Re: more serial woe

From: Christopher Phillips (
Date: 2004-08-25 16:09:22

On 25 Aug 2004, at 12:03, Spiro Trikaliotis wrote:
> Hello,
>> Next question - when working the job queue, is there any point doing a
>> SEEK ($B0) before a READ ($80)?
> Yes, there is a point in doing a seek before a read.
> <good explanation of action of annotated drive code>...
> So, we see the big difference: While a seek is executed nevertheless if
> we want to read, write, or do something else, there is a little bit
> functionality that is only inherent in a seek: It gets the ID of the
> disc and stores it into the "expected" ID (F410-F416). If this ID does
> not match for every other command (tested in F3F2-F402), then we get a
> 29, DISK ID MISMATCH error code.
> So, if there already was some activity on the disc and the disc was not
> changed in between, then a seek is not needed as it is executed
> nevertheless if needed. Anyway, if the disc might have changed or there
> was no activity before, then you have to do a seek.

Ah, thankyou.  So basically, if I am mid file-read there is not a lot 
of point (and it may even be bad!), as seek would just overwrite the ID 
of the disk I expect to be there with the ID of a replacement disk, and 
not let me know that the user has switched on me ;)  OTOH, if I am 
reading directory blocks or preparing for a file read, it is probably a 
good idea.

> HTH,
>    Spiro.

it does!


(ps - I'm replying to the copy of this message that Spiro forwarded to 
me, as I've yet to receive it from the list, so don't be surprised if 
you see this reply before the message it responds to ;)

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