.code16
.text
.org 0x0
.global _start
_start:
jmp _testing
nop
_testing:
mov $0x0E, %ah
mov the_byte, %al #the line in question
int $0x10
jmp .
the_byte: .byte 0x41
.fill (510-(.-_start)), 1, 0
.word 0xAA55
This simple 'bootloader', as it were, is supposed to print out A to the screen, but it fails to do so when I'm using VMWare Workstation 16 (unlike Bochs, which happily shows A on its screen). If I change the line in question to
mov $0x41, %al
I can see A on VMWare Workstation as well.
Have you by any chance got any idea what can be causing such strange behaviour?
PS. It is indeed loaded into 0x7C00 with a separate linker file.
Apparently, mov the_byte, %al is assembled into something akin to mov %ds:0x7C0C, %al, hence DS had to be zeroed out (along with other segment pointers):
cli
mov %cs, %ax # CS is set to 0x0
mov %ax, %es # ES = CS = 0x0
mov %ax, %ds # DS = CS = 0x0
mov %ax, %ss # SS = CS = 0x0
sti
Related
I have had this happen before and worked around it for a while but now it slowly becomes more and more unavoidable, because now I need them.
For some weird reason, my kernel crashes when I try to use a global variable in my code.
This works:
int global;
void kmain()
{
//do some stuff...
}
This does not work:
int global;
void kmain()
{
global = 1;
//do some stuff...
}
I have no idea why this is happening.
As some additional resources here is my linker script:
OUTPUT_FORMAT(binary)
phys = 0x0500;
SECTIONS
{
.text phys : AT(phys) {
code = .;
*(.text)
*(.rodata)
. = ALIGN(4096);
}
.data : AT(phys + (data - code))
{
data = .;
*(.data)
. = ALIGN(4096);
}
.bss : AT(phys + (bss - code)) {
bss = .;
*(.bss)
. = ALIGN(4096);
}
end = .;
/DISCARD/
: {
*(.comment)
*(.eh_frame)
*(.note.gnu.build-id)
}
}
and my makefile:
bin/UmbrellaOS.img: bin/boot.bin bin/kernel.bin bin/zeros.bin
cat $^ > $#
bin/kernel.bin: tmp/kernel_entry.o tmp/kernel.o
x86_64-elf-ld -o $# -T link.ld $^
tmp/kernel.o: src/kernel/main.c
x86_64-elf-gcc -ffreestanding -m64 -g -c $^ -o $#
Edit:
To be more specific I use QEMU to test my OS upon starting QEMU it instantly closes. It should also be noted that if I try something like this:
int global;
void kmain()
{
return;
global = 0;
}
it works for some reason.
I can see a green L printed to the screen which is the last thing my bootloader does before passing control to the kernel after long mode has been entered.
btw here is my bootloader:
[bits 16]
[org 0x7C00]
KERNEL_LOC equ 0x0500
_start:
mov [_BootDisk], dl
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov bp, 0x7BFF
mov sp, bp
push 0x7E00 ; buffer
push 1 ; sectors to read
push 2 ; sector num
call DiskRead
jc .error
push ebx
pushfd
pop eax
mov ebx, eax
xor eax, 0x200000
push eax
popfd
pushfd
pop eax
cmp eax, ebx
jnz .supported
push _CpuErrorString
call Print
jmp .error
.supported:
mov eax, 0x80000000
cpuid
cmp eax, 0x80000001
jb .no64
mov eax, 0x80000001
cpuid
test edx, 1 << 29
jnz .is64
.no64:
push _64ErrorString
call Print
jmp .error
.is64:
push 0x8000
call MapMem
push KERNEL_LOC ; buffer
push 8 ; sectors to read
