why the 3wire-spi-master have 4pins?

how to connect the net of spi-master ?

spoc u_sopc(

// 1) global signals:

.clk(clk_cpu),

.reset_n(cpu_reset_n),

// the_sdram_0

.zs_addr_from_the_sdram_0(SDRAM2_A[11:0]),

.zs_ba_from_the_sdram_0(SDRAM2_BA),

.zs_cas_n_from_the_sdram_0(SDRAM2_CAS_n),

.zs_cke_from_the_sdram_0(SDRAM2_CLKEN),

.zs_cs_n_from_the_sdram_0(SDRAM2_CS_n),

.zs_dq_to_and_from_the_sdram_0(SDRAM2_D),

.zs_dqm_from_the_sdram_0({SDRAM2_UDQM,SDRAM2_LDQM}),

.zs_ras_n_from_the_sdram_0(SDRAM2_RAS_n),

.zs_we_n_from_the_sdram_0(SDRAM2_WE_n),

// the_key

.in_port_to_the_key(KEY),

// the_led

.out_port_from_the_led(LED[2:0]),

// the_spi_master

.MISO_to_the_spi_master(), //----------------????? how to connect?

.MOSI_from_the_spi_master(SPI_SDIO),//-------?? how to connect?

.SCLK_from_the_spi_master(SPI_SCLK),

.SS_n_from_the_spi_master(SPI_SS),

);