Merge pull request #69 from vmttmv/bootloader

BL implementation

Makefile

@@ -1,64 +1,58 @@
 AS = nasm
+ASFLAGS = -f elf32 -g -F dwarf
 CC = gcc
-CFLAGS = -std=c11 -m32 -ffreestanding -c -fno-stack-protector -fno-pie
 LD = ld
-QEMU = qemu-system-i386
+QEMU= qemu-system-i386
-IMG_SIZE = 1440k
 
 BUILD_DIR = build
-
+DISK_IMG = $(BUILD_DIR)/disk.img
-BOOT_SRC = bootloader/boot.asm
+STAGE2_SIZE = 2048
-BOOT_OBJ = $(BUILD_DIR)/boot.o
-BOOT_ELF = $(BUILD_DIR)/boot.elf
-BOOT_IMG = $(BUILD_DIR)/boot.img
 
 KERNEL_C_SRC = $(wildcard kernel/*.c)
 KERNEL_ASM_SRC = $(wildcard kernel/*.asm)
 KERNEL_OBJ = $(patsubst kernel/%.c, $(BUILD_DIR)/%.o, $(KERNEL_C_SRC))
 KERNEL_OBJ += $(patsubst kernel/%.asm, $(BUILD_DIR)/asm_%.o, $(KERNEL_ASM_SRC))
-KERNEL_OBJ += $(BUILD_DIR)/boot1.o
-KERNEL_ELF = $(BUILD_DIR)/kernel.elf
-KERNEL_BIN = $(BUILD_DIR)/kernel.bin
 
-DISK_IMG = $(BUILD_DIR)/disk.img
+all: $(DISK_IMG)
 
-all: $(BOOT_IMG) $(KERNEL_BIN) $(DISK_IMG)
+.PHONY: stage1 stage2 kernel run gdb clean
+stage1: $(BUILD_DIR)
+	$(AS) $(ASFLAGS) -o $(BUILD_DIR)/$@.o bootloader/$@.asm
+	$(LD) -Ttext=0x7c00 -melf_i386 -o $(BUILD_DIR)/$@.elf $(BUILD_DIR)/$@.o
+	objcopy -O binary $(BUILD_DIR)/$@.elf $(BUILD_DIR)/$@.bin
+
+# NOTE: Stage2 final size should be checked against `$(STAGE2_SIZE)` by the build system to avoid an overflow.
+# Alternatively, convey the final stage2 size through other means to stage1.
+stage2: $(BUILD_DIR)
+	$(AS) $(ASFLAGS) -o $(BUILD_DIR)/stage2.o bootloader/stage2.asm
+	$(CC) -std=c11 -ffreestanding -nostdlib -fno-stack-protector -m32 -g -c -o $(BUILD_DIR)/stage2_load.o bootloader/stage2_load.c
+	$(LD) -Tbootloader/stage2.ld -melf_i386 -o $(BUILD_DIR)/$@.elf $(BUILD_DIR)/stage2.o $(BUILD_DIR)/stage2_load.o
+	objcopy -O binary $(BUILD_DIR)/$@.elf $(BUILD_DIR)/$@.bin
+	truncate -s $(STAGE2_SIZE) $(BUILD_DIR)/$@.bin
+
+$(BUILD_DIR)/asm_%.o: kernel/%.asm
+	$(AS) $(ASFLAGS) -o $@ $<
+
+$(BUILD_DIR)/%.o: kernel/%.c
+	$(CC) -std=c11 -ffreestanding -nostdlib -fno-stack-protector -m32 -g -c -o $@ $<
+
+kernel: $(KERNEL_OBJ) | $(BUILD_DIR)
+	$(LD) -melf_i386 -Tbootloader/linker.ld -o $(BUILD_DIR)/kernel.elf $(KERNEL_OBJ)
+
+$(DISK_IMG): stage1 stage2 kernel
+	dd if=$(BUILD_DIR)/stage1.bin of=$@
+	dd if=$(BUILD_DIR)/stage2.bin of=$@ oflag=append conv=notrunc
+	dd if=$(BUILD_DIR)/kernel.elf of=$@ oflag=append conv=notrunc
+	truncate -s 1M $@
 
 $(BUILD_DIR):
 	mkdir -p $@
 
-$(BOOT_OBJ): $(BOOT_SRC) | $(BUILD_DIR)
+run:
-	$(AS) -f elf32 -g -F dwarf -o $@ $<
+	qemu-system-i386 -s -S $(DISK_IMG)
 