push 3 ; sector num
call DiskRead
jc .error
cli
lgdt [GDT_descriptor]
mov eax, cr0
or eax, 1
mov cr0, eax
jmp CODE_SEG:protected_mode
.error:
jmp $
Print:
push bp
mov bp, sp
mov bx, [bp+4]
mov ah, 0x0E
.loop:
mov al, [bx]
cmp al, 0
je .end
int 0x10
inc bx
jmp .loop
.end:
mov sp, bp
pop bp
ret 2
DiskRead:
push bp
mov bp, sp
mov ah, 0x02
mov al, [bp+6]
mov ch, 0
mov cl, [bp+4]
mov dh, 0
mov dl, [_BootDisk]
mov bx, [bp+8]
int 0x13
cmp al, [bp+6]
je .end
jnc .end
push _DiskErrorString
call Print
.end:
mov sp, bp
pop bp
ret 6
MapMem:
push bp
mov bp, sp
mov si, [bp+4]
mov di, [bp+4]
add di, 4
xor ebx, ebx
mov edx, 0x0534D4150
mov eax, 0xE820
mov [di+20], dword 1
mov ecx, 24
int 0x15
jc .failed
mov edx, 0x0534D4150
cmp eax, edx
jne .failed
test ebx, ebx
je .failed
.loop:
mov eax, 0xE820
mov [di+20], dword 1
mov ecx, 24
int 0x15
jc .finish
mov edx, 0x0534D4150
.jmpin:
jcxz .skip
cmp cl, 20
jbe .notext
test byte [di+20], 1
je .skip
.notext:
mov ecx, [di+8]
or ecx, [di+12]
jz .skip
inc dword [si]
add di, 24
.skip:
test ebx, ebx
jne .loop
.finish:
clc
jmp .end
.failed:
push _MemErrorString
call Print
stc
jmp .end
.end:
mov sp, bp
pop bp
ret 2
_BootDisk: db 0
_DiskErrorString: db "Disk read error!", 13, 10, 0
_MemErrorString: db "Memory mapping failed!", 13, 10, 0
_CpuErrorString: db "CPUID not supported!", 13, 10, 0
_64ErrorString: db "x64 bits not supported!", 13, 10, 0
CODE_SEG equ GDT_code - GDT_start
DATA_SEG equ GDT_data - GDT_start
GDT_start:
GDT_null:
dd 0x0
dd 0x0
GDT_code:
dw 0xffff
dw 0x0
db 0x0
db 0b10011010
db 0b11001111
db 0x0
GDT_data:
dw 0xffff
dw 0x0
db 0x0
db 0b10010010
db 0b11001111
db 0x0
GDT_end:
GDT_descriptor:
dw GDT_end - GDT_start - 1
dd GDT_start
times 510-($-$$) db 0
dw 0xAA55
[bits 32]
protected_mode:
mov ax, DATA_SEG
mov ds, ax
mov ss, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ebp, 0x90000
mov esp, ebp
call Clear
mov ebx, VGA_MEM
mov byte [ebx], 'P'
inc ebx
mov byte [ebx], 14
mov eax, cr0
and eax, ~(1 << 31)
mov cr0, eax
mov edi, 0x1000
mov cr3, edi
xor eax, eax
mov ecx, 4096
rep stosd
mov edi, cr3
mov dword [edi], 0x2003
add edi, 0x1000
mov dword [edi], 0x3003
add edi, 0x1000
mov dword [edi], 0x4003
add edi, 0x1000
mov ebx, 0x00000003
mov ecx, 512
.set_entry:
mov dword [edi], ebx
add ebx, 0x1000
add edi, 8
loop .set_entry
mov eax, cr4
or eax, 1 << 5
mov cr4, eax
mov ecx, 0xC0000080
rdmsr
or eax, 1 << 8
wrmsr
mov eax, cr0
or eax, 1 << 31
mov cr0, eax
lgdt [GDT.Pointer]
jmp GDT.Code:long_mode
jmp $
Clear:
push ebp
mov ebp, esp
mov ecx, VGA_SIZE
mov eax, VGA_MEM
.loop:
mov byte [eax], 0
inc eax
loop .loop
mov esp, ebp
pop ebp
ret
PRESENT equ 1 << 7
NOT_SYS equ 1 << 4
EXEC equ 1 << 3
RW equ 1 << 1
ACCESSED equ 1 << 0
GRAN_4K equ 1 << 7
SZ_32 equ 1 << 6
LONG_MODE equ 1 << 5
GDT:
.Null: equ $ - GDT
dq 0
.Code: equ $ - GDT
dd 0xFFFF
db 0
db PRESENT | NOT_SYS | EXEC | RW
db GRAN_4K | LONG_MODE | 0xF
db 0
.Data: equ $ - GDT
dd 0xFFFF
db 0
db PRESENT | NOT_SYS | RW
db GRAN_4K | SZ_32 | 0xF
db 0
.TSS: equ $ - GDT
dd 0x00000068
dd 0x00CF8900
.Pointer:
