Add qemu UEFI firmware support.

README.md

@@ -10,7 +10,7 @@ git clone https://git.zxq5.dev/OsDev/FoundryOS.git
 * latest rust nightly release
 * all necessary rust components installed
 * xorriso: creates ISO images to be booted from.
-* (Optional / Recommended) Qemu: to run the kernel. (this may be packaged as qemu-desktop)
+* (Optional / Recommended) qemu: to run the kernel. (this may be packaged as qemu-desktop)
 * (Optional) GDB: for debugging the kernel.
 
 ```sh
@@ -20,12 +20,29 @@ rustup component add rust-src
 rustup component add llvm-tools-preview
 ```
 
-## Building & Running in Qemu:
+## Building & Running in qemu
 ```sh
 cargo run
 ```
 
-## Running in GDB:
+## Running in GDB
 ```sh
 USE_GDB=1 cargo run
 ```
+
+## Build errors
+
+If you see a qemu error like this:
+
+```
+qemu-system-x86_64: -drive if=pflash,format=raw,readonly=on,file=/home/jacob/Desktop/Code/FoundryOS/build/OVMF_CODE.fd: Could not open '/home/jacob/Desktop/Code/FoundryOS/build/OVMF_CODE.fd': No such file or directory
+```
+
+Simply delete the ./build directory and try to rebuild the program. Using the runner script or `cargo run` will download the required files for you, this is because the script only checks for the presence of one file and not the VARS file.
+
+Alternatively, you may disable using a UEFI firmware with qemu like so:
+```sh
+USE_LEGACY_BIOS=1 cargo run
+```
+
+If you have any other issues, feel free to create an issue or a PR.