-$(BOOT_ELF): $(BOOT_OBJ)
+gdb:
-	$(LD) -Ttext=0x7c00 -melf_i386 -o $@ $<
+	gdb -x gdb.txt
-
-$(BOOT_IMG): $(BOOT_ELF)
-	objcopy -O binary $< $@
-	truncate -s $(IMG_SIZE) $@
-
-$(BUILD_DIR)/boot1.o: bootloader/boot1.asm
-	$(AS) -f elf32 -o $@ $<
-
-$(BUILD_DIR)/asm_%.o: kernel/%.asm
-	$(AS) -f elf32 -o $@ $<
-
-$(BUILD_DIR)/%.o: kernel/%.c
-	$(CC) $(CFLAGS) $< -o $@
-
-$(KERNEL_BIN): $(KERNEL_OBJ) | $(BUILD_DIR)
-	$(LD) -melf_i386 --oformat binary -T bootloader/linker.ld -o $@ $(KERNEL_OBJ)
-
-$(DISK_IMG): $(BOOT_IMG) $(KERNEL_BIN)
-	dd if=$(BOOT_IMG) of=$@ bs=512 seek=4
-	dd if=$(KERNEL_BIN) of=$@ bs=512 seek=200
-
-run: $(DISK_IMG)
-	$(QEMU) -drive file=$<,format=raw,if=floppy
-
-.PHONY: stage1 clean
-
-stage1: $(BOOT_IMG)
 
 clean:
 	rm -rf $(BUILD_DIR)

bootloader/README.md

@@ -0,0 +1,27 @@
+# ClassicOS 2-stage bootloader
+
+Bootloader documentation for ClassicOS
+
+## Disk image organization:
+
+```
+[  512 B  ] [      2048 B      ] [    Unspecified    ]
+  Stage 1         Stage 2               Kernel
+```
+
+## Stage 1 (`stage1.asm`)
+
+Responsible for loading the second stage using BIOS routines, and switching to protected mode.
+
+- Queries CHS parameters from BIOS
+- Loads the second stage bootloader (2048 B) to `07c00h`
+- Sets up a GDT with descriptor entries for code and data both covering the whole 32-bit address space
+- Enables A20
+- Set CR0.PE (enable protected mode) and jump to stage 2
+
+## Stage 2 (`stage2.asm, stage2_load.c`)
+
+- Set up segment registers
+- Load the kernel ELF header
+- Parse the program headers, and load all `PT_LOAD` segments from disk
+- Jump to the kernel entry

