My puts function is not working in 16-bit code - gcc

Question

I'm writing a bootloader using GCC and a small assembler bootstrap routine. I've written a puts routine that prints a string to the display using BIOS interrupts that doesn't appear to write strings properly.
My bootstrap assembler file boot.s contains:
.code16 .section .text
.extern main
.globl start
start:
mov $0x7c0, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
mov %ax, %gs
jmp main
here:
hlt
jmp here
My C code in main.c is:
/*
* A 16 bit bootloader.
*/
void putchar_bios(unsigned char ch);
void set_videomode(unsigned short mode);
void puts(char str[]);
#define set_stack(ss, size) \
{ \
__asm__ __volatile__ ( \
"mov %%ax, %%ss\n" \
"mov $512, %%sp\n" : : "a" (ss), "r" (size)\
); \
}
#define set_videomode(mode) \
{ \
__asm__ __volatile__ ( \
"int $0x10\n" : : "a" (mode) \
); \
}
void putchar_bios(unsigned char ch)
{
__asm__ __volatile__ (
"int $0x10\n" : : "a" (0x0E | ch)
);
}
void puts(char *str)
{
while(*str)
putchar_bios(*str++);
}
void main()
{
set_stack(0x07C0, 512);
set_videomode(0x03);
char name[] = "0001234567890";
puts(name);
//This works fine.
// for(i=0; i<15; i++)
// putchar_bios(name[i]);
while(1);
}
I have successfully done this entirely in assembly, but now I'm trying to migrate it to GCC . I am using a cross-compiler (i386-gcc) and used -m16 flag also. I have used a custom linker script.
OUTPUT_FORMAT("binary");
ENTRY(start);
SECTIONS
{
. = 0x7C00;
.text : AT(0x7C00) {
*(.text);
}
.data : SUBALIGN(0) {
*(.data);
*(.rodata);
}
.bss : SUBALIGN(4) {
__bss_start = .;
*(.COMMON);
*(.bss)
. = ALIGN(4);
__bss_end = .;
}
__bss_sizel = SIZEOF(.bss)>>2;
__bss_sizeb = SIZEOF(.bss);
/* Boot signature */
.sig : AT(0x7DFE) {
SHORT(0xaa55);
}
}
The script I used to compile, link and run in QEMU are:
i386-elf-gcc -m16 -static -ffreestanding -nostdlib -c boot/boot.s
i386-elf-gcc -m16 -static -ffreestanding -nostdlib -c boot/main.c
i386-elf-ld -T link.ld -o b.bin -nostdlib --nmagic boot.o main.o
dd if=b.bin of=HD.img conv=notrunc
#add some noticable garbage to second sector since I also try to read it next
echo "This is the second sector..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................." | dd seek=512 bs=1 of=HD.img
qemu-system-i386 -hda HD.img # -boot a -s -S
Why does my program not display strings via my puts function properly?

Answer 1

I realized I set the values of segment registers to some garbage (0x7c0) which caused this to happen. I modified my assembly file to zero out the segment registers. The code now looks like:
.code16
.section .text
.extern main
.globl start
start:
xor %ax, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
mov %ax, %gs
jmp main
here:
hlt
jmp here
I had expected that the compiler would automatically initialize the segment registers, but it was not.