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I want to write system in FPC i have read Pascal Barebones, but at the end i have kernel.obj file.
How to have executable file ????

By following the instructions.

the last instruction "--gc-sections -s, in combination with -CX -XXs above, eliminates RTTI symbols from resulting binary" doesn't work.

The last sentence is not an instruction, but an explanation. I assume that you mean that ld --gc-sections -s -Tlinker.script -o kernel.obj stub.o kernel.o multiboot.o system.o console.o doesn't work. Then you need to make the appropriate changes to make that line work.

(See how it works? You ask a question without useful information, you get an answer without useful information.)

I have kernel.obj made from stub.o kernel.o multiboot.o system.o and console.o
I want to make executable file ( I mean *.elf) How to do this ?

There is a command line given that I even quoted. Apparently this command line has worked for at least one person. If it doesn't work for you, then something in your setup is different. There's probably a million ways to mess something up, do you expect me to guess the right one? (Sorry, I don't have my crystal ball handy right now)

Posting at least the error messages you get would be a start.