Re: [Qemu-devel] Re: fdc: support NEC PC-9821 family

[武田 俊也]

Dear Stuart and members,

>On Fri, Sep 11, 2009 at 08:36:02PM +0900, 武田 俊也 wrote:
>> This is new patch.
>
>Nice work -- looks a lot closer! :)
>
>I've a few questions, though.
>
>I notice that the 82078 has 'PC AT', 'PS/2' and 'PS/2 Model 30' modes.
>QEMU doesn't seem to implement this, but I don't know common each of the
>different modes is/was.
>
>Also, I notice that in fdctrl_write_dor(), the DR0 bit (in PS/2 mode)
>is set if drives 0 or 2 are selected, which looks like a bug -- I would
>expect that it should only be set if drive 0 is selected.
>
>AFAIK, in most IBM PC 'clones' (but not quite all), the FDC can support
>four drives, but only up to one/two drives can be physically attached.
>
>It seems that MAX_FD affects emulation of the FDC itself, and doesn't
>just determine the number of drives that may be connected.  Surely that
>can't be right?  I.e. if MAX_FD == 2, any attempt to select drive 2
>results in drive 0 being selected, which should not be the case...
>
>I'm not sure why QEMU limits certain machines to two drives...  Maybe
>the reason is that software for systems where only two drives can be
>physically connected might react badly to seeing unexpected drives.
>Is this the case?  TBH, I'm not sure whether I *really* need more than
>a single drive... :)
>
>Also, it seems to me that a floppy controller might have either one,
>two, three, four or no drives attached.  (I assume that software can
>distinguish between a missing drive and a drive with no disk inserted.)
>In that case, it seems to me that QEMU cannot provide emulation of a
>system with no drives connected to its FDC.
>
>Cheers,
>-- 
>Stuart Brady

I agree.

In PC-98x1 family case (uPD765A is used for fdc), bios checks which drive
the disket is inserted to, by the commands below:
        recalib to drive0
        recalib to drive1
        recalib to drive2
        recalib to drive3
        sence interrupt stat
Yes, if MAX_FD = 2, it is recognized the same drive is connected to
drive0 and drive2, drive1 and drive3.
So I need to change MAX_FD to 4 for PC-98x1 family.

I think we need to divide MAX_FD for 2 meanings,
the number of drives that floppy drive controller can control, is always 4,
and the number of drives that each machine has, is 2 or 4.
For example, MAX_FD used in FD_DOR_SELMASK definition is the former.
MAX_FD in save/load function is the latter.

(Sorry I don't enough about 82078, so I cannot comment about PC/AT and
other mode.)

Thanks,
TAKEDA, toshiya