Control Register implementation

I have a 8bit control register that in a PLD that resides at 0x330 on
an ISA bus.

I want to be able to have some bits in the control register read-only,
read/write, and write-only.

I have something like this

process (IOR)
begin
if (Falling_edge(IOR) and CRS = '0') then
-- Place the Control register onto the databus but ensure
the write-only
-- Data is correct
SD <= (reg and "11111110");
end if;
end process;

process (IOW)
begin
if (Falling_edge(IOW) and CRS = '0') then
-- Read the data from the databus but don't write over the read-only
bits
reg(4) <= SD(4);
reg(3) <= SD(3);
reg(2) <= SD(2);
reg(1) <= SD(1);
reg(0) <= SD(0);
end if;
end process;

However, doing it this way means that if I write data to the Data-bus I
need to tri-state the databus somehow? How can I do this/ when and
where???

Thanks

gommo