(Sorry about the last post but since then I've found out that it's nothing to do with the ISR, just the pointer assignment.
I'm trying to make a stack for recording physical pages that havn't been mapped into any any address spaces yet but I'm getting some very odd errors with a seemingly innocent pointer assignment.
Here's the code:
Code: Select all
/* ###################
; mem()
;
; Description: Sets up the SG-OS memory manager, it first
; retreves the size of the machine's ram from the
; Multiboot returned information, and then starts
; building up the unallocated page stack.
;
; C Prototype: int mem(multiboot_info_t *boot_info);
;
; Parameters: boot_info - Pointer to the Multiboot returned
; information
;
; Returns: 0 = Success, 1 = couldn't get available memory, 2 =
; ...
*/
int mem()
{
memory_map_t *mmap_ptr;
unsigned int count;
unsigned int *page_dir_ptr = (unsigned int*)
KERNEL_PAGE_DIR_LOCATION;
unsigned int next_unused_page;
/* 1. Get the amount of physical RAM installed in the
* machine. Check bit 6 of the flags to see if the mem map
* is valid.
* This is the profered method, but doesn't work on old
* machines
*/
if (boot_information->flags & 0x40)
{
/* It is, great, now to read the size/structure pairs */
mmap_ptr = (memory_map_t *)boot_information->mmap_addr;
/* Now to check each pair (ignoring the size and
* using 24 bytes for each one). If the structure is
* describing available memory, then add it's length
* to the total and then go to the next one!
*/
for (count = 0; count < boot_information->mmap_length ;
count += MMAP_PAIR_SIZE)
{
if (mmap_ptr->type == 1)
mem_amount += mmap_ptr->length_low;
mmap_ptr++;
}
}
else
{
/* Otherwise check bit 0 to see if we can get an
* older style memory size reading.
* This is the older, less reliable method.
*/
if (boot_information->flags & 1)
mem_amount = boot_information->mem_upper
+ 1024;/*KB, add the first 1MB to total*/
else
/* Oh well, just fail then */
return 1;
/* Current value is in Kilobytes, I want it in bytes */
mem_amount *= 1024;
}
/* 2. Setup the Unallocated page stack */
unallocated_stack_ptr = KERNEL_PAGE_DIR_LOCATION + 0x2000;
/* The current highest point of used memory is at the top
* of the unallocated page stack, we need to find out the
* next unused page
*/
next_unused_page = unallocated_stack_ptr + (4 * (mem_amount / PAGE_SIZE));
next_unused_page = (next_unused_page & 0xFFFFF000) + 0x1000;
/* Now add the addresses of the physical pages that
* are'nt allocated
*/
for (count = 0; count < (mem_amount / PAGE_SIZE) ; count++)
{
*unallocated_stack_ptr = next_unused_page;
unallocated_stack_ptr++;
next_unused_page += 0x1000;
}
return 0;
}
Anyway, when I make and assignment to the memory pointed to by this pointer, it could be any address, or anything being assigned to it, so this line:
*unallocated_stack_ptr = next_unused_page;
I just get this error from Bochs if I have interrupts dissabled:
[CPU ] fetch_raw_descriptor: LDTR.valid=0
Or, if i have interrupts enabled:
[CPU ] iret: AR byte indicated non code segment.
If I run this through VMWare, it just prints a pattern of characters on the screen,
The strangest thing is that on real hardware (AMD athlons at least) it works quite happily.
What am I doing wrong?
Thanks
srg
