[Info-vax] yet another sys$qiow question

[Stephen Hoffman]

On 2015-08-21 14:18:09 +0000, Craig A. Berry said:

> On 8/21/15 8:41 AM, Johnny Billquist wrote:
> 
>> What is wrong with checking the IOSB before the I/O completes? There is 
>> absolutely nothing wrong with that. And if it is non-zero, it means the 
>> I/O already have completed, so there is no point in waiting, since it 
>> will return immediately.
> 
> Until the I/O completes, the status of the IOSB is undefined. Show me 
> where there is a guarantee that if one longword of the IOSB has been 
> written, that the other longword has also been written, i.e., that the 
> entire IOSB is written atomically or in a single instruction. Without 
> that guarantee, non-zero does not mean complete.

Aside from the discussion of volatile and variable modifications 
out-of-compiler-scope being discussed else-thread, polling the IOSB is 
a documented and supported approach.

Quoth The Fine Manual: "The process polls the I/O status block. If the 
low-order word still contains zero, the I/O operation has not yet 
completed. In this example, the program loops until the request is 
complete."   
<http://h30266.www3.hp.com/odl/vax/opsys/vmsos73/vmsos73/5841/5841pro_062.html> 




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