Trouble interfacing FLASH pins to GPIO (retro game related)

You have to create a SIGNAL assignment for an additional signal in the design.

SIGNAL FL_ADDR_TEMP (21 DOWNTO 0);

Then let's say you want to access address location 0x24ffff (22 bit wide address though your code isn't using any of the upper bits for some reason):

FL_ADDR <= x"24ffff";

FL_ADDR_TEMP <= FL_ADDR;

Then the code you have would be:

GPIO_0(2) <= FL_ADDR_TEMP(17);

GPIO_0(24) <= FL_ADDR_TEMP(16);

GPIO_0(4) <= FL_ADDR_TEMP(15);

GPIO_0(29) <= FL_ADDR_TEMP(14);

GPIO_0(28) <= FL_ADDR_TEMP(13);

GPIO_0(5) <= FL_ADDR_TEMP(12);

GPIO_0(25) <= FL_ADDR_TEMP(11);

GPIO_0(23) <= FL_ADDR_TEMP(10);

GPIO_0(26) <= FL_ADDR_TEMP(9);

GPIO_0(27) <= FL_ADDR_TEMP(8);

GPIO_0(6) <= FL_ADDR_TEMP(7);

GPIO_0(7) <= FL_ADDR_TEMP(6);

GPIO_0(8) <= FL_ADDR_TEMP(5);

GPIO_0(9) <= FL_ADDR_TEMP(4);

GPIO_0(10) <= FL_ADDR_TEMP(3);

GPIO_0(11) <= FL_ADDR_TEMP(2);

GPIO_0(12) <= FL_ADDR_TEMP(1);

GPIO_0(13) <= FL_ADDR_TEMP(0);