From 2f2c824c987ec16ddc28a2f4784e6b9d3f7d99d8 Mon Sep 17 00:00:00 2001
From: Ruslan Isaev <ubijca16@gmail.com>
Date: Sun, 23 Mar 2025 13:39:29 +0300
Subject: [PATCH] add basic paging functionality

---
 Makefile         |  8 +++++-
 include/paging.h |  6 +++++
 src/kernel.c     |  3 +++
 src/page.asm     | 24 +++++++++++++++++
 src/paging.c     | 68 ++++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 108 insertions(+), 1 deletion(-)
 create mode 100644 include/paging.h
 create mode 100644 src/page.asm
 create mode 100644 src/paging.c

diff --git a/Makefile b/Makefile
index 8cfd563..a597331 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 # Define the object files
-OBJS = bin/boot.o bin/kernel.o bin/idt.o bin/isr.o bin/screen.o bin/utils.o bin/string.o bin/stdio.o bin/disk.o bin/pic.o bin/pit.o bin/keyboard.o
+OBJS = bin/boot.o bin/kernel.o bin/idt.o bin/isr.o bin/screen.o bin/utils.o bin/string.o bin/stdio.o bin/disk.o bin/pic.o bin/pit.o bin/keyboard.o bin/paging.o bin/page.o
 
 # Define the compiler and assembler
 CC = gcc -D__is_katauos
@@ -67,6 +67,12 @@ bin/pit.o: src/pit.c
 bin/keyboard.o: src/keyboard.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
+bin/paging.o: src/paging.c
+	$(CC) $(CFLAGS) -c $< -o $@
+
+bin/page.o: src/page.asm
+	$(AS) $(ASFLAGS) $< -o $@
+
 clean:
 	rm -rf bin/
 	rm -f $(OUTPUT)
diff --git a/include/paging.h b/include/paging.h
new file mode 100644
index 0000000..2c0583e
--- /dev/null
+++ b/include/paging.h
@@ -0,0 +1,6 @@
+#ifndef PAGING_H
+#define PAGING_H
+
+void paging_init();
+
+#endif
diff --git a/src/kernel.c b/src/kernel.c
index 71b57e8..1ace5ef 100644
--- a/src/kernel.c
+++ b/src/kernel.c
@@ -6,6 +6,7 @@
 #include "../include/string.h"
 #include "../include/pic.h"
 #include "../include/pit.h"
+#include "../include/paging.h"
 
 #include <stdint.h>
 
@@ -25,6 +26,8 @@ void kmain()
 	pit_init(100);
 
 	apply_pic_masks();
+
+	paging_init();
 	__asm__ __volatile__ ("sti");
 	printf("Kernel init sequence completed\n");
 
diff --git a/src/page.asm b/src/page.asm
new file mode 100644
index 0000000..6ca81b9
--- /dev/null
+++ b/src/page.asm
@@ -0,0 +1,24 @@
+
+
+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
diff --git a/src/paging.c b/src/paging.c
new file mode 100644
index 0000000..cb2cee4
--- /dev/null
+++ b/src/paging.c
@@ -0,0 +1,68 @@
+#include "../include/paging.h"
+#include <stdint.h>
+
+extern void loadPageDirectory(unsigned int*);
+extern void enablePaging();
+
+uint32_t page_directory[1024] __attribute__((aligned(4096)));
+uint32_t first_page_table[1024] __attribute__((aligned(4096)));
+
+void paging_init()
+{
+	//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
+		//   Not Present: The page table is not present
+		page_directory[i] = 0x00000002;
+	}
+
+	//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 ;)
+		first_page_table[i] = (i * 0x1000) | 3; // attributes: supervisor level, read/write, present.
+	}
+	page_directory[0] = ((unsigned int)first_page_table) | 3;
+
+	loadPageDirectory(page_directory);
+	enablePaging();
+}
+
+void *get_physaddr(void *virtualaddr) {
+    unsigned long pdindex = (unsigned long)virtualaddr >> 22;
+    unsigned long ptindex = (unsigned long)virtualaddr >> 12 & 0x03FF;
+
+    unsigned long *pd = (unsigned long *)0xFFFFF000;
+    // Here you need to check whether the PD entry is present.
+
+    unsigned long *pt = ((unsigned long *)0xFFC00000) + (0x400 * pdindex);
+    // Here you need to check whether the PT entry is present.
+
+    return (void *)((pt[ptindex] & ~0xFFF) + ((unsigned long)virtualaddr & 0xFFF));
+}
+
+void map_page(void *physaddr, void *virtualaddr, unsigned int flags) {
+    // Make sure that both addresses are page-aligned.
+
+    unsigned long pdindex = (unsigned long)virtualaddr >> 22;
+    unsigned long ptindex = (unsigned long)virtualaddr >> 12 & 0x03FF;
+
+    unsigned long *pd = (unsigned long *)0xFFFFF000;
+    // Here you need to check whether the PD entry is present.
+    // When it is not present, you need to create a new empty PT and
+    // adjust the PDE accordingly.
+
+    unsigned long *pt = ((unsigned long *)0xFFC00000) + (0x400 * pdindex);
+    // Here you need to check whether the PT entry is present.
+    // When it is, then there is already a mapping present. What do you do now?
+
+    pt[ptindex] = ((unsigned long)physaddr) | (flags & 0xFFF) | 0x01; // Present
+
+    // Now you need to flush the entry in the TLB
+    // or you might not notice the change.
+}