hw.ata.ata_dma="0": can I do this during bootup at the loader prompt?

[Rob]

Arjan Van Leeuwen wrote:
> On Thu, 09 Dec 2004 14:24:35 +0900, Rob <spamrefuse at yahoo.com> wrote:
> 
>>Hi,
>>
>>In /boot/loader.conf, I have
>>   hw.ata.ata_dma="0"
>>to prevent a WRITE_DMA failure crash at bootup.
>>Unfortunately, this forces my UDMA100 harddisk to operate
>>at PIO4 speed.
>>
>>There are patches flying around on this mailing list that might
>>solve the problem. I'm very keen on testing such patches, but I
>>should remove the line in loader.conf. However, if the patch does
>>not work, I end up with an unbootable disk.
>>
>>It would be nice if I can set hw.ata.ata_dma="0" at the loader
>>prompt during bootup, so that the system at least will boot from
>>harddisk. Is that possible?
>>
> 
> 
> I'm not sure if I understand what you want, but you can use eg
> 'atacontrol mode <channel> udma33 udma33' to set your hard drive to
> UDMA-33 after the system has booted.

No, that's no option. The situation is this: I have a harddisk with FreeBSD
5.3, which fails to boot. It would boot if I had hw.ata.ata_dma="0" in
/boot/loader.conf; however, that file is empty, so the bootup crashes
with a WRITE_DMA failure when the kernel loads.

I can get at the bootloader prompt during the boot. What I want to know is:
can I type something here that has the same effect as hw.ata.ata_dma="0" in
/boot/loader.conf? Then I can continue loading the kernel.

Thanks,
Rob.