bootloader/boot1.asm

@@ -1,194 +0,0 @@
-; ==============================================================================
-; boot1.asm - Second Stage Bootloader (Fixed Real Mode Transition)
-; ==============================================================================
-
-[BITS 32]
-global _start
-extern kmain
-
-_start:
-    ; Set up segments
-    mov ax, 0x10
-    mov ds, ax
-    mov es, ax
-    mov fs, ax
-    mov gs, ax
-    mov ss, ax
-
-    ; Stack (must be identity-mapped)
-    mov esp, 0x90000
-
-    ; CPU Feature Detection: check CPUID support
-    pushfd                      ; Save flags
-    pop eax
-    mov ecx, eax
-    xor eax, 1 << 21            ; Flip ID bit
-    push eax
-    popfd
-    pushfd
-    pop eax
-    xor eax, ecx
-    jz .no_cpuid                ; CPUID unsupported if no change
-
-    ; CPUID supported, verify features
-    mov eax, 1
-    cpuid
-    ; Check for paging support (bit 31 of edx)
-    test edx, 1 << 31
-    jz .no_paging_support
-
-    ; Additional CPU feature checks could be added here
-
-    jmp .cpuid_check_done
-
-.no_cpuid:
-    mov si, no_cpuid_msg
-    call print_string_16
-    jmp halt
-
-.no_paging_support:
-    mov si, no_paging_msg
-    call print_string_16
-    jmp halt
-
-.cpuid_check_done:
-
-    ; Temporarily switch back to real mode
-    cli
-    mov eax, cr0
-    and eax, 0x7FFFFFFE        ; Clear PE & PG bits
-    mov cr0, eax
-    jmp 0x18:real_mode_entry
-
-; ----------------------------------------------------------------
-[BITS 16]
-real_mode_entry:
-    ; Real mode for BIOS access (E820, VESA)
-    xor ax, ax
-    mov es, ax
-
-    ; VESA call
-    mov di, VbeControllerInfo
-    mov ax, 0x4F00
-    int 0x10
-    jc vesa_error
-
-    ; E820 memory map
-    xor ebx, ebx
-    mov edx, 0x534D4150
-    mov di, MemoryMapBuffer
-    mov [MemoryMapEntries], dword 0
-
-.e820_loop:
-    mov eax, 0xE820
-    mov ecx, 24
-    int 0x15
-    jc e820_error
-    add di, 24
-    inc dword [MemoryMapEntries]
-    test ebx, ebx
-    jnz .e820_loop
-    jmp e820_done
-
-e820_error:
-    mov si, e820_error_msg
-    call print_string_16
-    jmp halt
-
-vesa_error:
-    mov si, vesa_error_msg
-    call print_string_16
-
-    ; Fallback: set VGA text mode 3 and continue
-    mov ah, 0x00           ; BIOS Set Video Mode function
-    mov al, 0x03           ; VGA 80x25 text mode
-    int 0x10
-
-    ; Clear screen
-    mov ah, 0x06           ; Scroll up function
-    mov al, 0              ; Clear entire screen
-    mov bh, 0x07           ; Text attribute (gray on black)
-    mov cx, 0              ; Upper-left corner
-    mov dx, 0x184F         ; Lower-right corner
-    int 0x10
-
-    jmp e820_done         ; Continue booting without VESA graphics
-
-e820_done:
-    ; Back to protected mode
-    cli
-    mov eax, cr0
-    or eax, 1
-    mov cr0, eax
-    jmp 0x08:protected_entry
-
-; ----------------------------------------------------------------
-; removed duplicate print from here
-; suggested calling it from boot.asm
-
-e820_error_msg db "E820 Failed!", 0
-vesa_error_msg db "VESA Failed!", 0
-no_cpuid_msg db "No CPUID support detected!", 0
-no_paging_msg db "CPU lacks paging support!", 0
-
-; ----------------------------------------------------------------
-[BITS 32]
-protected_entry:
-    ; Paging setup
-    xor eax, eax
-    mov edi, page_directory
-    mov ecx, 1024
-    rep stosd
-    mov edi, page_table
-    rep stosd
-    mov eax, page_table
-    or eax, 0x3
-    mov [page_directory], eax
-    mov ecx, 1024
-    mov edi, page_table
-    mov eax, 0x00000003
-.fill_pages:
-    mov [edi], eax
-    add eax, 0x1000
-    add edi, 4
-    loop .fill_pages
-
-    mov eax, page_directory
-    mov cr3, eax
-    mov eax, cr0
-    or eax, 0x80000000
-    mov cr0, eax
-
-    jmp kmain
-
-halt:
-    cli
-
-.hang:
-    hlt
-    jmp .hang
-
-; ----------------------------------------------------------------
-; Data buffers and variables must be appropriately defined in your data section
-MemoryMapBuffer times 128 db 0          ; 128*24 bytes reserved for E820 memory map (adjust size as needed)
-MemoryMapEntries dd 0
-VbeControllerInfo times 512 db 0        ; Buffer for VESA controller info (adjust size as needed)
-
-; Define page directory and page table aligned as needed (in your data section)
-align 4096
-page_directory times 1024 dd 0
-align 4096
-page_table times 1024 dd 0
-
-%assign pad_size 4096
-%ifdef __SIZE__
-  %define size_current __SIZE__
-%else
-  %define size_current ($ - $$)
-%endif
-
-%if size_current < pad_size
-  times pad_size - size_current db 0
-%endif
-
-checksum_byte db 0

bootloader/stage1.asm

@@ -2,10 +2,16 @@
 ; boot.asm - First Stage Bootloader (CHS Based)
 ; ==============================================================================
 
-[BITS 16]
+; Params for stage2
-; [ORG 0x7C00]
+%define s2_addr 1               ; stage2 disk offset, in sectors
+%define s2_laddr 0x7e00         ; stage2 load address
+%define s2_size 2048            ; stage2 size
+%define s2_nsect s2_size / 512  ; stage2 size in sectors
 
-start:
+[BITS 16]
+global _start
+
+_start:
     cli                         ; Disable interrupts
 
     mov [bootdev], dl           ; Save boot device number (from BIOS in DL)
@@ -20,29 +26,16 @@ start:
     mov ds, ax
     mov es, ax
 
