I am writing a custom kernel, but I am getting an error when linking. I have taken the code and broken it down, but still no luck. Here's my code:
kernel.c:
#include <string.h>
// define our structure
typedef struct __attribute__ ((packed)) {
unsigned short di, si, bp, sp, bx, dx, cx, ax;
unsigned short gs, fs, es, ds, eflags;
} regs16_t;
// tell compiler our int32 function is external
extern void int32(unsigned char intnum, regs16_t *regs);
// int32 test
void int32_test()
{
int y;
regs16_t regs;
// switch to 320x200x256 graphics mode
regs.ax = 0x0013;
int32(0x10, ®s);
// full screen with blue color (1)
memset((char *)0xA0000, 1, (320*200));
// draw horizontal line from 100,80 to 100,240 in multiple colors
for(y = 0; y < 200; y++)
memset((char *)0xA0000 + (y*320+80), y, 160);
// wait for key
regs.ax = 0x0000;
int32(0x16, ®s);
// switch to 80x25x16 text mode
regs.ax = 0x0003;
int32(0x10, ®s);
}
kernel.asm:
[bits 32]
global int32, _int32
struc regs16_t
.di resw 1
.si resw 1
.bp resw 1
.sp resw 1
.bx resw 1
.dx resw 1
.cx resw 1
.ax resw 1
.gs resw 1
.fs resw 1
.es resw 1
.ds resw 1
.ef resw 1
endstruc
%define INT32_BASE 0x7C00
%define REBASE(x) (((x) - reloc) + INT32_BASE)
%define GDTENTRY(x) ((x) << 3)
%define CODE32 GDTENTRY(1)
%define DATA32 GDTENTRY(2)
%define CODE16 GDTENTRY(3)
%define DATA16 GDTENTRY(4)
%define STACK16 (INT32_BASE - regs16_t_size)
extern int32_test
global start
section .text
align 4
dd 0x1BADB002
dd 0x00
dd - (0x1BADB002 + 0x00)
int32: use32
_int32:
cli
pusha
mov esi, reloc
mov edi, INT32_BASE
mov ecx, (int32_end - reloc)
cld
rep movsb
jmp INT32_BASE
reloc: use32
mov [REBASE(stack32_ptr)], esp
sidt [REBASE(idt32_ptr)]
sgdt [REBASE(gdt32_ptr)]
lgdt [REBASE(gdt16_ptr)]
lea esi, [esp+0x24]
lodsd
mov [REBASE(ib)], al
mov esi, [esi]
mov edi, STACK16
mov ecx, regs16_t_size
mov esp, edi
rep movsb
jmp word CODE16:REBASE(p_mode16)
p_mode16: use16
mov ax, DATA16
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov eax, cr0
and al, ~0x01
mov cr0, eax
jmp word 0x0000:REBASE(r_mode16)
r_mode16: use16
xor ax, ax
mov ds, ax
mov ss, ax
lidt [REBASE(idt16_ptr)]
mov bx, 0x0870
call resetpic
popa
pop gs
pop fs
pop es
pop ds
sti
db 0xCD
ib: db 0x00
cli
xor sp, sp
mov ss, sp
mov sp, INT32_BASE
pushf
push ds
push es
push fs
push gs
pusha
mov bx, 0x2028
call resetpic
mov eax, cr0
inc eax
mov cr0, eax
jmp dword CODE32:REBASE(p_mode32)
p_mode32: use32
mov ax, DATA32
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
lgdt [REBASE(gdt32_ptr)]
lidt [REBASE(idt32_ptr)]
mov esp, [REBASE(stack32_ptr)]
mov esi, STACK16
lea edi, [esp+0x28]
mov edi, [edi]
mov ecx, regs16_t_size
cld
rep movsb
popa
sti
ret
resetpic:
push ax
mov al, 0x11
out 0x20, al
out 0xA0, al
mov al, bh
out 0x21, al
mov al, bl
out 0xA1, al
mov al, 0x04
out 0x21, al
shr al, 1
out 0xA1, al
shr al, 1
out 0x21, al
out 0xA1, al
pop ax
ret
stack32_ptr:
dd 0x00000000
idt32_ptr:
dw 0x0000
dd 0x00000000
gdt32_ptr:
dw 0x0000
dd 0x00000000
idt16_ptr:
dw 0x03FF
dd 0x00000000
gdt16_base:
.null:
dd 0x00000000
dd 0x00000000
.code32:
dw 0xFFFF
dw 0x0000
db 0x00
db 0x9A
db 0xCF
db 0x00
.data32:
dw 0xFFFF
dw 0x0000
db 0x00
db 0x92
db 0xCF
db 0x00
.code16:
dw 0xFFFF
dw 0x0000
db 0x00
db 0x9A
db 0x0F
db 0x00
.data16:
dw 0xFFFF
dw 0x0000
db 0x00
db 0x92
db 0x0F
db 0x00
gdt16_ptr:
dw gdt16_ptr - gdt16_base - 1
dd gdt16_base
int32_end:
start:
call int32_test
hlt
link.ld:
OUTPUT_FORMAT(elf32-i386)
ENTRY(start)
SECTIONS
{
. = 0x100000;
.text : { *(.text) }
.data : { *(.data) }
.bss : { *(.bss) }
}
Command Line & Output:
# gcc -fno-stack-protector -m32 -c kernel.c -o kc.o
# nasm -f elf32 kernel.asm -o kasm.o
# ld -m elf_i386 -T link.ld -o kernel kasm.o kc.o
kc.o: In function `int32_test':
kernel.c:(.text+0x2d): undefined reference to `memset'
kernel.c:(.text+0x5c): undefined reference to `memset'
A little bit of background: I am working on a custom OS for a custom project that I call AI Glasses. I am currently building a bootloader that will show a logo and some loading indicator. I am thinking that my indicator will be sort of like on Ubuntu. Anyways, I need to be able to go into a VGA mode and display stuff on the screen, while the rest of my (future) code can boot it up fully.
My question: How do I import memset's lib into my code so that ld can fully link my code.
The issue here is that linker doesn't find memset function in the object files you provide to ld. Neither kasm.o nor kc.o seem to have this function. In fact this is part of standard C library. So, you need to link standard C library, libc.so or libc.a. If you don't want to use standard C library in your kernel, you will need to provide your own implementation of memset