As a test, I just wanted to create a sort of "passthrough", where the virtual address is the same as its corresponding physical address. My code is below, is there something I'm missing?
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#include <stdint.h>
#include <stddef.h>
#include <lib/mem.h>
#include <multiboot.h>
#include <drivers/vga.h>
#include <lib/panic.h>
typedef struct {
unsigned int p : 1; // 0: Present
unsigned int rw : 1; // 1: Read/Write
unsigned int us : 1; // 2: User/Supervisor
unsigned int pwt : 1; // 3: Write-Through
unsigned int pcd : 1; // 4: Cache Disable
unsigned int a : 1; // 5: Accessed
unsigned int avl0 : 1; // 6: AVAILABLE
unsigned int ps : 1; // 7: Page Size (0)
unsigned int avl1 : 4; // 8-11: AVAILABLE
unsigned int addr : 20; // 12-31: Address
} dir_entry;
typedef struct {
unsigned int p : 1; // 0: Present
unsigned int rw : 1; // 1: Read/Write
unsigned int us : 1; // 2: User/Supervisor
unsigned int pwt : 1; // 3: Write-Through
unsigned int pcd : 1; // 4: Cache Disable
unsigned int a : 1; // 5: Accessed
unsigned int d : 1; // 6: Dirty
unsigned int pat : 1; // 7: Page Attribute Table
unsigned int g : 1; // 8: Global
unsigned int avl0 : 3; // 9-11: AVAILABLE
unsigned int addr : 20; // 12-31: Address
} table_entry;
dir_entry *dir;
table_entry *table;
void create_passthrough() {
dir = kmalloc();
table = kmalloc();
if (!dir || !table) panic("Not enough memory to inizialize paging.");
size_t kernel_size = &_kernel_end - &_kernel_start;
uint16_t kernel_pages = (kernel_size + PAGE_SIZE - 1) / PAGE_SIZE;
for (uint16_t i = 0; i < 1024; i++) {
dir[i].p = 0;
table[i].p = 0;
}
// vidmem passthrough
dir[0] = (dir_entry) {
.p = 1,
.rw = 1,
.us = 0,
.pwt = 0,
.pcd = 1,
.ps = 0,
.addr = ((uintptr_t) table >> 12) & 0xfffff
};
table[0] = (table_entry) {
.p = 1,
.rw = 1,
.us = 0,
.pwt = 0,
.pcd = 1,
.pat = 0,
.g = 1,
.addr = 0xb8000
};
// kernel passthrough
for (uint16_t i = 1; i <= kernel_pages; i++) {
dir[i] = (dir_entry) {
.p = 1,
.rw = 1,
.us = 0,
.pwt = 0,
.pcd = 0,
.ps = 0,
.addr = ((uintptr_t) table >> 12) & 0xfffff
};
table[i] = (table_entry) {
.p = 1,
.rw = 1,
.us = 0,
.pwt = 0,
.pcd = 0,
.pat = 0,
.g = 1,
.addr = (_kernel_start + (i * PAGE_SIZE)) & 0xfffff
};
}
// enable paging
asm volatile (
"mov %0, %%cr3\n\t" // load page directory address into CR3
"mov %%cr0, %%eax\n\t" // copy CR0 into EAX
"or $0x80000000, %%eax\n\t" // set the PG bit (bit 31) in EAX
"mov %%eax, %%cr0\n\t" // write modified value back to CR0
:
: "r" (dir) // pass in dir pointer
: "eax" // clobbers EAX register
);
printf("Test value: %d", dir[0].addr);
}
// when this is called, it causes the triple-fault
void vmalloc_init() {
// test function
create_passthrough();
}
. I'm actually glad you were able to answer this question. It does tell me that you do understand, but this was coded improperly.