+    ; Query bios for disk parameters
+    call get_disk_params
+
     ; Load second-stage bootloader (boot1.asm) to 0x7E00
     mov ax, 1                   ; LBA of boot1.asm (starts at sector 1)
     call lba_to_chs
-    mov al, 4                   ; Number of sectors to read
+    mov al, s2_nsect            ; Number of sectors to read
     mov bx, 0x7E00              ; Destination address offset ES = 0 (0x0000:0x7E00)
     call read_chs
 
-    ; Load kernel to 0x100000 (1 MB)
-    mov ax, 0xffff
-    mov es, ax                  ; Set ES for destination address (0xffff << 4 == 0xffff0)
-    mov ax, 5                   ; LBA of kernel start (boot1 is 4 sectors: LBA 1–4 → kernel at LBA 5)
-    call lba_to_chs
-    mov al, 16                  ; Number of sectors for kernel
-    mov bx, 0x10                ; Destination address offset (0xffff:0x10 = 0xffff << 4 + 0x10 = 0x100000)
-    call read_chs
-    jc disk_error
-
-    ; Memory Validation: Verify checksum of second stage bootloader
-    mov si, 0x7E00             ; Start of second stage
-    mov cx, 512 * 4            ; Size in bytes (adjust if more sectors loaded)
-    call verify_checksum
-    jc disk_error              ; Jump if checksum fails
-
     ; Enable A20 line
     call enable_a20
     jc a20_error                ; Jump if A20 enable fails
@@ -95,17 +88,32 @@ verify_checksum:
     pop ax
     ret
 
+get_disk_params:
+    mov  ah, 08h          ; BIOS: Get Drive Parameters
+    int  13h
+    ; TODO: error checking
+
+    ; CL bits 0–5 contain sectors per track
+    mov  al, cl
+    and  al, 3Fh           ; mask bits 0–5
+    mov  ah, 0
+    mov  [sectors_per_track], ax
+
+    ; DH = maximum head number (0-based)
+    mov  al, dh
+    inc  ax                ; convert to count (heads = maxhead + 1)
+    mov  [heads_per_cylinder], ax
+    ret
+
 ; ----------------------------------------------------------------
 ; CHS Disk Read Routine
 ; AL = number of sectors
 ; CL = starting sector (1-based)
-; SI = destination offset (Segment:ES already set)
 ; Inputs:
 ; AL = sector count
 ; CH = cylinder
 ; DH = head
 ; CL = sector (1–63, with top 2 bits as high cylinder bits)
-; SI = destination offset (segment ES must be set)
 
 ; ----------------------------------------------------------------
 ; Convert LBA to CHS
@@ -117,36 +125,29 @@ verify_checksum:
 ;   CL = sector (1-63, top 2 bits are upper cylinder bits)
 
 lba_to_chs:
-    pusha
+    ; Sector
-
     xor dx, dx
-    mov bx, [sectors_per_track]
+    mov bx, ax
-    div bx                     ; AX = LBA / sectors_per_track, DX = remainder (sector number)
+    div word [sectors_per_track] ; divide lba with max sectors
-    mov si, ax                 ; SI = temp quotient (track index)
+    add dl, 1 ; take the remainder, sectors start at 1
-    mov cx, [heads_per_cylinder]
+    mov cl, dl ; sector is in cl
-    xor dx, dx
-    div cx                     ; AX = cylinder, DX = head
-    mov ch, al                 ; CH = cylinder low byte
-    mov dh, dl                 ; DH = head
 
-    ; Now take sector number from earlier remainder
+    ; Head
-    mov cx, si                 ; Copy track index to CX to access CL
+    mov ax, bx
-    and cl, 0x3F               ; Mask to 6 bits (sector number)
+    mov dx, 0
-    inc cl                     ; Sector numbers are 1-based
+    div word [sectors_per_track] ; divide lba with max sectors
+    mov dx, 0
+    div word [heads_per_cylinder] ; divide quotient with heads
+    mov dh, dl ; take the remainder, head is in dh
 
