Date   : Sun, 28 Nov 2010 03:22:05
From   : mfirth@... (Michael Firth)
Subject: [BeebSoc] Killing Linux - but how?

On 28/11/2010 01:03, Rick Murray wrote:
> On 27/11/2010 18:34, Michael Firth wrote:
>
>> Why should the use of U-boot be destructive?
>
> Perhaps not the best phrasing... I consider u-boot to be "destructive" 
> as once I have instructed u-boot to TFTP an image from a remote 
> machine, it will keep at it until I tell it otherwise. I *cannot* 
> power-cycle to restore normal video recorder operation. I would need 
> to hook up to the other computer, then with hyperterminal and VNC, 
> talk to the other computer from this computer (!) to reconfig u-boot 
> to use the flash-based OS.
>
>
> It'd be rather less trouble if my eeePC had a serial port. But, alas, 
> it does not.
>
That's the bit that doesn't make sense to me.

The standard U-Boot setup has a "bootcmd" variable that defines what it 
automatically boots, and a "bootdelay" variable that defines how long it 
waits for a keypress before it executes "bootcmd".

Thus the normal test process is to connect up a serial port, press a key 
during the "bootdelay" wait, and you then get a U-Boot prompt at which 
you can run commands.

The standard "tftp" command will load a file from the network to RAM, 
and it can then be executed using "bootm". Unless the file you execute 
actually changes the flash on a power cycle U-Boot should go back to 
running "bootcmd" after the "bootdelay" time, which will load the Linux 
kernel from flash.

Does the Neuros U-Boot not support doing this?

I agree that without a serial port it would be very hard to use U-boot, 
but USB serial adapters are around ?6, so unlikely to break the bank. 
You can also use it for communicating with BBCs too (trying to make this 
not completely OT)

Most sensible systems these days use the standard 9-pin male DTE wired 
D-type as their serial port. It does mean that you always need a 
null-modem cable, but at least it makes things predictable. The original 
theory was that computers should have DTE male ports and peripherals 
(i.e. printers and modems) should have female DCE ports, and that you 
just connect them together with a straight cable. When people started 
wanting to connect computers directly together the null-modem cable was 
the solution. Unfortunately a lot of kit at one point broke the 
standards and either had female ports for DTE equipment (e.g. Sun 
workstations) or male ports for DCE equipment (e.g. some Ethernet 
switches). As a result, a lot of people get very confused about 
connecting serial kit together, but when you use kit that follows the 
rules it becomes pretty simple.

Regards

Michael