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|
`timescale 1us/1us
`include "eater_types.sv"
module eater_decoder (
input clk_i,
input wire enable_control_i,
input wire [7:0] instruction_i,
output CpuControlFlags flags_o
);
CpuState internal_state;
CpuState next_state;
CpuControlFlags internal_flags;
always_comb begin
if (enable_control_i) begin
flags_o = internal_flags;
end
end
initial begin
internal_state = INIT;
next_state = INIT;
// assigned in always_comb
// internal_flags = '{default: '0};
end
// next-state machine
always @(posedge clk_i) begin
next_state = INIT;
case (internal_state)
INIT: begin
next_state = PC_to_MAR;
end
PC_to_MAR: begin
next_state = MEM_to_INS;
end
MEM_to_INS: begin
next_state = PC_inc;
end
PC_inc: begin
next_state = PC_to_MAR;
end
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
default: begin
end
endcase
end
endmodule
|
