Bootloader issue - Githubissues

gbowne1 commented 1 year ago

The 2nd stage bootloader asm file loops.

Not sure what the cause is.

We are using nasm and QEMU.

Also not sure that we are dealing with bootloader properly.

Notes:

Not using EFI/UEFI, just BIOS
linked using ld
nasm assembly
using bios 0x10 and 0x13
starting in 16 bit real mode, switching to 32 protected
still need to initiaize the gdt to 0 values?
using QEMU
reading the kernel from disk (floppy, hd or cdr)
printing the booting message

gbowne1 commented 1 year ago

As far as I can tell, in a 2 stage bootloader system:

1st bootloader:

Starts out in 16 bit @ 07c00 offset
Loads 2nd stage bootloader using the disk [disk could be floppy or hard disk, etc]
Maybe prints a booting message?

2nd bootloader:
starts out with the 16 bit mode in nasm with [bits 16] @ 0x0000
Loads kernel
jumps [JMP] from 0x0000 to 0x2000 in order to get acceess to beyond 1MB.
switches to 32 bit mode in nasm with [bits 32]
initializes 16 bit GDT (Global Descriptor Table) to zero in preparation for kernel space GDT (gdt.c/gdt.h)
does segmentation
uses the A20 register by INT 15h in order to get past the 1MB limit by a mov INT 15h, AX=E601h,

Kernel does all the priting to screen using drivers, etc.

Using the A20 line in x86 IA32 requires setting the A20 gate bit in the processor's control register (CR0). To enable the A20 gate, software must first disable interrupts, then read the current value of CR0, set the A20 gate bit, and write the modified value back to CR0.

@Karutoh

gbowne1 commented 1 year ago

My reference is: https://github.com/gotoco/PE_Bootloader_x86/blob/master/src/Stage2.asm and https://github.com/gotoco/PE_Bootloader_x86/blob/master/src/Stage1FDD.asm

gbowne1 commented 1 year ago

In x86 IA-32 boot loaders, the common practice of starting at 0x7C00 and then jumping to either 0x0000, 0x1000, or 0x2000 by a jmp instruction in NASM serves several crucial purposes:

Real Mode Execution: When the BIOS finds a bootable floppy or hard disk, it loads the 512-byte boot sector into memory at physical addresses 0x7C00 through 0x7DFF. The jmp instruction is then used to set the CS:IP to 0000:7C00, passing control to the boot loader

Transition to Protected Mode: The boot loader switches the processor from real mode to 32-bit protected mode, enabling software to access more memory and utilize advanced features

Memory Addressing: The jmp instruction is used to transfer control to a specific memory address, allowing the boot loader to execute code located at different memory locations, such as 0x0000, 0x1000, or 0x2000, depending on the specific requirements of the bootstrapping process.

Serialization of the Processor: The jmp or call instruction immediately after the MOV CR0 instruction changes the flow of execution and serializes the processor, ensuring proper execution of subsequent instructions

gbowne1 commented 3 months ago

Here are some references for making bootloaders

https://medium.com/@prithivimaruthachalam/building-a-glorified-bootloader-in-c-part-1-bf0ffb5eb382 https://medium.com/@prithivimaruthachalam/building-a-glorified-bootloader-part-2-4dd17c9917f2 https://wiki.osdev.org/Bootloader https://github.com/tuhdo/os-study/blob/master/bootloader/boot_loader.asm https://github.com/tuhdo/os-study/blob/master/bootloader/stage2.asm https://stackoverflow.com/questions/2065370/how-to-load-second-stage-boot-loader-from-first-stage https://forum.osdev.org/viewtopic.php?p=305833 https://blog.ghaiklor.com/2017/10/21/how-to-implement-your-own-hello-world-boot-loader/ https://blog.ghaiklor.com/2017/11/02/how-to-implement-a-second-stage-boot-loader/ https://en.wikibooks.org/wiki/X86_Assembly/Bootloaders http://3zanders.co.uk/2017/10/13/writing-a-bootloader/ https://www.sqlpassion.at/archive/2022/02/03/the-boot-process-of-a-pc-and-how-to-write-your-own-boot-loader/ https://539kernel.com/book/Chapter_1:_Let%27s_Start_with_the_Bootloader.html https://www.cs.unc.edu/~porter/courses/comp630/s24/lab1.html https://medium.com/@g33konaut/writing-an-x86-hello-world-boot-loader-with-assembly-3e4c5bdd96cf https://www.cs.princeton.edu/courses/archive/fall17/cos318/projects/project1/project1 https://doc.redox-os.org/book/ch04-03-boot-process.html https://coditva.github.io/Jazz/boot-loader.html https://dev.to/frosnerd/writing-my-own-boot-loader-3mld https://www.robopenguins.com/x86-boot-loading/ https://github.com/matlo607/simple-x86-bootloader https://alamot.github.io/os_stage1/ https://alessandropellegrini.it/didattica/2018/aosv/3.x86-boot.pdf https://www.viralpatel.net/taj/tutorial/hello_world_bootloader.php http://www.cs.cmu.edu/~410-s07/p4/p4-boot.pdf https://arjunsreedharan.org/post/82710718100/kernels-101-lets-write-a-kernel