I'm trying to write a basic bootloader for my laptop, which uses UEFI. I've got the code working when I boot it from a usb, but it's just a pain to restart my computer every time I want to run my code. There seems to be loads of BIOS emulators, but I can't find any UEFI emulators for any OS.
I'm using nasm, and have an x86 processor.
Here's my code if you want to look at it
BITS 64
org 0x00100000 ;Space for a small stack?
;**************
;*** HEADER ***
;**************
section .header
DOS_HEADER:
dw 0x5a4d ;DOS Magic number
times 29 dw 0 ;Zeroes
dd 0x00000080 ;Address of PE header
DOS_STUB:
; I don't know how to write this as text, so you get zeroes
times 32 dw 0
PE_HEADER:
dd 0x00004550 ;PE Magic number
dw 0x8664 ;Building for x86 architecture
dw 2 ;Two sections (.text, .data)
dd 0x5ed4b58 ;number of seconds between 00:00:00 1st January 1970 and 00:00:00 1st July 2021
dd 0x0 ;No symbol table
dd 0x0 ;No symbols in the non-existent symbol table!
dw oSize ;The size of the entire optional header. See the OPTIONAL_HEADER_END label for the calculation.
dw 0x1002 ;Is a valid image file, is a system file. No other fancy characteristics.
oSize equ OPTIONAL_HEADER_END - OPTIONAL_HEADER_STANDARD_FIELDS
OPTIONAL_HEADER_STANDARD_FIELDS: ;Not actually optional
dw 0x020b ;PE32+ Executable. I want my 64-bit registers!
dw 0x0 ;What linker?
dd 1024 ;The size of the code segment
dd 1024 ;The size of the data segment
dd 0x0 ;No .bss section. All variables to be initialised.
dd 1024 ;The program's entry point
dd 1024 ;The program's first instruction. Same as the start of the code execution. Duh.
OPTIONAL_HEADER_WINDOWS_FIELDS: ;This is required for UEFI applications too. Trust me, plenty of debugging went into that discovery.
dq 0x00100000 ;The entry point of the image
dd 0x1024 ;The section alignment
dd 0x1024 ;The file alignment
dw 0x0 ;No operating system requirements
dw 0x0 ;Stil no operating system requirements
dw 0x0 ;Major image version number
dw 0x1 ;Minor image version number
dw 0x0 ;Major subsystem version. Doesn't matter, as long as it supports UEFI.
dw 0x0 ;Minor subsystem version. Doesn't matter, as long as it supports UEFI.
dd 0x0 ;A dedicated zero
dd 3072 ;Image size
dd 1024 ;Header size
dd 0x0 ;Checksum //TODO ADD LATER
dw 0x000A ;UEFI Subsystem number.
dw 0x0 ;Not a DLL, so this can be zero
;Using PE32+ file type, so the following are dqs, not dds
dq 0x8000 ;Amount of stack space to reserve
dq 0x8000 ;Amount of stack space to commit immediately
dq 0x8000 ;Amount of local heap space to reserve
dq 0x0 ;Amount of local heap space to commit immediately. Hopefully not needed.
dd 0x0 ;Another four bytes dedicated to being zeroes
dd 0x0 ;Number of data dictionary entries
;OPTIONAL_HEADER_DATA_DIRECTORIES: ;We don't have any special sections, so we don't need this header!
OPTIONAL_HEADER_END: ;This label is required for calculating value of oSize
SECTION_TABLE: ;as if you don't have enough information already :\
.1: ;text section
dq `.text` ;The name of the text section
dd 1024 ;virtual size.
dd 1024 ;virtual entry point address.
dd 1024 ;actual size.
dd 1024 ;actual entry point address.
dd 0 ;No relocations
dd 0 ;No line numbers
dw 0 ;No relocations
dw 0 ;No line numbers
dd 0x60000020 ;Contains executable code, can be executed as code, can be read.