-    ; Insert upper 2 bits of cylinder into CL
+    ; Cylinder
-    mov ah, al                 ; AH = cylinder again
+    mov ch, al ; take the quotient, cylinder is in ch
-    and ah, 0xC0               ; Get top 2 bits of cylinder
-    or cl, ah                  ; OR them into sector byte
-
-    popa
     ret
 
 read_chs:
     pusha
     push dx
 
-    mov cx, 5
 .retry:
     mov ah, 0x02         ; BIOS: Read sectors
     mov dl, [bootdev]    ; Boot device
@@ -262,7 +263,7 @@ switch_to_pm:
     mov eax, cr0
     or eax, 1
     mov cr0, eax
-    jmp 0x08:0x7E00
+    jmp 0x08:0x7E00             ; jump to S2
 
 ; ----------------------------------------------------------------
 print_string_16:
@@ -284,8 +285,8 @@ halt:
     hlt
 
 bootdev db 0
-sectors_per_track dw 63
+sectors_per_track dw 0
-heads_per_cylinder dw 255
+heads_per_cylinder dw 0
 
 times 510 - ($ - $$) db 0
 dw 0xAA55

bootloader/stage2.asm

@@ -0,0 +1,98 @@
+[BITS 32]
+global _start
+global ata_lba_read
+
+extern load_kernel
+
+_start:
+    ; Set up segments
+    ; Data segments
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+
+    ; Code segment
+    mov ax, 0x08
+    mov cs, ax
+
+    ; Stack (must be identity-mapped)
+    mov esp, 0x90000
+
+    call load_kernel
+
+    jmp eax
+
+; ----------------------------------------------------------------------------
+; ATA read sectors (LBA mode) 
+;
+; sysv32 abi signature:
+; void ata_lba_read(uint32_t lba, uint8_t nsect, void *addr);
+; ----------------------------------------------------------------------------
+ata_lba_read:
+    push ebp
+    mov ebp, esp
+
+    push ebx
+    push ecx
+    push edx
+    push edi
+
+    mov eax, [ebp+8]     ; arg #1 = LBA
+    mov cl, [ebp+12]     ; arg #2 = # of sectors
+    mov edi, [ebp+16]    ; arg #3 = buffer address
+    and eax, 0x0FFFFFFF
+
+    mov ebx, eax         ; Save LBA in RBX
+
+    mov edx, 0x01F6      ; Port to send drive and bit 24 - 27 of LBA
+    shr eax, 24          ; Get bit 24 - 27 in al
+    or al, 11100000b     ; Set bit 6 in al for LBA mode
+    out dx, al
+
+    mov edx, 0x01F2      ; Port to send number of sectors
+    mov al, cl           ; Get number of sectors from CL
+    out dx, al
+
+    mov edx, 0x1F3       ; Port to send bit 0 - 7 of LBA
+    mov eax, ebx         ; Get LBA from EBX
+    out dx, al
+
+    mov edx, 0x1F4       ; Port to send bit 8 - 15 of LBA
+    mov eax, ebx         ; Get LBA from EBX
+    shr eax, 8           ; Get bit 8 - 15 in AL
+    out dx, al
+
+    mov edx, 0x1F5       ; Port to send bit 16 - 23 of LBA
+    mov eax, ebx         ; Get LBA from EBX
+    shr eax, 16          ; Get bit 16 - 23 in AL
+    out dx, al
+
+    mov edx, 0x1F7       ; Command port
+    mov al, 0x20         ; Read with retry.
+    out dx, al
+
+    mov bl, cl          ; Save # of sectors in BL
+
+.wait_drq:
+    mov edx, 0x1F7
+.do_wait_drq:
+    in al, dx
+    test al, 8           ; the sector buffer requires servicing.
+    jz .do_wait_drq      ; keep polling until the sector buffer is ready.
+
+    mov edx, 0x1F0       ; Data port, in and out
+    mov ecx, 256
+    rep insw             ; in to [RDI]
+
+    dec bl               ; are we...
+    jnz .wait_drq        ; ...done?
+
+    pop edi
+    pop edx
+    pop ecx
+    pop ebx
+    pop ebp
+    ret

bootloader/stage2.ld

@@ -0,0 +1,14 @@
+SECTIONS {
+    . = 0x7e00;
+
+    .text : { *(.text*) }
+    .rodata : { *(.rodata*) }
+    .data : { *(.data*) }
+
+    .bss : {
+        *(.bss*)
+        *(COMMON)
+    }
+
+    read_buf = .;
+}

