+
+# Notes on UEFI
+
+Jonasforth runs without an underlying operating system, instead using the
+facilities provided by UEFI by running as a UEFI application. This section
+contains some notes about how this functionality is implemented.
+
+I also wrote an entire tutorial that descirbes how to write and compile a
+"Hello, World!" UEFI application, including how to run it on real hardware,
+which you can find here: [Getting started with bare-metal
+assembly](https://johv.dk/blog/bare-metal-assembly-tutorial.html).
+
+## Packaging and testing the image
+
+UEFI expects a UEFI application to be stored in a FAT32 file system on a
+GPT-partitioned disk.
+
+Luckily, QEMU has a convenient way of making a subdirectory availabe as a
+FAT-formatted disk (see [the relevant section in the QEMU User
+Documentation](https://qemu.weilnetz.de/doc/qemu-doc.html#disk_005fimages_005ffat_005fimages)
+for more information):
+
+ $ qemu-sytem-x86_64 ... -hda fat:/some/directory
+
+We use this to easily test the image in QEMU; see the Makefile for more
+information, or just run the `qemu` target to run the program inside of QEMU
+(of course, you must have QEMU installed for this to work):
+
+ $ make qemu
+
+## Interfacing with UEFI
+
+From [OSDev Wiki](https://wiki.osdev.org/UEFI#How_to_use_UEFI):
+
+> Traditional operating systems like Windows and Linux have an existing software
+> architecture and a large code base to perform system configuration and device
+> discovery. With their sophisticated layers of abstraction they don't directly
+> benefit from UEFI. As a result, their UEFI bootloaders do little but prepare
+> the environment for them to run.
+>
+> An independent developer may find more value in using UEFI to write
+> feature-full UEFI applications, rather than viewing UEFI as a temporary
+> start-up environment to be jettisoned during the boot process. Unlike legacy
+> bootloaders, which typically interact with BIOS only enough to bring up the OS,
+> a UEFI application can implement sophisticated behavior with the help of UEFI.
+> In other words, an independent developer shouldn't be in a rush to leave
+> "UEFI-land".
+
+For `JONASFORTH`, I have decided to run as a UEFI application, taking advantage
+of UEFI's features, including its text I/O features and general graphical device
+drivers. Eventually, we would like to add some basic graphical drawing
+capabilities to `JONASFORTH`, and it's my impression that this would be possible
+using what is provided to us by UEFI.
+
+A UEFI images is basically a windows EXE without symbol tables. There are three
+types of UEFI images; we use the EFI application, which has subsystem `10`. It
+is an x68-64 image, which has value `0x8664`.
+
+UEFI applications use [Microsoft's 64-bit calling
+convention](https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention)
+for x68-64 functions. See the linked article for a full description. Here is
+the short version:
+
+- Integer or pointer arguments are given in RCX, RDX, R8 and R9.
+- Additional arguments are pushed onto the stack from right to left.
+- Integer or pointer values are returned in RAX.
+- An integer-sized struct is passed directly; non-integer-sized structs are passed as pointers.
+- The caller must allocate 32 bytes of "shadow space" on the stack immediately
+ before calling the function, regardless of the number of parameters used, and
+ the caller is responsible for popping the stack afterwards.
+- The following registers are volatile (caller-saved): RAX, RCX, RDX, R8, R9, R10, R11
+- The following registers are nonvolatile (callee-saved): RBX, RBP, RDI, RSI, RSP, R12, R13, R14, R15
+
+When the application is loaded, RCX contains a firmware allocated `EFI_HANDLE`
+for the UEFI image, RDX contains a `EFI_SYSTEM_TABLE*` pointer to the EFI system
+table and RSP contains the return address. For more infromation about how a UEFI
+application is entered, see "4 - EFI System Table" in [the latest UEFI
+specification as of March 2020 (PDF)](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf).
+
+**Sources:**
+
+- [UEFI applications in detail - OSDev Wiki](https://wiki.osdev.org/UEFI#UEFI_applications_in_detail)
+- [Microsoft x64 calling convention](https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention)
+- [UEFI Specifications](https://uefi.org/specifications)
+
+### UEFI with FASM
+
+We might want to consider using something like this: https://wiki.osdev.org/Uefi.inc)
+
+FASM can generate UEFI application binaries by default. Use the following
+template to output a 64-bit UEFI application:
+
+ format pe64 dll efi
+ entry main
+
+ section '.text' code executable readable
+
+ main:
+ ;; ...
+ ret
+
+ section '.data' data readable writable
+
+ ;; ...
+
+Use `objdump -x` to inspect the assembled application binary.
+
+### UEFI documentation
+
+- [Latest specification as of March 2020 (PDF)](https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_A_Feb14.pdf)
+
+Notable sections:
+
+- 2\. Overview (14)
+- 4\. EFI System Table (89)
+- 7\. Services - Boot Services (140)
+- 8\. Services - Runtime Services (228)
+- 12\. Protocols - Console Support (429)
+- 13\. Protocols - Media Access (493)
+- Appendix B - Console (2201)
+- Appendix D - Status Codes (2211)
+
+## Resources
+
+- [UEFI - OSDev Wiki](https://wiki.osdev.org/UEFI)
+- [Unified Extensible Firmware Interface (Wikipedia)](https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface)
+- [UEFI Specifications](https://uefi.org/specifications)