You can instantiate your block RAM as follows:
# Sub Components
# ###############################################################################
# PortB connected to microblaze and PortA connected to user externals
# PORTB is auto-calculated due to bus_interface connection to the bram_cntlr
BEGIN bram_block
PARAMETER INSTANCE = opb_bram1
PARAMETER HW_VER = 1.00.a
BUS_INTERFACE PORTB = bram_portb
PORT BRAM_Clk_A = sys_clk
PORT BRAM_EN_A = my_bram_en_a
PORT BRAM_WEN_A = my_bram_wen_a
PORT BRAM_Addr_A = my_bram_addr_a
PORT BRAM_Din_A = my_bram_din_a
PORT BRAM_Dout_A = my_bram_dout_a
END
And, define your global ports as follows:
PORT BRAM_Rst_A = BRAM_Rst, DIR = I
PORT BRAM_Clk_A = BRAM_Clk, DIR = I
PORT BRAM_EN_A = my_bram_en_a, DIR = I
PORT BRAM_WEN_A = my_bram_wen_a, DIR = I, VEC = [0:3]
PORT BRAM_Addr_A = my_bram_addr_a, DIR = I, VEC = [0:31]
PORT BRAM_Din_A = my_bram_din_a, DIR = O, VEC = [0:31]
PORT BRAM_Dout_A = my_bram_dout_a, DIR = I, VEC = [0:31]