Re: [Tack-devel] Accessing to unaligned data in records/structures

[David G. <dg...@co...>]

EM requires word-aligned accesses (or short-aligned for shorts) (which just
recently came as a nasty shock when I set the i80 alignment to 1 and
everything started failing obscurely). Only architectures that don't
require it you *can* get away with code like the above but it's not
guaranteed to work, and the C compiler will insert padding in structures to
make alignment work.

To load unaligned fields you have to read bytes and then assemble a word
with arithmetic. For some reason there aren't any EM instructions to do
unaligned reads.

On Tue, 11 Jun 2019 at 17:35, Carl Eric Codere via Tack-devel <
tac...@li...> wrote:

> Greetings,
>               I am slowly working on the em instruction testsuite, and I
> am a bit stuck, when trying to test LOI from the below code, I get a bad
> pointer error in the interpreter at LOI 2, because the address is not
> aligned. Hence my question, how do you access the second field of this
> structure? What happens in pascal if the record is declared as packed, or
> is this just a problem associated with the interpreter?
>
>
> /* Unaligned structure containing the data to read. */
> struct01
>     rom    255I1
>     rom -32767I2
>     rom 2147483647I4
> #if EM_WSIZE >= 4
>     rom 9223372036854775807I8
> #endif
>
>     exp $_m_a_i_n
>     pro $_m_a_i_n, 0
>
>     /* Unaligned structure access */
>
> 1
>     lae    struct01+1            /* stack address of structure */
>     loi    2                    /* stack data from structure    */
>     loc    -32767
>     cmi    EM_WSIZE            /* check if value is what is expected. */
>     zeq *2
>
>     ldc __LINE__
>     cal $fail
>     asp 4
>
> Thanks for all your help,
> Carl
>
> _______________________________________________
> Tack-devel mailing list
> Tac...@li...
> https://lists.sourceforge.net/lists/listinfo/tack-devel
>


-- 
┌─── http://www.cowlark.com ───
│ "I have always wished for my computer to be as easy to use as my
│ telephone; my wish has come true because I can no longer figure out
│ how to use my telephone." --- Bjarne Stroustrup