The SBA decoder is an main IP core, the function of this core is enable (logic "1") the strobe (enable/chip select) signals based upon the value of the address bus.
For achieve that, the SBA decoder use the constants defined in SBA config.
The signal STB_O will have a bit in "1" determinate by the value of ADR_I, the others bits will be "0". The port STB_I works as a global enabler and it is asserted by the SBA controller when it gives a valid address during a bus cycle.
For the above example, the SBA decoder gives the following results:
Then when the SBA controller put an address of x"0000" or x"0001" the SBA decoder enables the bit 0 of the STB_O signal. If the address is x"0002" the SBA decoder enables the bit 1 of the STB_O signal.