protected mode

CMakeLists.txt

@@ -0,0 +1,30 @@
+cmake_minimum_required(VERSION 3.15)
+project(my_os_bootloader ASM)
+
+set(CMAKE_CXX_FLAGS "-nostdinc -m32 -nostdlib")
+set(CMAKE_ASM_FLAGS "-m32")
+
+set(EXTRACT_DIR ${PROJECT_BINARY_DIR}/extract)
+file(MAKE_DIRECTORY ${EXTRACT_DIR})
+
+set(BOOTLOADER_SOURCES src/boot.s
+                       )
+
+add_library(bootloader STATIC ${BOOTLOADER_SOURCES})
+
+add_custom_command(OUTPUT extracted_bootloader
+    DEPENDS bootloader
+    COMMAND ar xf ${PROJECT_BINARY_DIR}/libbootloader.a --output=${EXTRACT_DIR}
+)
+
+add_custom_target(boot.img
+    DEPENDS extracted_bootloader
+    COMMAND ld -T ${CMAKE_SOURCE_DIR}/ldscript.ld ${EXTRACT_DIR}/*.o
+    -melf_i386 --oformat=binary -o ${CMAKE_BINARY_DIR}/boot.img
+)
+
+add_custom_command(OUTPUT run
+    POST_BUILD
+    DEPENDS boot.img
+    COMMAND bochs -f ${CMAKE_SOURCE_DIR}/bochs.conf
+)

Makefile

@@ -1,36 +1,3 @@
-BUILD_DIR=build
-MBR_NAME=$(BUILD_DIR)/mbr.bin
-BOOT_IMAGE_NAME=$(BUILD_DIR)/boot.img
-
-BOOT_SOURCE=src/boot.s
-
-AS=as
-CC=gcc
-DD=dd
-LD=ld
-
-$(BOOT_IMAGE_NAME): $(MBR_NAME)
-	$(DD) if=$(MBR_NAME) of=$(BOOT_IMAGE_NAME)
-	$(DD) if=/dev/zero of=$(BOOT_IMAGE_NAME) seek=1 bs=512 count=2879
-
-$(MBR_NAME): $(BUILD_DIR)/boot.o
-	$(LD) -t ldscript.ld $(BUILD_DIR)/boot.o -o $(MBR_NAME) --oformat=binary
-
-$(BUILD_DIR)/boot.o: $(BOOT_SOURCE)
-	$(AS) $< -o $@
-
-%.o: %.s
-	$(AS) $< -o $(BUILD_DIR)/$@
-
-%.o: %.c
-	$(CC) -c $< -o $(BUILD_DIR)/$@
-
 .PHONY: run
-run: $(BOOT_IMAGE_NAME)
+run:
 	-bochs -f bochs.conf
-
-.PHONY: clean
-clean:
-	-rm $(BUILD_DIR)/*.o
-	-rm $(MBR_NAME)
-	-rm $(BOOT_IMAGE_NAME)

bochs.conf

@@ -1,4 +1,4 @@
-megs: 4
+megs: 64
 
 # floppya: 1_44=build/boot.img, status=inserted
 ata0: enabled=1, ioaddr1=0x1f0, ioaddr2=0x3f0, irq=14

src/boot.s

@@ -1,6 +1,8 @@
 .text
 .code16
 
+# mbr
+
 .globl _start
 
 _start:
@@ -19,6 +21,8 @@ real_start:
 
     call read_data
 
+    ljmp $0x0050, $(loader_start-loader_start)
+
 die:
     hlt
     jmp die
@@ -35,10 +39,10 @@ string_hello:
 
 read_data_pack:
     .byte 0x10, 0
-    .word 2      # block count
+    .word 16     # block count (read 8k)
     .word 0x0000 # offset address
     .word 0x0050 # segment address
-    .long  0     # LBA to read
+    .long 1      # LBA to read
 
 stack_edge:
 .space 128
@@ -46,3 +50,78 @@ stack_base:
 
 .space 510 - (.-_start)
 .word 0xaa55
+
+
+# loader
+
+loader_start:
+# set segment registers
+    movw %cs, %ax
+    movw %ax, %ds
+
+# load gdt
+    cli
+    lgdt (asm_gdt_descriptor-loader_start)
+
+# enable protection enable (PE) bit
+    movl %cr0, %eax
+    orl $1, %eax
+    movl %eax, %cr0
+
+    ljmp $0x08, $0x0500 + (start_32bit-loader_start)
+
+.code32
+
+start_32bit:
+    movw $16, %ax
+    movw %ax, %ds
+    movw %ax, %ss
+
+# set up stack
+    movl $0x03ffffff, %ebp
+    movl $0x03ffffff, %esp
+
+# breakpoint
+#ifdef _DEBUG
+    xchgw %bx, %bx
+#endif
+
+# load gdt
+    cli
+    lgdt (gdt_descriptor-loader_start)
+    nop
+
+loader_halt:
+    hlt
+    jmp loader_halt
+
+asm_gdt_descriptor:
+    .word (3 * 8) - 1 # size
+    .long 0x0500+(asm_gdt_table-loader_start)  # address
+
+.globl asm_gdt_descriptor
+.type asm_gdt_descriptor @object
+.size asm_gdt_descriptor, (.-asm_gdt_descriptor)
+
+asm_gdt_table:
+    .8byte 0         # null descriptor
+
+    # code segment
+    .word 0x1000     # limit 0 :15
+    .word 0x0000     # base  0 :15
+    .byte 0x00       # base  16:23
+    .byte 0x9a       # access
+    .byte 0b11000000 # flag and limit 16:20
+    .byte 0x00       # base 24:31
+
+    # data segment
+    .word 0x1000     # limit 0 :15
+    .word 0x0000     # base  0 :15
+    .byte 0x00       # base  16:23
+    .byte 0x92       # access
+    .byte 0b11000000 # flag and limit 16:20
+    .byte 0x04       # base 24:31
+
+.space (512 * 31) - (.-loader_start)
+
+.space (512 * 2016) # fill 1m