Mallocation #30

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1. emilkovacev

   self-assigned this

   on Apr 21, 2023
2. emilkovacev commented on Sep 30, 2023

   

   emilkovacev

   on Sep 30, 2023

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   Ok, so I'm a bit stumped - I've been stuck on this issue for a while, in particular trying to get the 0x15 eax=0xe820 bios interrupt to work. I am executing the bios call right before the bootloader, in an attempt to detect upper segments of memory that are useable. It seems that in the following snippet of code (most of which is referenced from osdev.org):

   Here's my fork

   [bits 16]
   detect_mem:
   	mov di, 0x8004
   	xor ebx, ebx
   	xor bp, bp
   	mov edx, 0x534D4150
   	mov eax, 0xe820
   	mov [es:di + 20], dword 1 ; force a valid ACPI 3.X entry
   	mov ecx, 24 ; ask for 24 bytes
   	int 0x15
   

   The line with mov edx, 0x534D4150 seems to be crashing, before the instruction is even made (or if it is made at all). My guess is either that it's overriding a register either from the 0x13 interrupt or for something down the line that isn't reset. Are there any calls that depend on a specific value for edx that I can set it back to later?

   This is my first exposure to writing code in a bootloader, so I'm really enjoying this :)
3. Sploder12 commented on Sep 30, 2023

   

   Sploder12

   on Sep 30, 2023

   Last edited by Sploder12

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   On my machine the crash appears to come from executing garbage memory (It jumps to 0x1000 [_start] and proceeds to NOP slide a great distance [gdb displays zeros as add %al(%eax)] ). I agree with your suspicion, It seems some registers that aren't explicitly set for int 0x13 are being changed here (ES:BX maybe? Definitely dl) which is causing the OS to load somewhere in the aether (if at all).

   ^^^ for historical purposes (forgive me, it's late at night)

   Yes, your suspicion is correct. dl needs to be reset.

   Luckily dl is preserved in [BOOT_DRIVE] already!

   🎉React with 🎉1Reacted by Emil Kovacev
4. emilkovacev commented on Oct 19, 2023

   

   emilkovacev

   on Oct 19, 2023

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   Just an update: I'm able to get the first few hundred kilobytes mapped! I was able to verify the mapping using gdb, and I am working on getting it to work for many mappings. I believe we talked about using bitmaps, so I think I can get a simple page allocator in a week or so!

   🎉React with 🎉1Reacted by trevor-shoe