[J-core] Illegal instruction handling (was: Re: PC-relative loads and delay slots)

[Robert Ou]

On Tue, Jul 19, 2016 at 10:01 AM, Geoff Salmon
<gsalmon at se-instruments.com> wrote:
> On 16-07-19 11:01 AM, Rich Felker wrote:
>>
>> I agree and I would assume this was the motivation for the sh4 change.
>> This is a good time to mention that it would be really nice if illegal
>> slot instruction and undefined instruction exceptions actually worked.
>
>
> The illegal slot instruction detection is implemented. I don't know why it's
> not working. Someone will have to look in simulation to see why.
>
> It uses this function to detect instructions that don't belong in delay
> slots.
>
>     function check_illegal_delay_slot (code : std_logic_vector(15 downto 0))
> return std_logic is
>     begin
>         -- Check for instructions that assign to PC:
>         -- RTE, RTS, JMP @Rm, JSR @Rm, BRAF Rm, BSRF Rm, BF label, BF /S
> label, BT label, BT /S label, BRA label, BSR label, TRAPA #imm
>         if ((code(15 downto 12) = "0000" and code(3 downto 2) = "00" and
> code(0) = '1') or (code(15 downto 14) = "10" and code(12 downto 11) = "01"
> and code(8) = '1') or code(15 downto 13) = "101" or (code(15) = '1' and
> code(13 downto 8) = "000011") or (code(15) = '0' and code(13 downto 12) =
> "00" and code(4 downto 0) = "01011")) then
>             return '1';
>         else
>             return '0';
>         end if;
>     end;
>
> That may be an incomplete list.
>
>
> The illegal/undefined instruction test is woefully incomplete.
>
>     function check_illegal_instruction (code : std_logic_vector(15 downto
> 0)) return std_logic is
>     begin
>         -- TODO: Improve detection of illegal instructions
>         if code(15 downto 8) = x"ff" then
>             return '1';
>         else
>             return '0';
>         end if;
>     end;
>
> We weren't sure how to implement it without a large amount of logic. At one
> point I was trying to generate it from the instruction spreadsheet. I forget
> why that didn't work, or maybe it did work but the result was too large. If
> the instruction space is full enough, it may be easy to hand-write the
> check_illegal_instruction function to cover all the gaps. Whatever is
> preventing the illegal delay slot test from having an effect will also
> probably stop the illegal instruction test as well though.
>
> - Geoff
>
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I just tested illegal instruction handling, and it does "work" with
some caveats:

a) Illegal instruction checking is very limited as Geoff just explained.

b) The vector used by illegal instructions is always hardcoded to 0.
This happens on line 78 of decode_core.vhm:
cd := x"0" & system_instr_codes(instr) & x"00";
If you change the x"00" to something else, then all "system operations
that are not external events" (slot illegal, general illegal, and
break) will then use that new vector.

c) You need to explicitly set VBR to 0 in your program if you are
simulating (this should not be necessary on real hardware). The reason
for this is quite interesting. If you look at the microcode
spreadsheet for the reset logic, the CPU tries to clear VBR to 0 by
xor-ing TEMP1 with itself and storing that in VBR. On a real device,
this should work. However, the register file in simulation is not
initialized with anything, so it gets a special value of "U" for
uninitialized. Apparently, xor-ing "U" with "U" doesn't give 0 but
instead gives another "U". When an illegal instruction happens and the
CPU tries to read the vector table, it then adds some constant to the
value of VBR. "U" plus something then gives "X" for unknown/invalid,
and the CPU then tries to read from an address "XXXXXXXX" which
obviously doesn't work.

Robert