[Superseding prior response, typo in example.]
Tim Frink <plfriko@[EMAIL PROTECTED]
> writes:
> Hi,
>
> the ISA of my processor can handle immediate operands of max.
> 16bits. So, to store a 32bit base pointer into a 32bit address
> register, the manual suggest the usage of a combination of two
> instructions:
>
> MOVH AREG, ((absolute address+0x8000)>>16) & 0xFFFF and
> LEA AREG, AREG (off16) # off16 = (absolute address & 0xFFFF)
>
> where
> * the absolute address is 32bit,
> * MOVH moves the 16bit value of the second operand to the
> most-significant half-word of AREG and set the least-significant
> 16 bits to zero.
> * LEA computes the effective address with
> effective address = second AREG + sign_ext(off16);
^^^^^^^^
> first AREG = EA[31:0];
>
> The thing I don't understand here is the addition of 0x8000 in
> the MOVH instruction before ****fting. Why is this done? If the
> 16th bit is already 1 in the value of absolute address, then
> the addition would also modify the most-significant 16 bits.
If bit 15 (value 8000) is 1, then sign extension would add
ffff0000, which is subtracting 10000. So you need to add
1 to the value loaded into the top 16 bits to ensure it's
10000 higher.
Just try it with the example 00008000 to see why it works.
Phil
--
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