Update README

README.md

@@ -1,2 +1,78 @@
-# grub-mod-setup_var
-A modded grub allowing tweaking hidden BIOS settings.
+# A modified grub allowing tweaking hidden BIOS settings.
+based on grub with [setup_var patch (invalid link now)](http://luna.vmars.tuwien.ac.at/~froemel/insydeh2o_efi/grub2-add-setup_var-cmd.patch) and [setup_var2 patch](https://habr.com/post/190354/) with setup\_var\_3 patch as a wordaround to duplicate Setup vairable.
+
+As said in a [guide](https://github.com/acidanthera/AptioFixPkg#verifymsre2) about changing hidden "CFG Lock" BIOS setting, by using a modified GRUB shell, we can change any hidden UEFI BIOS settings. But here comes some errors on my Dell XPS 8930, so I have the shell patched and added a new command `setup_var_3` for this situation.
+
+## The problem
+On my PC, when I try to read/write values using the method above. It obtained a Setup variable but producing an error saying the offset is out of range. With extra investigation, I found there are two Setup variables in BIOS and the error comes from the very small (only 9 bytes in my situation) Setup variable.
+
+With `setup_var` (without parameters), it said:
+
+```
+var name: Setup, var size: 12, var guid: 80e1202e-2697-4264 - ...
+
+...
+
+var name: Setup, var size: 12, var guid: ec87d643-eba4-4bb5 - ...
+```
+
+So there are two Setup variables, and if using `setup_var 0xC2` (the offset could be any value larger than first Setup variable size):
+
+```
+var name: Setup, var size: 12, var guid: 80e1202e-2697-4264 - ...
+
+...
+
+successfully obtained "Setup" variable from VSS (got 9 (0x9) bytes).
+
+error: offset is out of range
+
+```
+
+Here comes the error, but if using a smaller offset (smaller than first variable's size):
+
+```
+var name: Setup, var size: 12, var guid: 80e1202e-2697-4264 - ...
+
+...
+
+successfully obtained "Setup" variable from VSS (got 9 (0x9) bytes).
+offset 0x01 is: 0x15
+
+var name: Setup, var size: 12, var guid: ec87d643-eba4-4bb5 - ...
+
+...
+
+successfully obtained "Setup" variable from VSS (got 4304 (0x10d0) bytes).
+
+offset 0x01 is: 0x00
+```
+
+So without the error, we can get correct value from the "real", second "Setup" variable. This is what the patch comes for.
+
+## The patch
+This patch on two patches above adds the third `setup_var` command: `setup_var_3`. It supresses the error when the Setup variable is too small and doesn't look like a real Setup variable to avoid error terminating the program resulting in cannot access real Setup variable with large offset.
+
+USE WITH CAUTION AND ENSURE YOU HAVE EXAMINED YOU ARE ACCESSING RIGHT SETUP VARIABLE OR YOU WILL RISK BRICKING YOUR COMPUTER!!!
+
+## Build Notes
+As this is based on a relatively old grub version (2.00+20130519), I used Ubuntu 16.04 LTS with gcc 4.8 to compile it. (I haven't tested gcc 4.9 but gcc 5 won't work).
+
+Build:
+
+```
+cd grub-2.00+20130519
+./autogen.sh
+./configure --with-platform=efi --prefix=<temporary install prefix>
+make
+make install
+
+```
+
+Generating modified GRUB shell:
+
+```
+cd <temporary install prefix>
+./bin/mkstandalone -O x86_64-efi -o modGRUBShell.efi
+
+```