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create IDT in Assembly
Posted: Sun Jan 10, 2021 10:12 pm
by growlnx
Hello everybody!
I know in C is more easy but i'm trying to implement the IDT in pure assembly for learning purposes.
Code: Select all
%ifndef IDT
%define IDT
extern K_int_handler
idt_start:
%assign i 0
%rep 32
irq %+i:
dw <ADDRESS OF ISR> & 0FFFFh
dw DATA_SEG ; 0x08
db 0
db 08eh
dw <ADDRESS OF ISR> & 0FFFF0000h) >> 16
%assign i i+1
%endrep
times (255-31) resb 0 ; fill rest of idt
idt_end:
idtr:
dw idt_end - idt_start - 1
dd idt_start
%assign i 0
%rep 32
isr %+i:
cli
pusha
call K_int_handler
popa
sti
iret
%assign i i+1
%endrep
idt_setup:
pusha
cli
lidt [idtr]
sti
popa
ret
%endif ; IDT
I want put the address of isr* in "<ADDRESS OF ISR>", does anyone have any idea how to do?
Re: create IDT in Assembly
Posted: Sun Jan 10, 2021 10:33 pm
by Octocontrabass
growlnx wrote:I want put the address of isr* in "<ADDRESS OF ISR>", does anyone have any idea how to do?
If you're assembling a flat binary, you can perform arithmetic on labels.
If you're assembling a proper object file to eventually link with other object files into a complete kernel binary, it's not possible. There are no x86 object file formats that support relocations for half of an address.
Re: create IDT in Assembly
Posted: Sun Jan 10, 2021 10:48 pm
by growlnx
it will be a elf_i386 binary kernel to be loaded with GRUB.
Re: create IDT in Assembly
Posted: Sun Jan 10, 2021 10:53 pm
by growlnx
Any ideas on how to get around this problem without resorting to high level?
Re: create IDT in Assembly
Posted: Mon Jan 11, 2021 7:33 am
by bzt
growlnx wrote:Any ideas on how to get around this problem without resorting to high level?
You can calculate the IDT entries in run-time instead of using macros. But I'd like to point out that it is very uncommon not to load the kernel at fixed address, therefore chances are good no relocation needed no matter the executable format (elf, pe, coff, aout etc.). In i386 elf (if it's not compiled as shared library) then the program headers tells the loader (GRUB) where to load the segments.
Anyway, if you really want a relocatable kernel, then something like:
Code: Select all
mov edi, idt_start
mov eax, label1 ; relocateable, as it has the entire label
call store_idt
mov eax, label2
call store_idt
; ... etc.
; --- store one IDT entry IN: edi=ptr to idt, eax=label OUT: edi=incremented ---
store_idt:
mov ebx, eax ; do the shifting and masking in run-time
stosw ; store first part in IDT
mov eax, 08e0008
stosd
mov eax, ebx
shr eax, 16
stosw ; store second part in IDT
ret
Note these are not necessarily valid IDT operations, just examples. Do not copy'n'paste
store_idt as-is.
Cheers,
bzt
Re: create IDT in Assembly
Posted: Mon Jan 11, 2021 10:48 am
by Octocontrabass
growlnx wrote:Any ideas on how to get around this problem without resorting to high level?
Everything you can do in a high-level language you can do in assembly.
But if you're sure you'll never want to generate IDT entries at runtime, you can always rearrange the IDT entries. For example, exchange the ISR segment and the upper 16 bits of the ISR offset in your code (that way the ISR offset isn't split), then write some code that puts them back where they belong when your kernel boots.
bzt wrote:But I'd like to point out that it is very uncommon not to load the kernel at fixed address, therefore chances are good no relocation needed no matter the executable format (elf, pe, coff, aout etc.).
The linker still requires relocations in order to insert the correct fixed address at the correct location in the binary.
Re: create IDT in Assembly
Posted: Mon Jan 11, 2021 11:25 am
by bzt
Octocontrabass wrote:The linker still requires relocations in order to insert the correct fixed address at the correct location in the binary.
True, unless you use one compilation unit with an ORG directive. So it's safe to say you'll need run-time code, I guess? (I always do it in run-time anyway because I only install ISRs for which the drivers register an IRQ during boot)
Cheers,
bzt
Re: create IDT in Assembly
Posted: Tue Jan 12, 2021 10:56 am
by growlnx
Well, then the easiest way to do this directly in the assembly is to imitate the result obtained in C.