update freestanding study

free/.gitignore

@@ -0,0 +1,2 @@
+free
+*.o

free/build.sh

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+#!/bin/sh
+
+g++ -m32 -c -o free.o free.cpp -ffreestanding && ld -m elf_i386 -o free free.o

free/free.cpp

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+extern "C" void _start() {
+    asm volatile (
+        "mov $1, %eax\n" 
+        "mov $0, %ebx\n" 
+        "int $0x80" 
+    );
+}

free/readme.md

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+# freestanding
+
+```bash
+g++ -c -o free.o free.cpp
+ld -o free free.o
+
+objdump -x free
+
+# g++ -c -nostdlib -o free free.cpp
+```