bootloader/stage2_load.c

@@ -0,0 +1,118 @@
+#include <stdint.h>
+
+// ELF Ident indexes
+#define EI_NIDENT 16
+
+// Program header types
+#define PT_NULL    0
+#define PT_LOAD    1
+
+// ELF Header (32-bit)
+typedef struct {
+    uint8_t  e_ident[EI_NIDENT];
+    uint16_t e_type;
+    uint16_t e_machine;
+    uint32_t e_version;
+    uint32_t e_entry;     // Entry point
+    uint32_t e_phoff;     // Program header table offset
+    uint32_t e_shoff;     // Section header table offset
+    uint32_t e_flags;
+    uint16_t e_ehsize;
+    uint16_t e_phentsize;
+    uint16_t e_phnum;
+    uint16_t e_shentsize;
+    uint16_t e_shnum;
+    uint16_t e_shstrndx;
+} __attribute__((packed)) Elf32_Ehdr;
+
+// Program Header (32-bit)
+typedef struct {
+    uint32_t p_type;
+    uint32_t p_offset;
+    uint32_t p_vaddr;
+    uint32_t p_paddr;
+    uint32_t p_filesz;
+    uint32_t p_memsz;
+    uint32_t p_flags;
+    uint32_t p_align;
+} __attribute__((packed)) Elf32_Phdr;
+
+// Load an ELF executable into memory.
+static int elf_load(const void* data, void (*load_segment)(uint8_t *vaddr, uint32_t src, uint32_t size)) {
+    const Elf32_Ehdr* header = (const Elf32_Ehdr*)data;
+    const Elf32_Phdr* ph = (const Elf32_Phdr*)((uint8_t*)data + header->e_phoff);
+
+    for (int i = 0; i < header->e_phnum; i++) {
+        if (ph[i].p_type != PT_LOAD)
+            continue;
+
+        uint32_t offset = ph[i].p_offset;
+        uint32_t vaddr = ph[i].p_vaddr;
+        uint32_t filesz = ph[i].p_filesz;
+        uint32_t memsz = ph[i].p_memsz;
+
+        // Copy data segment
+        //load_segment((uint8_t *)vaddr, offset, filesz);
+        load_segment((uint8_t *)vaddr, offset, filesz);
+
+        // Zero remaining BSS (if any)
+        if (memsz > filesz) {
+            uint8_t* bss_start = (uint8_t*)(vaddr + filesz);
+            for (uint32_t j = 0; j < memsz - filesz; j++) {
+                bss_start[j] = 0;
+            }
+        }
+    }
+
+    return header->e_entry;
+}
+
+#define KERN_START_SECT 5
+#define MAX(a, b) ((a)>(b) ? (a) : (b))
+
+extern void ata_lba_read(uint32_t lba, uint8_t nsect, void *addr);
+extern uint8_t read_buf[];
+
+static uint32_t
+total_header_size(const Elf32_Ehdr *header) {
+    uint32_t phend = header->e_phoff + header->e_phentsize*header->e_phnum;
+
+    // Align to 512
+    return (phend + 511) & ~511;
+}
+
+static void read_sectors(uint8_t *vaddr, uint32_t offset, uint32_t size) {
+    // # of sectors to read
+    uint32_t rem_nsect = ((size + 511) & ~511) / 512;
+
+    // Current lba address, offset by the first sector already read
+    uint32_t lba = KERN_START_SECT + offset / 512;
+
+    // Max 255 sectors at a time
+    while (rem_nsect) {
+        uint8_t nsect = rem_nsect > 255 ? 255 : rem_nsect;
+        ata_lba_read(lba, nsect, vaddr);
+
+        vaddr += nsect * 512;
+        rem_nsect -= nsect;
+        lba += nsect;
+    }
+}
+
+void *load_kernel(void) {
+    // Read the first sector
+    ata_lba_read(KERN_START_SECT, 1, read_buf);
+
+    const Elf32_Ehdr* header = (const Elf32_Ehdr*)read_buf;
+
+    // Remaining data size, subtract the first 512B already read
+    uint32_t rem = total_header_size(header) - 512;
+
+    // Read the rest if necessary
+    if (rem)
+        read_sectors(read_buf+512, 512, rem);
+
+    elf_load(read_buf, read_sectors);
+
+    return (void *)header->e_entry;
+}

gdb.txt

@@ -0,0 +1,6 @@
+target remote :1234
+add-symbol-file build/stage1.elf
+add-symbol-file build/stage2.elf
+add-symbol-file build/kernel.elf
+hbreak *0x7c00
+c