Re: [cc65] Why does this work in 128 mode but not 64 mode?

On Fri, Apr 30, 2010 at 1:08 AM, Ullrich von Bassewitz <uz@musoftware.de>wrote:

>
> On Thu, Apr 29, 2010 at 10:52:46PM -0500, Payton Byrd wrote:
> >    After a lot of digging, it appears to be a problem in the libraries.
>
> Actually not. cbm_open is identical for all CBM platforms. It returns what
> the
> kernal returns. If you don't get the expected error, this means that the
> kernal doesn't return it. This is the code for cbm_open:
>
> _cbm_open:
>        pha
>        txa
>        pha                     ; Save name
>
>        jsr     popa            ; Get sec_addr
>        jsr     _cbm_k_setlfs   ; Call SETLFS, pop all args
>
>        pla
>        tax
>        pla                     ; Get name
>        jsr     _cbm_k_setnam
>
>        jsr     _cbm_k_open
>        sta     __oserror
>
>        rts
>
> And here is cbm_k_open:
>
> _cbm_k_open:
>        jsr     OPEN            ; <- KERNAL call
>        ldx     #0              ; Clear high byte
>        bcs     @NotOk
>        txa
> @NotOk: rts
>
> If the carry is set after a call to the kernal, the error code is
> definitely
> stored in _oserror. If _oserror doesn't contain an error code, it is
> because
> the kernal doesn't return it.
>
>
What is the correct way to determine if a drive exists on the IEC bus?  I've
tried simply doing an open using the cbm_open routine but the kernel does
not return an error as expected when attempting to open the drive.  From
BASIC the following happens:

OPEN 15,15,15,""

READY
PRINT #15, "UJ"

?DEVICE NOT PRESENT
READY

So it appears that with BASIC you have to attempt to write to the channel to
get the error, but how does it know about the error if the kernel didn't set
the error in the accumulator when open was called?  When I attempt to read
or write to the channel of a non-existent drive the 64 locks up as is
evidenced by the sample I posted.  Obviously lots of people have overcome
this problem or the C64 would have been useless from the get-go.

-- 
Payton Byrd
<http://www.paytonbyrd.com>
<http://it.toolbox.com/blogs/paytonbyrd>

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