Enable paging, run kernel in high half.

Makefile

@@ -13,7 +13,7 @@ CFLAGS = -fno-pic -ffreestanding -static -fno-builtin -fno-strict-aliasing \
 		 -mno-sse \
 		 -Wall -ggdb -m32 -Werror -fno-omit-frame-pointer
 CFLAGS += $(shell $(CC) -fno-stack-protector -E -x c /dev/null >/dev/null 2>&1 && echo -fno-stack-protector)
-ASMFLAGS = -m32 -ffreestanding -c -g
+ASMFLAGS = -m32 -ffreestanding -c -g -I.
 
 ifeq ($(LLVM),on)
 #AS=llvm-as
@@ -24,7 +24,7 @@ ASMFLAGS = -target elf-i386 -ffreestanding -c -g
 LDKERNELFLAGS = --script=script.ld
 endif
 
-OBJECTS = kernel.o console.o drivers/vga.o drivers/uart.o drivers/keyboard.o \
+OBJECTS = kernel/kstart.o kernel.o console.o drivers/vga.o drivers/uart.o drivers/keyboard.o \
 	cpu/idt.o cpu/gdt.o cpu/swtch.o cpu/vectors.o lib/mem.o proc.o lib/string.o \
 	fs/fs.o drivers/ata.o lib/mem.o lib/string.o proc.o drivers/pit.o
 
@@ -91,13 +91,13 @@ fs.img: kernel.bin tools/mkfs user/false user/greet user/div0 user/shout
 LDFLAGS=-m elf_i386
 
 user/%: user/%.o user/crt.o
-	$(LD) $(LDFLAGS) -o $@ -Ttext 0x1000 $^
+	$(LD) $(LDFLAGS) -o $@ -Ttext 0x10000 $^
 
 image.bin: mbr.bin fs.img
 	cat $^ >$@
 
 kernel.bin: $(OBJECTS)
-	$(LD) $(LDFLAGS) $(LDKERNELFLAGS) -o $@ -Ttext 0x9000 $^
+	$(LD) $(LDFLAGS) $(LDKERNELFLAGS) -o $@ -Ttext 0xf0009000 $^
 
 bootmain.o: bootmain.c
 	$(CC) $(CFLAGS) -Os -c $< -o $@

bootmain.c

@@ -40,7 +40,7 @@ bootmain(void)
   ph = (Elf32_Phdr*)((uchar*)elf + elf->e_phoff);
   eph = ph + elf->e_phnum;
   for(; ph < eph; ph++) {
-    pa = (uchar*)ph->p_paddr;
+    pa = (uchar*)(ph->p_paddr & 0x0fffffff);
     readseg(pa, ph->p_filesz, ph->p_offset);
     if(ph->p_memsz > ph->p_filesz)
       stosb(pa + ph->p_filesz, 0, ph->p_memsz - ph->p_filesz);
@@ -48,7 +48,7 @@ bootmain(void)
 
   // Call the entry point from the ELF header.
   // Does not return!
-  entry = (void(*)(void))(elf->e_entry);
+  entry = (void(*)(void))(elf->e_entry & 0x0fffffff);
   entry();
 }
 

cpu/memlayout.h

@@ -0,0 +1,3 @@
+#pragma once
+
+#define KERNBASE 0xf0000000

kernel.c

@@ -14,7 +14,7 @@ asm(".asciz \"kernel start\\n\"");
 #include "proc.h"
 
 
-void _start() {
+void kmain() {
     load_gdt();
     init_keyboard();
     init_pit();

kernel/kstart.S

@@ -0,0 +1,71 @@
+#include "cpu/memlayout.h"
+
+/*
+    Memory layout at this point (see https://wiki.osdev.org/Memory_Map_(x86) for more details):
+       0x00500 - 0x08fff: usable memory
+       0x09000 – 0x14fff: kernel code and global data
+       0x15000 - 0x7ffff: usable memory
+       0x80000 - 0xfffff: BDA and upper memory
+       0x100000 - 0x8000000 (1 MiB - 128 MiB): usable memory
+*/
+    .intel_syntax noprefix
+    .global _start
+    .asciz "kernel start\n"
+_start:
+.code32
+    mov dword ptr [patch_addr - KERNBASE], offset gdt32_begin - KERNBASE
+    lgdt gdtr32 - KERNBASE
+
+    // zero our PD at 0x1000 and PT at 0x2000
+    xor eax, eax
+    mov ecx, 2048
+    rep stosd
+
+    // Set up the page table for the lowest 4 MiB: PD[0] -> PT
+    mov dword ptr [0x1000], 0x2000 | 0x3
+
+    // KERNBASE = 0xF000_0000
+    // Use the same page table for the first 4 MiB after KERNBASE: PD[0x3c0] -> PT
+    mov dword ptr [0x1000 + (0x3c0 * 4)], 0x2000 | 0x3
+
+    // Identity map the first 2 MiB
+    mov edi, 0x2000      // Start of the PT
+    mov ebx, 0x00000003  // Present, R/W
+    mov ecx, 512         // 2 MiB / 4 KiB = 512
+SetEntry:
+    mov dword ptr [edi], ebx
+    add ebx, 0x1000
+    add edi, 4
+    loop SetEntry
+
+    // Load physical address of PD into CR3
+    mov edi, 0x1000
+    mov cr3, edi
+
+    // Enable paging
+    mov eax, cr0
+    or eax, 1 << 31      // Set the PG bit
+    mov cr0, eax
+
+    .att_syntax
+    jmp $0x8, $loadseg - KERNBASE
+    .intel_syntax noprefix
+
+loadseg:
+    // jump to the high half
+    add esp, KERNBASE
+    lea eax, kmain
+    jmp eax
+
+.align 16
+gdtr32:
+    .word gdt32_end - gdt32_begin - 1
+patch_addr:
+    .long gdt32_begin - KERNBASE
+
+.align 16
+gdt32_begin:
+    .long 0x00000000, 0x00000000      # Null descriptor
+    .long 0x0000ffff, 0x00cf9a00      # Code segment
+    .long 0x0000ffff, 0x00cf9200      # Data segment
+gdt32_end: