Merge branch 'paging' into 'master'

Paging

Enables paging. I have yet to determine exactly how this affects the rest of the system.

See merge request !7

src/kern.c

-#if !defined(__cplusplus)
 #include <stdbool.h> /* C doesn't have booleans by default. */
-#endif
 #include <stddef.h>
-
-/* Check if the compiler thinks we are targeting the wrong operating system. */
-#if defined(__linux__)
-#error "You are not using a cross-compiler, you will most certainly run into trouble"
-#endif
-
-/* This tutorial will only work for the 32-bit ix86 targets. */
-#if !defined(__i386__)
-#error "This tutorial needs to be compiled with a ix86-elf compiler"
-#endif
+#include <stdint.h>
 
 #include "serial.h"
 #include "term.h"
+#include "mem.h"
 
 #if defined(__cplusplus)
 extern "C" /* Use C linkage for kernel_main. */
@@ -31,4 +21,14 @@ void kernel_main() {
     serial_writestring("First Serial Test!\n");
     terminal_writestring("Hello, kernel World!\n");
     serial_writestring("Second Serial Test\n");
+
+    // Enable paging of memory. For now there is only one virtual
+    // memory space defined by page_dir
+    page_directory_entry* page_dir;
+    page_dir = initiate_directory();
+    initiate_page_table(page_dir);
+    loadPageDirectory(page_dir);
+    enablePaging();
+
+    serial_writestring("Finished Initilizing memory\n");
 }

src/makefile

@@ -2,6 +2,7 @@ C=/home/adam/college/kernel/cross/bin/i686-elf-gcc
 
 all:
 	/usr/bin/nasm -felf32 head.asm -o boot.o
+	/usr/bin/nasm -felf32 paging.asm
 	$C *.c -std=gnu99 -ffreestanding -O2 -Wall -Wextra -c
 	$C -T linker.ld -o myos.bin -ffreestanding -O2 -nostdlib *.o -lgcc
 

src/mem.c

+/*
+  mem.c is the main file for dealing with virtual memory and paging.
+*/
+
+#include "mem.h"
+#include "serial.h"
+
+// page_directory is currently defined once and should not be touched
+// later. It must be page aligned for the MMU to read properly
+page_directory_entry page_directory[1024] __attribute__((aligned(4096)));
+
+// page_table_one is the first page table, and at the beginning the
+// only page table in the page directory
+page_table_entry page_table_one[1024] __attribute__((aligned(4096)));
+
+page_directory_entry* initiate_directory() {
+    // set each entry to not present
+    int i;
+    for(i = 0; i < 1024; i++)
+    {
+        // This sets the following flags to the pages:
+        //   Supervisor: Only kernel-mode can access them
+        //   Write Enabled: It can be both read from and written to
+        //     (this is the 2 bit)
+        //   Not Present: The page table is not present
+        page_directory[i] = 0;
+        page_directory[i] |= PAGE_WRITABLE;
+    }
+
+    return page_directory;
+}
+
+void initiate_page_table(page_directory_entry* page_dir) {
+    // holds the physical address where we want to start mapping these pages to.
+    // in this case, we want to map these pages to the very beginning of memory.
+    unsigned int i;
+
+    //we will fill all 1024 entries in the table, mapping 4 megabytes
+    for(i = 0; i < 1024; i++)
+    {
+        // As the address is page aligned, it will always leave 12 bits zeroed.
+        // Those bits are used by the attributes:
+        //    -> supervisor level, read/write, present.
+        page_table_one[i] = 0;
+        page_table_one[i] |= i << 12;
+        page_table_one[i] |= PAGE_WRITABLE | PAGE_PRESENT;
+    }
+
+    // attributes: supervisor level, read/write, present
+    page_dir[0] = (uint32_t) page_table_one;
+    page_dir[0] |= PAGE_WRITABLE | PAGE_PRESENT;
+
+}

src/mem.h

+/*
+  mem.h
+*/
+
+#ifndef _MEM_INC
+#define _MEM_INT
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// Flags for pages directory entries and table entries
+#define PAGE_PRESENT        0x1
+#define PAGE_WRITABLE       0x2
+#define PAGE_USER           0x4
+#define PAGE_WRITE_THROUGH  0x8
+#define PAGE_CACHE_DISABLED 0x10
+#define PAGE_ACCESSED       0x20
+#define PAGE_DIRTY          0x80
+#define PAGE_SIZE_FLAG      0x80
+
+typedef uint32_t page_directory_entry;
+typedef uint32_t page_table_entry;
+
+// These two are defined in the assembly file paging.asm
+extern void loadPageDirectory(page_directory_entry*);
+extern void enablePaging();
+
+// Defined in the file mem.c
+page_directory_entry* initiate_directory();
+void initiate_page_table(page_directory_entry* page_dir);
+
+#endif

src/paging.asm

+section .text
+	global loadPageDirectory
+
+loadPageDirectory:
+	push ebp
+	mov ebp, esp
+	mov eax, [esp+8]
+	mov cr3, eax
+	mov esp, ebp
+	pop ebp
+	ret
+
+	global enablePaging
+enablePaging:
+	push ebp
+	mov ebp, esp
+	mov eax, cr0
+	or eax, 0x80000000
+	mov cr0, eax
+	mov esp, ebp
+	pop ebp
+	ret

src/serial.c

@@ -32,3 +32,18 @@ void serial_writestring(char* s) {
         s++;
     }
 }
+
+const char* hexchars = "0123456789abcdef";
+void serial_hexword(uint32_t x) {
+    for (int i = 8; i != 0; i--) {
+        serial_writechar(hexchars[x>>i*4&0xf]);
+    }
+}
+
+void serial_hexstring(void* s, uint32_t n) {
+    char* d = (char*)s;
+    for (uint32_t i = 0; i < n; i++) {
+        serial_writechar(hexchars[(d[i]>>4)&0xf]);
+        serial_writechar(hexchars[d[i]&0xf]);
+    }
+}

src/serial.h

@@ -2,7 +2,11 @@
   serial.h contains headers for the serial interaction functions
 */
 
+#ifndef _SERIAL_INC
+#define _SERIAL_INC
+
 #include "asm.h"
+#include <stdint.h>
 
 // Initilizes the first serial port
 void serial_init();
@@ -15,3 +19,9 @@ void serial_writechar(char a);
 
 // Write a null terminated string to serial
 void serial_writestring(char* s);
+
+// Function to write a 4 byte word toserial in hex
+void serial_hexword(uint32_t x);
+void serial_hexstring(void* s, uint32_t n);
+
+#endif