This underwent some crazy scope creep. Just one more feature...
- Changed primary assembler to NASM. Specifically so I could use - - Swapped start.s to intel syntax. I find it prettier. Learned a lot doing it tbh. - Changed the name of the multiboot structures in C.
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d97ec36f8c
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4 changed files with 96 additions and 48 deletions
6
Makefile
6
Makefile
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@ -1,7 +1,7 @@
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###CONFIGURATION
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ARCH := i686
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CROSS := $(ARCH)-elf-
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AS := $(CROSS)as
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AS := nasm
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CC := $(CROSS)gcc
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LD := $(CROSS)ld
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QEMU := qemu-system-i386
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@ -9,6 +9,8 @@ INCLUDES:= include/
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CFLAGS := -I $(INCLUDES) -O2 -Wall -Wextra -ffreestanding -nostdlib -lgcc -g
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LDFLAGS := -T linker.ld -nostdlib
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ASFLAGS := -f elf32
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SRC_DIR := src
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OBJ_DIR := obj
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@ -39,7 +41,7 @@ $(OBJ_DIR)/%.o: $(SRC_DIR)/%.c
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$(OBJ_DIR)/%.o: $(SRC_DIR)/%.s
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@mkdir -p $(dir $@)
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$(AS) $< -o $@
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$(AS) $(ASFLAGS) $< -o $@
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#Related targets
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@ -34,6 +34,5 @@ SECTIONS
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{
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*(COMMON)
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*(.bss)
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*(.tss_sect)
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}
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}
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@ -6,7 +6,7 @@
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#include "kmultiboot.h"
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//finally, main.
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void kern_main(uint32_t multiboot_magic, multiboot_info_t* multiboot_info)
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void kern_main(uint32_t multiboot_magic, mb_info_t* multiboot_info)
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{
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//wipe the screen
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vga_clear();
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135
src/start.s
135
src/start.s
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@ -1,60 +1,107 @@
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//C main function for our kernel
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//See: kernel.c
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.extern kern_main
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//This will be our entrypoint function name - gotta initialize it now as global so the linker knows later.
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.global start
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;C main function for our kernel
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;See: kernel.c
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bits 32
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extern kern_main
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;This will be our entrypoint function name - gotta initialize it now as global so the linker knows later.
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global start
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//multiboot for GRUB to boot it. Ideally stage01_bootloader will be able to support the multiboot standard.
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//regardless of who's doing it, we have to set the required stuff
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.set MB_MAGIC, 0x1BADB002 // bytes that bootloader will use to find this place
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.set MB_FLAGS, (1 << 0) | (1 << 1) // flags request the following from the bootloader: maintain page boundaries + provide a memory map
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.set MB_CHECKSUM, (0 - (MB_MAGIC + MB_FLAGS)) // Fails if checksum doesn't pass. Kind of arbitrary, but required.
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;multiboot for GRUB to boot it. Ideally stage01_bootloader will be able to support the multiboot standard.
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;regardless of who's doing it, we have to set the required stuff
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MB_MAGIC equ 0x1BADB002 ; bytes that bootloader will use to find this place
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MB_FLAGS equ (1 << 0) | (1 << 1) ; flags request the following from the bootloader: maintain page boundaries + provide a memory map
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MB_CHECKSUM equ (0 - (MB_MAGIC + MB_FLAGS)) ; Fails if checksum doesn't pass. Kind of arbitrary, but required.
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//Now we actually place the multiboot stuff into the resulting executable...
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.section .multiboot
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.align 4 // 4 byte alignment
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.long MB_MAGIC
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.long MB_FLAGS
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.long MB_CHECKSUM
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;Now we actually place the multiboot stuff into the resulting executable...
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section .multiboot align=4
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dd MB_MAGIC
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dd MB_FLAGS
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dd MB_CHECKSUM
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// Set up for C code. Practically the only requirement for C-generated assembly to work properly is alignment and the presence of a stack.
