While loop not writing to video memory [duplicate] - c

Question

This question already has answers here:
Unexpected output when printing directly to text video memory
(3 answers)
Closed 3 years ago.
I'm writing my own OS as a homework assignment and I'm having some trouble with my C code. I've set up my boot headers and linker files as well as a simple assembler script which prints 'H' and then calls my C code. The assembler code prints to video memory fine (I'm printing to 0xb8000) but my while loop in my C code is not.
const char *str = "My first kernel";
char *vidptr = (char*)0xb8002; //My ASM code writes to 0xb8000 for testing purposes
while(*str){
*vidptr++ = 0x07;
*vidptr++ = *str++;
}
vidptr[0] = 0x07;
vidptr[1] = 'f';
The 'H' and 'f' characters print just fine, but my loop isn't printing anything to the screen. I've tried multiple ways of writing this loop such as
unsigned int j = 0;
char c;
while(c = *(str++)){
vidptr[j] = 0x07;
vidptr[j+1] = c;
j+=2;
}
and of course I've tried
int i, j;
while(str[i] != '\0'){
vidptr[j] = 0x07;
vidptr[j+1] = str[i];
i++;
j+=2;
}
but nothing seems to work. I'm rather confused.
EDIT:
I made a slight change which resulted in the characters in str to be printed to the screen. I simply changed vidptr to a pointer to an integer.
int *vidptr = (int*)0xb8000;
This seems to have worked. But, I'm still confused as to why it wasn't working when vidptr was declared as a char pointer. Even when I declared it volatile I still was not getting output to the screen.
EDIT 2:
By request, here is the working ASM code.
global start
extern main
section .text
bits 32
start:
mov word [0xb8000], 0x0748
mov esp, stack_space
call main
hlt
section .bss
resb 8192
stack_space:
main is the function in my C code that I have provided above.
A complete copy of my source code can be found in my Github Repository. My Makefile is:
default: build
.PHONY: clean
build/boot_header.o: multiboot_header.asm
mkdir -p build
nasm -f elf64 multiboot_header.asm -o build/boot_header.o
build/asmk.o: kernel.asm
mkdir -p build
nasm -f elf64 kernel.asm -o build/asmk.o
build/ckern.o: kernel.c
gcc -m64 -c kernel.c -o build/ckern.o
build/kernel.bin: build/boot_header.o build/asmk.o build/ckern.o link.ld
ld -n -o build/kernel.bin -T link.ld build/boot_header.o build/ckern.o build/asmk.o
build/os.iso: build/kernel.bin grub.cfg
mkdir -p build/isofiles/boot/grub
cp grub.cfg build/isofiles/boot/grub
cp build/kernel.bin build/isofiles/boot/
grub-mkrescue -o build/os.iso build/isofiles
run: build/os.iso
qemu-system-x86_64 -curses -cdrom build/os.iso
build: build/os.iso
clean:
rm -rf build

Answer 1

From the comments above:
In your update you did mov word [0xb8000], 0x0748. That moves the 16-bit word 0x0748 to 0xb8000 . Because x86 is little endian a word is stored least significant byte to most significant byte when placed in memory. So 0xb8000 would have 0x48 in it and 0x07 at 0b8001 . See my comment above about you having the bytes reversed. – Michael Petch Nov 21 '16 at 19:33