[Info-vax] Volatile, was: Re: yet another sys$qiow question

[Johnny Billquist]

On 2015-08-21 15:17, "VAXman- "@SendSpamHere.ORG wrote:
> In article <mr753o$1jg$3 at Iltempo.Update.UU.SE>, Johnny Billquist <bqt at softjar.se> writes:
>> On 2015-08-20 22:57, "VAXman- "@SendSpamHere.ORG wrote:
>>> In article <mr5au9$d3s$1 at Iltempo.Update.UU.SE>, Johnny Billquist <bqt at softjar.se> writes:
>>>> On 2015-08-20 21:41, John Reagan wrote:
>>>>> On Thursday, August 20, 2015 at 3:15:38 PM UTC-4, JF Mezei wrote:
>>>>>> On 15-08-20 12:12, Simon Clubley wrote:
>>>>>>
>>>>>>> Will yes, that's how all compilers work because sys$qiow doesn't return
>>>>>>> control back to the program until it's completed so it's just like a
>>>>>>> normal subroutine call.
>>>>>>
>>>>>> The compiler doesn't know that. The compiler would/should treat both QIO
>>>>>> and QIOW as possibly modifying a value during their execution. Even if
>>>>>> QIO only modifies it later, that variable should be marked as "possibly
>>>>>> modified" by the compiler for the next time you access it, whether the
>>>>>> instriuction right after a QIOW or 10 instructions after QIO. Right ?
>>>>>>
>>>>>
>>>>> Well, yes, the FIRST access after $QIO will be a memory read.  However, since the asynchronous update may occur later, the compiler won't re-read the memory.
>>>>>
>>>>> For something like
>>>>>
>>>>> while (iosb == 0) do;
>>>>>
>>>>> It is not a loop of memory fetches (well, it might be at /NOOPT).  The optimizer will quickly turn that into:
>>>>>
>>>>> tmp = iosb;
>>>>> while (tmp == 0) do;
>>>>
>>>> I think an even better optimization would be:
>>>>
>>>> if (iosb == 0) while (1) {};
>>>>
>>>> And of course, when this ends up in assembler, it would actually be
>>>> (excuse my MACRO-11):
>>>>
>>>> 	TST	IOSB
>>>> 	BNE	10$
>>>> 20$:	BR	20$
>>>> 10$:
>>>>
>>>> I'm sure you can translate that to Macro-32 easily. :-)
>>>
>>> How did you define IOSB in the C?
>>
>> The assumption was that the IOSB was not declared as volatile. With the
>> above examples, iosb would have to be something int-like.
>>
>> 	Johnny
>>
>
> int-like... then,...
>
> 	TSTL	IOSB
> 	BNEQ	10$
> 20$:	BRB	20$
> 10$:

Yeah, I suspected it should look like that, it's just that I have not 
programmed in Macro-32 in many years, and instead of feeling foolish for 
doing something stupid with some mnemonic, I stayed with what I know 
even in my sleep, and expect you to rescue me. :-)

(There are just so many variants of the instructions on the VAX...)

	Johnny