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.section .bss
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.align 16
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; Set up for C code. Practically the only requirement for C-generated assembly to work properly is alignment and the presence of a stack.
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section .bss align=16
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stack_bottom:
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.skip 4096 // 4096 bytes (4kb) large stack. by skipping some amount of data (and eventually filling it with zeroes?), we've essentially just reserved space for our stack.
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//Remember, stack grows DOWNWARD! So the last thing in the section -> the highest memory address -> the very first thing on the stack!
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//Therefore, we put a label here to represent the top of our stack for later.
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resb 4096 ; 4096 bytes (4kb) large stack. by skipping some amount of data (and eventually filling it with zeroes?), we've essentially just reserved space for our stack.
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;Remember, stack grows DOWNWARD! So the last thing in the section -> the highest memory address -> the very first thing on the stack!
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;Therefore, we put a label here to represent the top of our stack for later.
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stack_top:
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//Allocation of some space for our TSS.
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.section .tss_sect:
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.align 8
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tss:
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.skip 104
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resb 104
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.section .gdt_sect:
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.align 8
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;We're putting the GDT here. I'll be thoroughly commenting the first entry for my own understanding. Rest will be minimal.
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section .gdt_sect
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gdt:
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.skip 48
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;Null descriptor
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dd 0x00000000
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dd 0x00000000
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;Kernel code segment.
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;Limit: in 4kib pages. 0xFFFFF * 4K = full address space.
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dw 0xFFFF
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;Base: Start at 0. We want the whole thing.
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dw 0
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;ALSO BASE: bits 16-23. All zeroes, still.
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db 0
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;Access byte. Defines flags for access permissions to the segment. This segment is:
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;RX, and code/data segment
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db 0b10011010
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;Next two segments are nibbles so I put them together (cant db only a nibble at once).
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;Upper limit bits (right hand nibble) is all ones to fill out the full 4gib in pages
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;Flags (left hand nibble) are set to say that the limit is meant to be read as pages, and we're working in 32bit.
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db 0b11001111
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;Final upper base bits. Still zero lol.
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db 0
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;Done! Now we move onto our next table entries, they are back to back.
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;Kernel Data Segment
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dw 0xFFFF
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dw 0
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db 0
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db 0b10010010
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db 0b11001111
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db 0
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;User Code Segment
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dw 0xFFFF
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dw 0
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db 0
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db 0b11111010
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db 0b11001111
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db 0
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;User Data Segment
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dw 0xFFFF
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dw 0
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db 0
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db 0b11110010
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db 0b11001111
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db 0
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;Task State Segment
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;For a lot of this it's going to be zeroes so we can do it dynamically later (such as finding &tss).
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;Really, we just want to set the access bytes correctly.
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dd 0
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db 0
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db 0b10001001
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dw 0
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gdtr:
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dw gdtr - gdt - 1
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dd gdt
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//Actual code. Entry point goes here!
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.section .text
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//Here it is!
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;Actual code. Entry point goes here!
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section .text
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;Here it is!
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start:
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//Lets set up the stack. Stack grows downward on x86. We did the work earlier of defining where the top of the stack is, so just tell esp.
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mov $stack_top, %esp //set the stack pointer
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pushl %ebx
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pushl %eax
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//To C-land!
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;We made a GDT! Let's use it!
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lgdt [gdt]
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;Lets set up the stack. Stack grows downward on x86. We did the work earlier of defining where the top of the stack is, so just tell esp.
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mov esp, stack_top ;set the stack pointer
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push ebx
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push eax
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;To C-land!
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call kern_main
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//You should never get here, but in case you do, we will just hang.
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;You should never get here, but in case you do, we will just hang.
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hang:
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cli //Interrupts: off
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hlt //Halt!
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jmp hang //just in case...
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cli ;Interrupts: off
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hlt ;Halt!
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jmp hang ;just in case...
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