Syslinux - Sep 2015 - quesiton about the mem=size option

I"m sorry if this is not the correct place for this question but i have not
been able to locate any documentation on the mem=size  option for memdisk

The only thing  i have found is
mem=size           Mark available memory above this point as Reserved.

But i'm not sure if this is accurate and i'd like to understand the
purpose of this option

I am PXE booting a LENOVO  machine which fails unless i add  mem=2048m  and i am
trying to better understand what this is doing so I can better understand why
this works but leaving it out fails.





sincerely,
John Cutler

On Sep 9, 2015 6:21 PM, "john via Syslinux" <syslinux at
zytor.com> wrote:
>
> I"m sorry if this is not the correct place for this question but i
have
not been able to locate any documentation on the mem=size  option for
memdisk
>
> The only thing  i have found is
> mem=size           Mark available memory above this point as Reserved.
Actually this is more a SYSLINUX option that has common use with memdisk.
> But i'm not sure if this is accurate and i'd like to understand the
purpose of this option
>
> I am PXE booting a LENOVO  machine which fails unless i add  mem=2048m
and i am trying to better understand what this is doing so I can better
understand why this works but leaving it out fails.

https://en.m.wikipedia.org/wiki/E820

It sets the highest address usable for loading a kernel and initrd and
adjusts the INT 15h calls so that the memory is reserved.

--Gene

Reply All should keep it on the list.

On Wed, Sep 9, 2015 at 9:40 PM,  <john_cutler at dell.com>
wrote:
> Gene,
>
> Thank you for your response I have a follow up to that.
>
> I have 2 different machines    using the same pxe boot  for both machines
> machine A fails at pxe boot  if i don?t have mem=2048m   while machine b
> will fail if it contains mem=2048m
Not entirely surprising though certainly disappointing.  How it fails
would be nice to know.  Have you considered other values too, perhaps
"mem=1024m"?
> Since both machines have 4+GB of ram  I am trying to understand what this
I'm presuming one has 4GiB and the other has more?  This distinction
might be crucial.
> option is doing and what what it affects so I can understand what i should
> be looking at (bios i[?m guessing ] to understand why one wont work without
> this setting and one wont work with it.
They each use RAM differently.  meminfo.c32 will print the results
from INT 15h AH=88h, INT 15h AX=E801h, and the map from INT 15h
AX=E820h.

com32/lib/syslinux/load_linux.c:bios_boot_linux() line 180 is the code
that deals with the option.  Line 298 is where it's then added to the
memory map.

Details like make, model, BIOS/firmware revision, whether the machine
is (U)EFI-based in CSM-mode versus a traditional BIOS system, exactly
how much RAM and how each fails would probably be useful, especially
if someone else has run into a similar issue.

-- 
-Gene
> From: Gene Cumm [mailto:gene.cumm at gmail.com]
> Sent: Wednesday, September 09, 2015 9:14 PM
> To: Cutler, John
> Cc: For discussion of Syslinux and tftp-hpa
> Subject: Re: [syslinux] quesiton about the mem=size option
>
> On Sep 9, 2015 6:21 PM, "john via Syslinux" <syslinux at
zytor.com> wrote:
>>
>> I"m sorry if this is not the correct place for this question but i
have
>> not been able to locate any documentation on the mem=size  option for
>> memdisk
>>
>> The only thing  i have found is
>> mem=size           Mark available memory above this point as Reserved.
>
> Actually this is more a SYSLINUX option that has common use with memdisk.
>
>> But i'm not sure if this is accurate and i'd like to understand
the
>> purpose of this option
>>
>> I am PXE booting a LENOVO  machine which fails unless i add  mem=2048m
>> and i am trying to better understand what this is doing so I can better
>> understand why this works but leaving it out fails.
>
> https://en.m.wikipedia.org/wiki/E820
>
> It sets the highest address usable for loading a kernel and initrd and
> adjusts the INT 15h calls so that the memory is reserved.