3)
- `funct3_ecall_ebreak:
- if((rs1 != 0) | (rd != 0) | ((immediate & ~32'b1) != 0))
- calculate_action = `decode_action_trap_illegal_instruction;
- else
- calculate_action = `decode_action_trap_ecall_ebreak;
- `funct3_csrrw,
- `funct3_csrrs,
- `funct3_csrrc,
- `funct3_csrrwi,
- `funct3_csrrsi,
- `funct3_csrrci:
- calculate_action = `decode_action_csr;
- default:
- calculate_action = `decode_action_trap_illegal_instruction;
- endcase
- end
- `opcode_load_fp,
- `opcode_custom_0,
- `opcode_op_imm_32,
- `opcode_48b_escape_0,
- `opcode_store_fp,
- `opcode_custom_1,
- `opcode_amo,
- `opcode_op_32,
- `opcode_64b_escape,
- `opcode_madd,
- `opcode_msub,
- `opcode_nmsub,
- `opcode_nmadd,
- `opcode_op_fp,
- `opcode_reserved_10101,
- `opcode_rv128_0,
- `opcode_48b_escape_1,
- `opcode_reserved_11010,
- `opcode_reserved_11101,
- `opcode_rv128_1,
- `opcode_80b_escape: begin
- calculate_action = `decode_action_trap_illegal_instruction;
- end
- default:
- calculate_action = `decode_action_trap_illegal_instruction;
- endcase
- end
- endfunction
-
- assign decode_action = calculate_action(funct7,
- funct3,
- rd,
- rs1,
- rs2,
- immediate,
- opcode);
-
- endmodule
-"""
+ this is a first level case statement that calls down to 2nd
+ level case (and in some cases if logic) mostly using funct3
+ (funct7 in the case of arith ops).
+ """
+ c = {}
+ c[OP.load ] = self.calculate_load_action()
+ c[OP.misc_mem] = self.calculate_misc_action()
+ c[OP.op_imm ] = self.calculate_op_action()
+ c[OP.op ] = self.calculate_op_action()
+ c[OP.lui ] = self.decode_action.eq(DA.lui_auipc)
+ c[OP.auipc ] = self.decode_action.eq(DA.lui_auipc)
+ c[OP.store ] = self.calculate_store_action()
+ c[OP.branch ] = self.calculate_branch_action()
+ c[OP.jalr ] = self.calculate_jalr_action()
+ c[OP.jal ] = self.decode_action.eq(DA.jal)
+ c[OP.system ] = self.calculate_system_action()
+
+ # big batch of unrecognised opcodes: throw trap.
+ for o in [ OP.load_fp, OP.custom_0, OP.op_imm_32,
+ OP.op_48b_escape_0, OP.store_fp, OP.custom_1,
+ OP.amo, OP.op_32, OP.op_64b_escape,
+ OP.madd, OP.msub, OP.nmsub,
+ OP.nmadd, OP.op_fp, OP.reserved_10101,
+ OP.rv128_0, OP.op_48b_escape_1, OP.reserved_11010,
+ OP.reserved_11101, OP.rv128_1, OP.op_80b_escape,
+ "default", ]:
+ c[o] = self.decode_action.eq(DA.trap_illegal_instruction)
+ return Case(self.opcode, c)
if __name__ == "__main__":
example = CPUDecoder()
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