Re: IO delay, port 0x80, and BIOS POST codes

[John Heil]

On Thu, 14 Mar 2002, Pedro M. Rodrigues wrote:

> Date: Thu, 14 Mar 2002 22:19:23 +0100
> From: Pedro M. Rodrigues <[e-mail]>
> To: John Heil <[e-mail]>, [e-mail]
> Cc: Linus Torvalds <[e-mail]>, [e-mail],
>      Martin Wilck <[e-mail]>
> Subject: Re: IO delay, port 0x80, and BIOS POST codes
>
>
>    This piece of code is taken from an old Minix source code tree, the file being
> boothead.s . Notice the port 0xED usage and the comment.
>
>
> ! Enable (ah = 0xDF) or disable (ah = 0xDD) the A20 address line.
> gate_A20:
>         call    kb_wait
>         movb    al, #0xD1       ! Tell keyboard that a command is coming
>         outb    0x64
>         call    kb_wait
>         movb    al, ah          ! Enable or disable code
>         outb    0x60
>         call    kb_wait
>
>
>         mov     ax, #25         ! 25 microsec delay for slow keyboard chip
> 0:      out     0xED            ! Write to an unused port (1us)
>         dec     ax
>         jne     0b
>
>         ret
> kb_wait:
>         inb     0x64
>         testb   al, #0x02       ! Keyboard input buffer full?
>         jnz     kb_wait         ! If so, wait
>         ret
>
>
>
> /Pedro
>
> On 14 Mar 2002 at 16:03, Richard B. Johnson wrote:
>
> >
> >
> > Well I can see why he's an EX-Phoenix BIOS developer. A port at 0xed
> > does not exist on any standard or known non-standard Intel/PC/AT
> > compatible.
> >
> > Remember DOS debug?
> >
> > C:\>debug
> >
> > -i ed
> > FF
> > -o ed aa
> > -i ed
> > FF
> > -o ed 55
> > -i ed
> > FF
> > -q
> >
> >
> > This is not a DOS emulation. This is a real-mode boot where any ports
> > will be visible. If you used it with success, it means that you didn't
> > need the I/O delay of writing to a real port. Instead you got the few
> > hundred nanoseconds of delay you get by writing to nowhere.
> >
> > Cheers,
> > Dick Johnson
>
>

We did not want the I/O delay based on the port itself.
We specifically wanted an unused port, and avoid the 0x80 conflict.

Perhaps this should be a kernel hacking/debug option, due to the
difference in environments and needs.

Johnh

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John Heil
South Coast Software
Custom systems software for UNIX and IBM MVS mainframes
1-714-774-6952
[e-mail]
http://www.sc-software.com
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