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Re: m68k bootstrapping broken
Andreas Schwab <schwab@suse.de> writes:
> Bernardo Innocenti <bernie@develer.com> writes:
>
>> Another thing. I'm playing with libffi to see if I can get it to work
>> on plain 68000. Do you know a simple instruction sequence to replace
>> this bit-field operation?
>>
>> bfins %d0,(%a1){#0,%d2}
>>
>>
>> I was thinking of something like this (in pseudo-C):
>>
>> mask = (1 << (d2 & 0x1f)) - 1;
>> *a1 = (*a2 & ~mask) | (d0 & mask);
>
> The mask is wrong. The bitfield insns number the bits from MSB to LSB, not
> the other way round, as the single bit insns do. Also, a width of 0 is
> replaced by 32.
>
> shift = 32 - (d2 & 0x1f);
> if (shift == 32) mask = 0, shift = 0;
> else mask = (1 << shift) - 1;
> *a1 = (*a2 & mask) | (d0 << shift);
Here is a better version:
shift = d2 & 0x1f;
if (shift == 0) mask = 0, shift = 32;
else mask = 0xFFFFFFFF >> shift;
*a1 = (*a2 & mask) | (d0 << (32 - shift));
(If the offset is nonzero it gets even more complicated.)
Andreas.
--
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux AG, Maxfeldstraße 5, 90409 Nürnberg, Germany
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