I don't recommend GCC for 16-bit code. A GCC alternative may be the separate IA16-GCC project which is a work in progress and is experimental.
It is hard to get GCC to emit proper real-mode code because of the need for inline assembly. GCC's inline assembly is difficult to get right if you wish to avoid subtle bugs especially when optimizations are enabled. It is possible to write such code but I strongly advise against it.
You don't have a linker script so your compiled C code was placed after the bootloader signature. The BIOS only reads one sector into memory. Your jmp kmain ends up jumping to memory where the kernel would have been had it actually been loaded into memory, but it wasn't loaded so it fails to work as expected. You need to add code to call BIOS Int 13/AH=2 to read additional disk sectors starting from Cylinder, Head, Sector (CHS) = (0,0,2) which is the sector right after the bootloader.
Your bootloader doesn't properly set up the segment registers. Because you are using GCC, it expects CS=DS=ES=SS. Since we need to load data into memory we need to put the stack somewhere safe. The kernel will be loaded to 0x0000:0x7e00, so we can place the stack below the bootloader at 0x0000:0x7c00 where they won't conflict. You need to clear the direction flag (DF) with CLD before calling GCC as it is a requirement. Many of these issues are captured in my General Bootloader Tips. A more complex bootloader that determines the size of the kernel (stage2) and reads the appropriate number of sectors from disk can be found in my other Stackoverflow answer.
We need a linker script to properly lay things out in memory and ensure the instruction(s) at the very beginning jumps to the real C entry point kmain. We also need to properly zero out the BSS section because GCC expects that. The linker script is used to determine the beginning and the end of the BSS section. The function zero_bss clears that memory to 0x00.
The Makefile could be cleaned up a bit to make adding code easier in the future. I've amended the code so the object files get built in the src directory. This simplifies the make processing.
When the real-mode code support was introduced and support added to GNU assembler it was enabled in GCC by using asm (".code16gcc");. For quite some time now GCC has supported the -m16 option that does the same thing. With -m16 you don't need to add the .code16gcc directive to the top of all the files.
I haven't modified your inline assembly that prints a to the screen. Just because I didn't modify it, doesn't mean that it doesn't have problems. Since registers are clobbered and the compiler isn't told of that it can lead to strange bugs especially when optimizations are on. The second part of this answer shows a mechanism to use the BIOS to print characters and strings to the console with proper inline assembly.
I recommend the compiler options -Os -mregparm=3 -fomit-frame-pointer to optimize for space.
Makefile:
CROSSPRE=i686-elf-
CC=$(CROSSPRE)gcc
LD=$(CROSSPRE)ld
OBJCOPY=$(CROSSPRE)objcopy
DD=dd
NASM=nasm
DIR_SRC=src
DIR_BIN=bin
DIR_BUILD=build
KERNEL_NAME=jasos
KERNEL_BIN=$(DIR_BIN)/$(KERNEL_NAME).bin
