Re: [Qemu-devel] [PATCH 08/10] target-avr: adding instruction translation

[Michael Rolnik]

On Mon, Jun 6, 2016 at 10:17 PM, Richard Henderson <address@hidden> wrote:

> On 06/05/2016 11:52 PM, Michael Rolnik wrote:
>
>> truth table shows that these computations are different.
>>
>
> You're not giving the right inputs to the truth table.
>
> you can't look onto 4th bit because 4th bits in the input were not 0s.
>>
>
> What did you think the xor's do?  They remove the non-zero input bits.
>
> #include <stdio.h>
>
> static int orig(int r, int d, int s)
> {
>   return (((d & s) | (d & ~r) | (s & ~r)) & 2) != 0;
> }
>
> static int mine(int r, int d, int s)
> {
>   return (((d ^ s) ^ r) & 4) != 0;
> }
>
> int main()
> {
>   int s, d;
>   for (s = 0; s < 8; ++s)
>     for (d = 0; d < 8; ++d)
>       {
>         int r = d + s;
>         int o = orig(r, d, s);
>         int m = mine(r, d, s);
>
>         if (o != m)
>           printf("%2d = %d + %d  (o=%d, m=%d)\n", r, d, s, o, m);
>       }
>   return 0;
> }
>
> This performs tests on 3-bit inputs, testing for carry-out on bit 1, just
> like Hf computes carry-out on bit 3.

you are right I can look onto 4th bit, however I do the whole computation
of C flag already.

>
>
>
>     Then you've got the order of the stores wrong.  Your code pushes the
>> LSB
>>     before pushing the MSB, which results in the MSB at the lower address,
>>     which means big-endian.
>>
>> this is right. However as far as I understand AVR is neither little nor
>> big
>> endian because there it's 8 bit architecture (see
>> here http://www.avrfreaks.net/forum/endian-issue). for time being I
>> defined the
>> platform to be little endian with ret address exception
>>
>
> True, AVR is an 8-bit core, where endianness doesn't (normally) apply.
> And you are right that ADIW does treat the registers as little-endian.
>
> But the only multi-byte store to memory is in big-endian order.  So why
> wouldn't you want to take advantage of that fact?

I will.

>
>
>     You have swapped the overflow conditions for INC and DEC.
>>
> ...
>
>> this is how it's defined in the document.
>>
>
> No, it isn't.  Look again, you've swapped them.

checking again.

>
>
>
> r~
>



-- 
Best Regards,
Michael Rolnik