Hello UEFI!

.gitignore

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+build/
+vdir/
+.vscode/

Makefile

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+MINGW_GCC = x86_64-w64-mingw32-gcc
+QEMU = qemu-system-x86_64
+MINGW_GCC_FLAGS = -Wall -Wextra -e efi_main -nostdinc -nostdlib  -fno-builtin -Wl,--subsystem,10
+
+all: 
+	mkdir -p build
+	x86_64-w64-mingw32-gcc $(MINGW_GCC_FLAGS) -o build/BOOTX64.EFI boot.c
+
+vdir: all
+	mkdir -p vdir
+	mkdir -p vdir/EFI/BOOT
+	cp build/BOOTX64.EFI vdir/EFI/BOOT
+
+run: vdir
+	$(QEMU) -bios /usr/share/ovmf/OVMF.fd -net none -drive file=fat:rw:vdir,index=0,format=vvfat -serial stdio
+
+clean:
+	rm -rf build vdir

boot.c

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+#define ull unsigned long long
+
+struct EFI_SYSTEM_TABLE {
+    char _buf[60];
+    struct EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL {
+        ull _buf;
+        ull (*OutputString)(
+            struct EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL *This,
+            unsigned short *String);
+        ull _buf2[4];
+        ull (*ClearScreen)(
+             struct EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL *This);
+    } *ConOut;
+};
+  
+void efi_main(void *ImageHandle __attribute__ ((unused)), struct EFI_SYSTEM_TABLE *SystemTable)
+{
+    SystemTable->ConOut->ClearScreen(SystemTable->ConOut);
+    SystemTable->ConOut->OutputString(SystemTable->ConOut, L"Hello UEFI!\n");
+    while (1);
+}
+