dw $ - GDT - 1
dq GDT
[bits 64]
long_mode:
cli
mov ax, GDT.Data
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov rbp, 0x0007FFFF
mov rsp, rbp
mov rbx, VGA_MEM
mov byte [rbx], 'L'
inc rbx
mov byte [rbx], 2
jmp KERNEL_LOC
VGA_MEM equ 0xB8000
VGA_WIDTH equ 80
VGA_HEIGHT equ 25
VGA_STRIDE equ 2
VGA_SIZE equ VGA_WIDTH * VGA_STRIDE * VGA_HEIGHT
VGA_LENGTH equ VGA_WIDTH * VGA_HEIGHT
times 1024-($-$$) db 0
And for anyone wanting to see the big picture here's the Github repository I made.
The problem was that I simply forgot that I put my page table structures at 0x1000 and accidentally overrode them when loading my kernel at 0x0500.
I ended up leaving the structures at 0x1000 and moved my kernel to 0x5000.
This was a rather simple problem but I would recommend that you still take a look at the comments because there's still a lot of useful information and things to consider.
First: ALWAYS Initialize variables especially global ones.
Second: The problem is surely from your bootloader, can you edit the post to show us how you load your kernel?
Try objdump to see if the variable is declared, use -monitor stdio with QEMU and check the value of CR2 Register, it may be a page fault due to the second problem.
Here is a solution to check if the variable really has a valid pointer:
You can remove these edits after everything is ok.
instead of :
jmp KERNEL_LOC
do:
call KERNEL_LOC ; RAX Has the pointer of the global variable
jmp $
In kmain just type:
return &global
Then run it on QEMU and type in the console info registers, RAX should contain the pointer of the variable named global.
So I have this code here were I'm trying to add two numbers in it but I can't seem to get the output from this code that I've been trying do for a while. :
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
mov edx, '6'
sub edx,'0'
mov ecx ,'7'
sub ecx,'0'
add edx,ecx
mov [math_sum], edx
mov eax,msg
mov ebx, len
mov ecx,1
mov edx,4
int 0x80
mov eax,math_sum
mov ebx,1
mov ecx,1
mov edx,4
int 0x80
mov edx,1
int 0x80
section .data
msg db "Sum of 6 and 7 is:"
len equ $ - msg
segment .bss
math_sum resb 2
I'm getting Segmentation fault but I don't know how to fix it.
I am trying to make the Lisp function cons in x86_84 assembly on MacOS. Below I am trying to make a pair of 2 and 3, but it is not working; I am getting a segmentation fault.
.global _main
.extern _malloc
.text
.macro make_node register
mov rdi, 8 # 64-bit number
call _malloc # failed on malloc
mov [rax], \register # contents of register in address of rax
mov \register, [rax]
.endm
cons:
push rbp
mov rbp, rsp
mov r8, [rbp + 16]
make_node r8
mov r9, [rbp + 24]
make_node r9
mov rsp, rbp
pop rbp
ret
_main:
push 3
push 2
call cons
add rsp, 16
# I should now be able to do whatever I want with r8 (2) and r9 (3)
mov rdi, 0
mov rax, 0x2000001
syscall
I stepped through it with GDB and I see that it failed on calling malloc, but to me, there doesn't seem to be a problem since malloc only takes one argument (the number of bytes to allocate) in the rdi register.
