C Writing to absolute address does not update value - c

Question

Im trying to write a basic page manager for my C kernel. The code goes like this:
#define NUM_PAGES 1024
#define PAGE_SIZE 4096
#define NULL 0
#define IMPORTANT_SEGMENT 0xC0900000
struct page {
void * addr;
int in_use;
};
struct page CORE_FILE[NUM_PAGES];
void mem_init() {
for (int i = 0; i < NUM_PAGES; i++) {
CORE_FILE[i].addr = (void*)IMPORTANT_SEGMENT+PAGE_SIZE*i;
CORE_FILE[i].in_use = 0;
}
}
void * allocate() {
for (int i = 0; i < NUM_PAGES; i++) {
if (!CORE_FILE[i].in_use) {
CORE_FILE[i].in_use = 1;
return CORE_FILE[i].addr;
}
}
return NULL;
}
int deallocate(void* p) {
for (int i = 0; i < NUM_PAGES; i++) {
if (CORE_FILE[i].addr == p && CORE_FILE[i].in_use) {
CORE_FILE[i].in_use = 0;
return 0;
}
}
return -1;
}
CORE_FILE is an array of structs containing just one field for telling if the page is in use and an address (im using contiguous addresses growing from IMPORTANT_SEGMENT = 0xC0900000).
When i call allocate() it returns me the correct void* which i cast for example to char, but when i write to the address it simply does nothing.
I have checked the address to which is pointing with GDB and is the correct one.
But when i examine its contents they haven't been updated (Still 0).
void kmain(void) {
mem_init();
int * addr = (int*)allocate();
*addr = 5;
}
Im giving qemu 4 GB of RAM executing with:
qemu-system-i386 -m 4G -kernel kernel -gdb tcp::5022
Perhaps im writing to non-existent memory or maybe im overwriting the address contents after. I don't know.
Any ideas will be appreciated.
Thank you in advance.
[edit] This is the bootloader im using:
bits 32
section .text
;multiboot spec
align 4
dd 0x1BADB002 ;magic
dd 0x00 ;flags
dd - (0x1BADB002 + 0x00) ;checksum. m+f+c should be zero
global start
global keyboard_handler
global read_port
global write_port
global load_idt
extern kmain ;this is defined in the c file
extern keyboard_handler_main
read_port:
mov edx, [esp + 4]
;al is the lower 8 bits of eax
in al, dx ;dx is the lower 16 bits of edx
ret
write_port:
mov edx, [esp + 4]
mov al, [esp + 4 + 4]
out dx, al
ret
load_idt:
mov edx, [esp + 4]
lidt [edx]
sti ;turn on interrupts
ret
keyboard_handler:
call keyboard_handler_main
iretd
start:
cli ;block interrupts
mov esp, stack_space
call kmain
hlt ;halt the CPU
section .bss
resb 8192; 8KB for stack
stack_space:
My link.ld
OUTPUT_FORMAT(elf32-i386)
ENTRY(start)
SECTIONS
{
. = 0x100000;
.text : { *(.text) }
. = 0x200000;
.data : { *(.data) }
. = 0x300000;
.bss : { *(.bss) }
}
Edit2: I compile with this
nasm -f elf32 kernel.asm -o kasm.o
gcc -g -fno-stack-protector -fno-builtin -m32 -c memory.c -o memory.o
gcc -g -fno-stack-protector -fno-builtin -m32 -c kernel.c -o kc.o
ld -m elf_i386 -T link.ld -o kernel kasm.o memory.o kc.o

Answer 1

The problem is with protected and real mode, when the computer boots it does so in 16 bit real mode, which makes you able to address 1 MB of data. Everything over that wont be suitable for reading/writing. If i changed the IMPORTANT_SEGMENT to 0x300000 it works.
Now i have to create and load my gdt, enable the a20 line, enable protected mode, set the registers and jump to my code.