Whats this line mean? (system:call getfreediskex) - nsis

Question

System::Call '${sysGetDiskFreeSpaceEx}(r0,.,,.r1)'
If i'm right, r0: directoryname, free bytes, number of bytes, number oof free bytes, but what actually means the r0,.,.,r1?
Thx for the help!

Answer 1

${...} is a define so when you want to know how some things work then the first thing you should do is to find out what the define does: !error "${sysGetDiskFreeSpaceEx}" will print !error: kernel32::GetDiskFreeSpaceEx(t, *l, *l, *l) i
In the System readme you will find this nugget:
PARAMS, RETURN and OPTIONS can be repeated many times in one Get/Call
line. When repeating, a lot can be omitted, and only what you wish to
change can be used. Type, source and/or destination can be omitted for
each parameter, even the return value. Options can be added or
removed. This allows you to define function prototypes and save on
some typing.
So ${sysGetDiskFreeSpaceEx} is a prototype that specifies the parameter count and types but it does not specify parameter source and destination.
What is the parameter syntax?
The parameters list is separated by commas. Each parameter is combined
of three values: type, source and destination. Type can be an integer,
a string, etc. Source, which is the source of the parameter value, can
be a NSIS register ($0, $1, $INSTDIR), the NSIS stack, a concrete
value (5, "test", etc.) or nothing (null). Destination, which is the
destination of the parameter value after the call returns, can be a
NSIS register, the NSIS stack or nothing which means no output is
required. Either one of source or destination can also be a dot (`.')
if it is not needed.
We can now expand the entire call !error 'System::Call "${sysGetDiskFreeSpaceEx}(r0,.,,.r1)"' and this gives us !error: System::Call 'kernel32::GetDiskFreeSpaceEx(t, *l, *l, *l) i(r0,.,,.r1)'
If we merge the repeated parameter definitions we get kernel32::GetDiskFreeSpaceEx(tr0, *l., *l, *l.r1)i.
So parameter 1 is a string (LPTSTR on MSDN) with the source r0 (NSIS register $0).
Parameter 2 and 3 have no source and no destination, only parameter 2 uses a . (dot) but the end result is the same; no input and no output. The only important part here is *l so the system plugin knows how large the parameter is.
The final parameter is a pointer (*) to a 64 bit number (l) with no input (.) and we request the output to be stored in $1 (r1).
The system plugin calls the native Windows API so it is often useful to look at MSDN to see what it has to say about the function you are calling.

Answer 2

Given that $0..$9 and $R0..$R9 are NSIS registers, the notation r0 -> r9 (resp. R10..R19 or also R0..R9) is used by the System plugin to specify the $0..$9 (resp. $R0..$R9) registers as a source and / or a destination with system API or other dll function calls.
Either one of source or destination can also be a dot (.) if it is not needed.
Look for the "Calling functions" and "Available sources and destinations" sections in the system plugin documentation.