[syslinux] Reports from 5.00

[Gert Hulselmans]

2013/1/16 H. Peter Anvin <hpa at zytor.com>:
> On 01/16/2013 04:30 AM, Matt Fleming wrote:
>>
>>
>>> E_ Under FreeDOS, SYSLINUX.COM doesn't seem to install ldlinux.*
>>> (specially when used with "--directory"), and there are no ([error])
>>> messages; not even when executed with no parameters, or with
>>> "--help", or with "--version". The messages disappeared _after_
>>> 5.00-pre7.
>>
>>
>> Right, as Peter pointed out on IRC, it seems unreasonable to expect
>> 5.00's 496K syslinux.com to be loaded under real-mode DOS (-pre7 is when
>> we started installing ldlinux.c32 automatically instead of requiring the
>> user to manually copy it).
>>
>> At this point I'm leaning towards marking real-mode DOS as Not Supported
>> from Syslinux-5.00 onwards since 5.x's execution revolves around
>> ldlinux.c32, which at 428K, is unlikely to ever execute in 1MB of RAM.
>> Even if we compress ldlinux.c32, it has to be uncompresed in RAM at some
>> point so that we can execute it, unless we only uncompressed chunks of
>> ldlinux.c32 as and when it was needed. But partial decompression would
>> be a major project.
>>
>
> There is actually two other ways to do it which would be easier, however, it
> would really help if someone who cares about DOS could do the work.
>
> Alternative #1:
>
> DOS allows the filename of the running executable to be discovered by
> looking at the environment pointer in the PSP; the environment block is a
> series of null-terminated strings followed by a null string (similar to Unix
> systems); *after* the null string is the full pathname of the executable
> itself as a full pathname; this is what MS-DOS C compilers provide as
> argv[0].
>
> One can then open the file and seek to an offset from the end of the file,
> alternatively (probably more robust) read the header and look at the
> _exe_edata_low and _exe_edata_blocks fields to find the end of the EXE data
> proper and then seek to that offset.
>
> At that point, the data beyond the end of the file is simply handled like a
> normal file under DOS, which one can read with the normal read system call
> and write out to the target file.  It also means this data is *not*
> compressed by UPX; until we compress modules this will create a large
> installer.
>
> Alternative #2:
>
> Use DJGPP as the compiler and compile the DOS installer as an "extended DOS"
> (32-bit executable.)
>
> The downside there is additional tools dependencies and the need for a DPMI
> provider (such as cwsdpmi.exe) in order to run.
>
>         -hpa
>
> --
> H. Peter Anvin, Intel Open Source Technology Center
> I work for Intel.  I don't speak on their behalf.
>

The size of the DOS (and other installers) can be reduced a lot by
stripping the Syslinux binaries:
$ ll ldlinux.c32
-rwxrwxr-x. 1 gert gert 428468 Jan 16 13:25 ldlinux.c32
$ strip ldlinux.c32
$ ll ldlinux.c32
-rwxrwxr-x. 1 gert gert 115728 Jan 16 21:19 ldlinux.c32

- Gert