KERNEL_ELF=$(DIR_BIN)/$(KERNEL_NAME).elf
BOOTLOADER_BIN=$(DIR_BIN)/bootloader.bin
BOOTLOADER_ASM=$(DIR_SRC)/bootloader.asm
DISK_IMG=$(DIR_BUILD)/disk.img
CFLAGS=-g -fno-PIE -static -std=gnu99 -m16 -Os -mregparm=3
-fomit-frame-pointer -nostdlib -ffreestanding -Wall -Wextra
LDFLAGS=-melf_i386
# List all object files here
OBJS=$(DIR_SRC)/god.o
.PHONY: all clean
all: $(DISK_IMG)
$(BOOTLOADER_BIN): $(BOOTLOADER_ASM)
$(NASM) -f bin $< -o $@
%.o: %.c
$(CC) -c $(CFLAGS) $< -o $@
$(KERNEL_ELF): $(OBJS)
$(LD) $(LDFLAGS) -Tlink.ld $^ -o $@
$(KERNEL_BIN): $(KERNEL_ELF)
$(OBJCOPY) -O binary $< $@
$(DISK_IMG): $(KERNEL_BIN) $(BOOTLOADER_BIN)
$(DD) if=/dev/zero of=$@ bs=1024 count=1440
$(DD) if=$(BOOTLOADER_BIN) of=$@ conv=notrunc
$(DD) if=$(KERNEL_BIN) of=$@ conv=notrunc seek=1
clean:
rm -f $(DIR_BIN)/*
rm -f $(DIR_BUILD)/*
rm -f $(DIR_SRC)/*.o
link.ld:
OUTPUT_FORMAT("elf32-i386");
ENTRY(kmain);
SECTIONS
{
. = 0x7E00;
.text.main : SUBALIGN(0) {
*(.text.bootstrap);
*(.text.*);
}
.data.main : SUBALIGN(4) {
*(.data);
*(.rodata*);
}
.bss : SUBALIGN(4) {
__bss_start = .;
*(.COMMON);
*(.bss)
}
. = ALIGN(4);
__bss_end = .;
__bss_sizel = ((__bss_end)-(__bss_start))>>2;
__bss_sizeb = ((__bss_end)-(__bss_start));
/DISCARD/ : {
*(.eh_frame);
*(.comment);
}
}
src/god.c:
#include <stdint.h>
/* The linker script ensures .text.bootstrap code appears first.
* The code simply jumps to our real entrypoint kmain */
asm (".pushsection .text.bootstrap
"
"jmp kmain
"
".popsection");
extern uintptr_t __bss_start[];
extern uintptr_t __bss_end[];
/* Zero the BSS section */
static inline void zero_bss()
{
uint32_t *memloc = __bss_start;
while (memloc < __bss_end)
*memloc++ = 0;
}
/* JASOS kernel C entrypoint */
void kmain()
{
/* We need to zero out the BSS section */
zero_bss();
asm (
"movb $0, %dl;"
"inc %dh;"
"movb $2, %ah;"
"movb $0, %bh;"
"int $0x10;"
"movb $'a', %al;"
"movb $10, %ah;"
"movw $1, %cx;"
"int $0x10;"
);
return;
}
src/bootloader.asm:
; Allows our code to be run in real mode.
BITS 16
ORG 0x7c00
_start:
xor ax, ax ; DS=ES=0
mov ds, ax
mov es, ax
mov ss, ax ; SS:SP=0x0000:0x7c00
mov sp, 0x7c00
cld ; Direction flag = 0 (forward movement)
; Needed by code generated by GCC
; Read 17 sectors starting from CHS=(0,0,2) to 0x0000:0x7e00
; 17 * 512 = 8704 bytes (good enough to start with)
mov bx, 0x7e00 ; ES:BX (0x0000:0x7e00) is memory right after bootloader
mov ax, 2<<8 | 17 ; AH=2 Disk Read, AL=17 sectors to read
mov cx, 0<<8 | 2 ; CH=Cylinder=0, CL=Sector=2
mov dh, 0 ; DH=Head=0
int 0x13 ; Do BIOS disk read
jmp 0x0000:Start ; Jump to start set CS=0
; Moves the cursor to row dl, col dh.
MoveCursor:
mov ah, 2
mov bh, 0
int 10h
ret
; Prints the character in al to the screen.
PrintChar:
mov ah, 10
mov bh, 0
mov cx, 1
int 10h
ret
; Set cursor position to 0, 0.
ResetCursor:
mov dh, 0
mov dl, 0
call MoveCursor
ret
Start:
call ResetCursor
; Clears the screen before we print the boot message.
; QEMU has a bunch of crap on the screen when booting.
Clear:
mov al, ' '
call PrintChar
inc dl
call MoveCursor
cmp dl, 80
jne Clear
mov dl, 0
inc dh
call MoveCursor
cmp dh, 25
jne Clear
; Begin printing the boot message.