.2: ;data section
dq `.data` ;The name of the data section
dd 1024 ;virtual size.
dd 2048 ;virtual entry point address.
dd 1024 ;actual size.
dd 2048 ;actual entry point address.
dd 0 ;No relocations
dd 0 ;No line numbers
dw 0 ;No relocations
dw 0 ;No line numbers
dd 0xc0000040 ;Contains initialised data, can be read, can be written to.
times 1024 - ($-$$) db 0 ;alignment
;*****************
;*** MAIN CODE ***
;*****************
section .text follows=.header
vars:
;Function return codes.
EFI_SUCCESS equ 0
;Offsets for loading function addresses
OFFSET_TABLE_BOOT_SERVICES equ 96
OFFSET_TABLE_ERROR_CONSOLE equ 80
OFFSET_TABLE_OUTPUT_CONSOLE equ 64
OFFSET_TABLE_RUNTIME_SERVICES equ 88
OFFSET_BOOT_EXIT_PROGRAM equ 216
OFFSET_BOOT_STALL equ 248
OFFSET_CONSOLE_OUTPUT_STRING equ 8
start:
sub rsp, 6*8+8 ; Copied from Charles AP's implementation, fix stack alignment issue (Thanks Charles AP!)
;Start moving handoff variables.
mov [EFI_HANDLE], rcx
mov [EFI_SYSTEM_TABLE], rdx
mov [EFI_RETURN], rsp
;Set up necessary boot services functions
add rdx, OFFSET_TABLE_BOOT_SERVICES ;get boot services table
mov rcx, [rdx]
mov [BOOT_SERVICES], rcx
add rcx, OFFSET_BOOT_EXIT_PROGRAM ;get exit function from boot services table
mov rdx, [rcx]
mov [BOOT_SERVICES_EXIT], rdx
mov rcx, [BOOT_SERVICES]
add rcx, OFFSET_BOOT_STALL ;get stall function from boot services table
mov rdx, [rcx]
mov [BOOT_SERVICES_STALL], rdx
;Set up necessary console functions
mov rdx, [EFI_SYSTEM_TABLE]
add rdx, OFFSET_TABLE_ERROR_CONSOLE ;get error console table
mov rcx, [rdx]
mov [CONERR], rcx
add rcx, OFFSET_CONSOLE_OUTPUT_STRING ;get output string function from console table
mov rdx, [rcx]
mov [CONERR_PRINT_STRING], rdx
mov rdx, [EFI_SYSTEM_TABLE]
add rdx, OFFSET_TABLE_OUTPUT_CONSOLE ;get output console table
mov rcx, [rdx]
mov [CONOUT], rcx
add rcx, OFFSET_CONSOLE_OUTPUT_STRING ;get output string function from console table
mov rdx, [rcx]
mov [CONOUT_PRINT_STRING], rdx
;Set up necessary runtime services functions
mov rdx, [EFI_SYSTEM_TABLE]
add rdx, OFFSET_TABLE_RUNTIME_SERVICES ;get runtime services table
mov rcx, [rdx]
mov [RUNTIME_SERVICES], rcx
;Clear some registers for use.
xor rcx, rcx
xor rdx, rdx
xor r8, r8
;Print a string
mov rcx, [CONOUT]
lea rdx, [hello]
call [CONOUT_PRINT_STRING]
;Wait a second so that the user can read the string
mov rcx, 5000000
call [BOOT_SERVICES_STALL]
;Return back to the UEFI with success!
mov rcx, [EFI_HANDLE]
mov rdx, EFI_SUCCESS
mov r8, 1
call [BOOT_SERVICES_EXIT]
ret
times 1024 - ($-$$) db 0 ;alignment
;************
;*** DATA ***
;************
section .data follows=.text
dataStart:
;Handover variables
EFI_HANDLE dq 0
EFI_SYSTEM_TABLE dq 0
EFI_RETURN dq 0
;Accessing functions of EFI system table
BOOT_SERVICES dq 0
BOOT_SERVICES_EXIT dq 0 ;This one exits the program, not just stop boot services!
BOOT_SERVICES_STALL dq 0
CONERR dq 0
CONERR_PRINT_STRING dq 0
CONOUT dq 0
CONOUT_PRINT_STRING dq 0
RUNTIME_SERVICES dq 0
;Strings used in the program.
hello db __utf16__ `Hello world\r\n\0`
times 1024 - ($-$$) db 0 ;alignment
Thanks in advance :)