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Developer notes

  • big fluffy goal: boot and run FreeDOS[1] on a UEFI[2] PC

Relevant standards (and "standards")

  • the hope is that biefircate can implement at least the BIOS interrupts and parts of the BIOS data area described in RBIL[3]

    • intrs. 0x00—0x1f (incl. 0x08—0x0f for IRQs 0—7)

    • intrs. 0x70—0x77 for IRQs 8—15

    • the BIOS data area 0x40:0—0x50:0

    • maybe also intrs. 0x40 (relocated diskette I/O) and 0x42 (relocated video output services)

    • no need for biefircate to implement MS-DOS intrs. (0x20—0x33 etc.)

  • biefircate should also properly initialize any option ROMs for PCI devices — PCI firmware spec.[4] says how

  • we assume the PC firmware

    • implements UEFI[2]

    • has some PCI support

    • has a legacy-style PCI option ROM for the VGA device

    • has some ACPI tables[5] which report what features the underlying system has

      • biefircate will likely not support ACPI in its full generality; the spec. is extremely complex

  • some PCs — but not all — allow bootloaders to access the firmware volume protocol;[6] this can be used to discover some option ROMs

Code structure (?)

  • stage 1 is for stuff that happens before exiting UEFI boot services; stage 2 is for stuff after that

    • other than the above, there are (currently) no hard and fast rules for delineating the two

    • stage 1 passes a pointer to a linked list of boot parameters (see bparm.h) to stage 2

  • stage 2

    • currently takes the form of an ELF executable[7]

      • 32-bit, but can access 64-bit addr. space via PAE paging[8]

    • calling convention used is -mregparm=3 -mrtd

      • when calling non-variadic function: first few arguments go in eax, edx, ecx; callee pops any stack arguments

      • callee must preserve ebx (!), esi, edi, ebp

Other notes

  • UEFI stuff

    • note: boot services will be unavailable once bootloader calls BS→ExitBootServices(, )

      • might be possible to implement a BIOS without exiting boot services, but this is non-trivial — UEFI expects to be in control of most system resources, incl. memory, page tables, and IRQs

  • how to correctly interface with PC hardware legacy-style

    • IBM’s published BIOS source code, e.g. PC AT BIOS listings from 1986,[9] might be useful

    • also potentially useful is Eyal Abraham’s new-xt-bios,[10] a rewrite of the BIOS for an (older) PC XT with partially custom hardware; code is under an MIT License

  • Xv6 will panic if it cannot find MP tables per Intel’s (deprecated) MultiProcessor Specification[11]

    • biefircate produces dummy MP tables if XV6_COMPAT is defined

    • other OSes probably do not need MP tables to work

1. https://www.freedos.org/. Retrieved on 15 August 2021.
2. UEFI Forum. Unified Extensible Firmware Interface (UEFI) Specification: Version 2.9. March 2021. https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf.
3. Brown, Ralf. The x86 Interrupt List. 16 July 2000. http://www.cs.cmu.edu/~ralf/files.html.
4. PCI-SIG. PCI Firmware Specification: Revision 3.0. June 2005.
5. UEFI Forum. Advanced Configuration and Power Interface (ACPI) Specification: Version 6.4. January 2021. https://uefi.org/sites/default/files/resources/ACPI_Spec_6_4_Jan22.pdf.
6. UEFI Forum. UEFI Platform Initialization (PI) Specification: Version 1.7 Errata A. April 2020. https://uefi.org/sites/default/files/resources/PI_Spec_1_7_A_final_May1.pdf.
7. TIS Committee. Tool Interface Standard (TIS) Executable and Linking Format (ELF) Specification: Version 1.2. May 1995. http://refspecs.linuxbase.org/elf/elf.pdf.
8. Intel Corporation. Intel® 64 and IA-32 Architectures Software Developer’s Manual: Volume 3A, pp. 2-17, 4-14—4-19. June 2021.
9. International Business Machines Corporation. IBM: Personal Computer Hardware Reference Library: Technical Reference 6183355, pp. 5-14 et seq. March 1986. https://archive.org/details/bitsavers_ibmpcat618ferenceMar86_25829277/page/n245/mode/2up.
10. https://github.com/eyalabraham/new-xt-bios. Retrieved on 28 August 2021.
11. Intel Corporation. MultiProcessor Specification: Version 1.4. May 1997.