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`timescale 1us/1us
`include "eater_types.sv"
module eater_decoder (
input clk_i,
input wire [7:0] instruction_i,
output CpuControlFlags flags_o
);
CpuState internal_state /* verilator public_flat_rd */;
CpuState next_state;
CpuControlFlags internal_flags;
wire [3:0] actual_instruction = instruction_i[7:4];
assign flags_o = internal_flags;
initial begin
internal_state = INIT;
next_state = INIT;
// assigned in always_comb
// internal_flags = '{default: '0};
end
function CpuState insdep_state;
case (actual_instruction)
NOP: insdep_state = PC_to_MAR;
LDA: insdep_state = LDA_INS_to_MAR;
ADD: insdep_state = ADD_INS_to_MAR;
SUB: insdep_state = SUB_INS_to_MAR;
OUT: insdep_state = OUT_A_to_OUT;
HALT_op: insdep_state = HALT_st;
default: insdep_state = INIT;
endcase
endfunction
// next-state machine
always @(posedge clk_i) begin
next_state = INIT;
case (internal_state)
INIT: next_state = PC_to_MAR;
PC_to_MAR: next_state = MEM_to_INS;
MEM_to_INS: next_state = PC_inc;
PC_inc: next_state = insdep_state();
LDA_INS_to_MAR: next_state = LDA_MEM_to_A;
LDA_MEM_to_A: next_state = PC_to_MAR;
ADD_INS_to_MAR: next_state = ADD_MEM_to_B;
ADD_MEM_to_B: next_state = ADD_ALU_to_A;
ADD_ALU_to_A: next_state = PC_to_MAR;
SUB_INS_to_MAR: next_state = SUB_MEM_to_B;
SUB_MEM_to_B: next_state = SUB_ALU_to_A;
SUB_ALU_to_A: next_state = PC_to_MAR;
OUT_A_to_OUT: next_state = PC_to_MAR;
HALT_st: next_state = HALT_st;
default: begin
next_state = INIT;
end
endcase
internal_state <= next_state;
end
// current-state-to-flags machine
// TODO: This is kinda weird, I *can* set "all flags" to zero "in the beginning",
// and then only set the required flags "later".
// how does that work combinatorically???
always_comb begin
`ifdef IVERILOG
internal_flags = 'b0;
`else
internal_flags = '{default: '0};
`endif
case (internal_state)
// INIT: // left out
PC_to_MAR: begin
internal_flags.PC_out = 1;
internal_flags.MAR_in = 1;
end
MEM_to_INS: begin
internal_flags.RAM_out = 1;
internal_flags.INS_in = 1;
end
PC_inc: begin
internal_flags.PC_count = 1;
end
LDA_INS_to_MAR: begin
internal_flags.INS_out = 1;
internal_flags.MAR_in = 1;
end
LDA_MEM_to_A: begin
internal_flags.RAM_out = 1;
internal_flags.A_in = 1;
end
ADD_INS_to_MAR: begin
internal_flags.INS_out = 1;
internal_flags.MAR_in = 1;
end
ADD_MEM_to_B: begin
internal_flags.RAM_out = 1;
internal_flags.B_in = 1;
end
ADD_ALU_to_A: begin
internal_flags.ALU_out = 1;
internal_flags.A_in = 1;
end
SUB_INS_to_MAR: begin
internal_flags.INS_out = 1;
internal_flags.MAR_in = 1;
end
SUB_MEM_to_B: begin
internal_flags.RAM_out = 1;
internal_flags.B_in = 1;
end
SUB_ALU_to_A: begin
internal_flags.ALU_out = 1;
internal_flags.ALU_subtract_nadd = 1;
internal_flags.A_in = 1;
end
OUT_A_to_OUT: begin
internal_flags.A_out = 1;
internal_flags.OUT_in = 1;
end
HALT_st: begin
internal_flags.halt = 1;
end
default: begin
end
endcase
end
endmodule
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