Msg:
call ResetCursor
mov si, BootMessage
NextChar:
lodsb
call PrintChar
inc dl
call MoveCursor
cmp si, End
jne NextChar
call dword 0x7e00 ; Because GCC generates code with stack
; related calls that are 32-bits wide we
; need to specify `DWORD`. If we don't, when
; kmain does a `RET` it won't properly return
; to the code below.
; Infinite ending loop when kmain returns
cli
.endloop:
hlt
jmp .endloop
BootMessage: db "Booting..."
End:
; Zerofill up to 510 bytes
times 510 - ($ - $$) db 0
; Boot Sector signature
dw 0AA55h
A 1.44MiB floppy disk image called build/disk.img is created. It can be run in QEMU with a command like:
qemu-system-i386 -fda build/disk.img
The expected output should look similar to:
Proper use of Inline Assembly to Write a String Using the BIOS
A version of the code that uses more complex GCC extended inline assembly is presented below. This answer is not meant to be a discussion on GCC's extended inline assembly usage, but there is information online about it. It should be noted that there is a lot of bad advice, documentation, tutorials, and sample code fraught with problems written by people who may not have had a proper understanding of the subject. You have been warned!1
Makefile:
CROSSPRE=i686-elf-
CC=$(CROSSPRE)gcc
LD=$(CROSSPRE)ld
OBJCOPY=$(CROSSPRE)objcopy
DD=dd
NASM=nasm
DIR_SRC=src
DIR_BIN=bin
DIR_BUILD=build
KERNEL_NAME=jasos
KERNEL_BIN=$(DIR_BIN)/$(KERNEL_NAME).bin
KERNEL_ELF=$(DIR_BIN)/$(KERNEL_NAME).elf
BOOTLOADER_BIN=$(DIR_BIN)/bootloader.bin
BOOTLOADER_ASM=$(DIR_SRC)/bootloader.asm
DISK_IMG=$(DIR_BUILD)/disk.img
CFLAGS=-g -fno-PIE -static -std=gnu99 -m16 -Os -mregparm=3
-fomit-frame-pointer -nostdlib -ffreestanding -Wall -Wextra
LDFLAGS=-melf_i386
# List all object files here
OBJS=$(DIR_SRC)/god.o $(DIR_SRC)/biostty.o
.PHONY: all clean
all: $(DISK_IMG)
$(BOOTLOADER_BIN): $(BOOTLOADER_ASM)
$(NASM) -f bin $< -o $@
%.o: %.c
$(CC) -c $(CFLAGS) $< -o $@
$(KERNEL_ELF): $(OBJS)
$(LD) $(LDFLAGS) -Tlink.ld $^ -o $@
$(KERNEL_BIN): $(KERNEL_ELF)
$(OBJCOPY) -O binary $< $@
$(DISK_IMG): $(KERNEL_BIN) $(BOOTLOADER_BIN)
$(DD) if=/dev/zero of=$@ bs=1024 count=1440
$(DD) if=$(BOOTLOADER_BIN) of=$@ conv=notrunc
$(DD) if=$(KERNEL_BIN) of=$@ conv=notrunc seek=1
clean:
rm -f $(DIR_BIN)/*
rm -f $(DIR_BUILD)/*
rm -f $(DIR_SRC)/*.o
link.ld:
OUTPUT_FORMAT("elf32-i386");
ENTRY(kmain);
SECTIONS
{
. = 0x7E00;
.text.main : SUBALIGN(0) {
*(.text.bootstrap);
*(.text.*);
}
.data.main : SUBALIGN(4) {
*(.data);
*(.rodata*);
}
.bss : SUBALIGN(4) {
__bss_start = .;
*(.COMMON);
*(.bss)
}
. = ALIGN(4);
__bss_end = .;
__bss_sizel = ((__bss_end)-(__bss_start))>>2;
__bss_sizeb = ((__bss_end)-(__bss_start));
/DISCARD/ : {
*(.eh_frame);
*(.comment);
}
}
src/biostty.c:
#include <std
