>> I believe what it *actually* does is yell "hey, somebody changed this!"
>
>  How does it do that? The bios cannot have sufficient info about the
> graphics card in order to show a message on screen, especially if the
> graphics card is currently in non-vga mode.

I would imagine it goes something like this:

Interrupt 13 handler (file I/O) code inside BIOS:

Check if trying to write to MBR (always on the same place)
If not, write as usual to the drive, return
Change screen mode to 720x480 (or whatever that one is the BIOS uses at 
startup) using an interrupt 10 call
Display some text on the sceeen using the same routines as the BIOS setup 
screen etc
Wait for keyboard input (note this totally hangs the system)
If user agrees, write the data as normal to the disc
If not, just exit the interrupt handler

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On Thu, 13 Mar 2008 09:51:41 +0100, scott wrote:
> I would imagine it goes something like this:
>
> Interrupt 13 handler (file I/O) code inside BIOS:

No modern OS uses 16-bit BIOS interrupts.

I have understood so that the MBR write protection simply
discards any writes addressing the MBR, silently.

-- 
Joel Yliluoma - http://iki.fi/bisqwit/

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>> I would imagine it goes something like this:
>>
>> Interrupt 13 handler (file I/O) code inside BIOS:
>
> No modern OS uses 16-bit BIOS interrupts.

Well yeh I doubt it does, but that's how I understood the BIOS MBR 
write-protect feature to work that I'd seen in the past on my system.

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Jim Henderson <nos### [at] nospamcom> wrote:
> POST = Power On Self Test.  I'm really surprised you don't know that, 
> it's a very common thing for people who work with computers to know. 

  I program computers, I don't build them.

-- 
                                                          - Warp

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scott wrote:
>>> I believe what it *actually* does is yell "hey, somebody changed this!"
>>
>>  How does it do that? The bios cannot have sufficient info about the
>> graphics card in order to show a message on screen, especially if the
>> graphics card is currently in non-vga mode.
> 
> I would imagine it goes something like this:
> 
> Interrupt 13 handler (file I/O) code inside BIOS:
> 
> Check if trying to write to MBR (always on the same place)
> If not, write as usual to the drive, return
> Change screen mode to 720x480 (or whatever that one is the BIOS uses at 
> startup) using an interrupt 10 call

You can't call an interrupt from within an interrupt (at least, you 
couldn't when I was messing with them, but that was 12 years ago... of 
course, I doubt they've changed much since then).

-- 
...Ben Chambers
www.pacificwebguy.com

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scott <sco### [at] laptopcom> wrote:
> Interrupt 13 handler (file I/O) code inside BIOS:

  Any current OS will override the default interrupt handlers created
by BIOS. There's just no way BIOS can catch a write attempt to the disk
by simply having an interrupt handler. If it needs to catch it, there
has to be some other way. It probably nees cooperation from the disk
controller logic or something.

-- 
                                                          - Warp

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Chambers wrote:
> You can't call an interrupt from within an interrupt (at least, you 
> couldn't when I was messing with them, but that was 12 years ago... of 
> course, I doubt they've changed much since then).

I think this is what interrupt priority levels are all about. Depends on 
the CPU of course.

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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On Thu, 13 Mar 2008 09:42:55 -0500, Warp wrote:

> Jim Henderson <nos### [at] nospamcom> wrote:
>> POST = Power On Self Test.  I'm really surprised you don't know that,
>> it's a very common thing for people who work with computers to know.
> 
>   I program computers, I don't build them.

When I was starting out in the computer business, I was programming and 
using just IBM PS/2s, not building them.  But I knew what it was, and 
everyone I knew knew what it was.

It's just so surprising that such a fundamental element of computers is 
something you don't know.  I thought everyone knew what it was - even my 
mother knows what POST stands for, and she is functionally computer-
illiterate....(OK, not really, but you get my point)

Jim

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Jim Henderson <nos### [at] nospamcom> wrote:
> It's just so surprising that such a fundamental element of computers is 
> something you don't know.

  I'm feeling like Andrew like now. ;)

  Of course I know that the bios makes tests (at least by default) when
booting. I just didn't know what "POST" means.

-- 
                                                          - Warp

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Warp wrote:
> Jim Henderson <nos### [at] nospamcom> wrote:
>> It's just so surprising that such a fundamental element of computers is 
>> something you don't know.
> 
>   I'm feeling like Andrew like now. ;)
> 
>   Of course I know that the bios makes tests (at least by default) when
> booting. I just didn't know what "POST" means.

Mmm, humility? >:-)

Nobody knows *everything*, even about a specific topic. So let's all 
just chill, k people? ;-)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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