//外圍模塊 
//--------------地址譯碼器----------------------
module decoder( addr, rom_sel, ram_sel);
output rom_sel, ram_sel;
input [12:0] addr;
reg rom_sel, ram_sel;

always @( addr )
begin
casex(addr)
13'b1_1xxx_xxxx_xxxx:{rom_sel,ram_sel}<=2'b01;
13'b0_xxxx_xxxx_xxxx:{rom_sel,ram_sel}<=2'b10;
13'b1_0xxx_xxxx_xxxx:{rom_sel,ram_sel}<=2'b10;
default:{rom_sel,ram_sel}<=2'b00;
endcase
end
endmodule

/* 地址譯碼器用于產生選通信號，選通ROM或RAM。
FFFFH---1800H RAM
1800H---0000H ROM -----------------------------*/