Dump of assembler code for function cons:
0x0000000100003f48 <+0>: push %rbp
0x0000000100003f49 <+1>: mov %rsp,%rbp
0x0000000100003f4c <+4>: mov 0x10(%rbp),%r8
0x0000000100003f50 <+8>: mov $0x8,%rdi
=> 0x0000000100003f57 <+15>: callq 0x100003f96
0x0000000100003f5c <+20>: mov %r8,(%rax)
0x0000000100003f5f <+23>: mov (%rax),%r8
0x0000000100003f62 <+26>: mov 0x18(%rbp),%r9
0x0000000100003f66 <+30>: mov $0x8,%rdi
0x0000000100003f6d <+37>: callq 0x100003f96
0x0000000100003f72 <+42>: mov %r9,(%rax)
0x0000000100003f75 <+45>: mov (%rax),%r9
0x0000000100003f78 <+48>: mov %rbp,%rsp
0x0000000100003f7b <+51>: pop %rbp
0x0000000100003f7c <+52>: retq
End of assembler dump.
(gdb) ni
Thread 2 received signal SIGSEGV, Segmentation fault.
I am assembling on a Mac like this: clang -masm=intel cell.asm.
Does anyone familiar with x86 assembly know the source of my error?
(Also, in case anyone asks, I know that it's important to call free after malloc but this code is the only code necessary to demonstrate my problem.)
This is a part of a bootloader that I am studying from
`[ORG 0x00]
[BITS 16]
SECTION .text
jmp 0x07c0:START ; set CS(segment register) to 0x07C0 and jump to START label.
TOTALSECTORCOUNT:
dw 0x02
KERNEL32SECTORCOUNT:
dw 0x02
START:
mov ax, 0x07c0
mov ds, ax ; set DS(segment register) to the address of the bootloader.
mov ax, 0xb800
mov es, ax ; set ES(segment register) to the address of the video memory starting address.
; stack initialization
mov ax, 0x0000
mov ss, ax
mov sp, 0xfffe
mov bp, 0xfffe
; clear the screen
mov si, 0
CLEARSCREEN:
mov byte[es:si], 0
mov byte[es:si + 1], 0x0a
add si, 2
cmp si, 80 * 25 * 2
jl CLEARSCREEN
; print welcome message`
I don't understand the beginning: jmp 0x07C0:START How does it set the CS register?
And what are the two variables TOTALSECTORCOUNT and KERNEL32SECTORCOUNT for? They don't appear anywhere in the bootsector file and if I remove them, the bootloader fails to load the welcome message.
Removing the parts causes the OS to fail to load. So what is the significance of that jmp statement and the two variables?
``[ORG 0x00]
[BITS 16]
jmp START
START:
mov ax, 0x07c0
mov ds, ax ; set DS(segment register) to the address of the bootloader.
mov ax, 0xb800
mov es, ax ; set ES(segment register) to the address of the video memory starting address.
; stack initialization
mov ax, 0x0000
mov ss, ax
mov sp, 0xfffe
mov bp, 0xfffe
`
I am not great with assembly and usually use the AT&T syntax also. I have however written a bootloader before.
Hopefully you have learnt about the segmented addressing system used in 16 bit applications. The cs register holds the code segment. http://wiki.osdev.org/Segmentation
jmp 0x07C0:START ;This is a long jump
jmp segment:offset
A long jump sets the cs register to segment parameter and then does a jump to the offset parameter. When you do a short jump the cs register doesn't change. I assume that it would contain 0x0. You can use a short jump but you must tell your assembler or linker where the code will be run.
EDIT: After reading the code again there is the [org 0x00] line. This sets the cs register to 0x00 by default. If you wanted to use the short jump try changing this line to [org 0x7c00]
CS should already be set to 0x7c00 by the BIOS so the line:
jmp 0x07c0:START
can be replaced by:
jmp START
The two variables you mention must be use elsewhere in the code to load the kernel. However, it appears you haven't posted the whole code here.
Without seeing the rest of the bootsector code, we cannot help.
Closed. This question is not reproducible or was caused by typos. It is not currently accepting answers.
This question was caused by a typo or a problem that can no longer be reproduced. While similar questions may be on-topic here, this one was resolved in a way less likely to help future readers.
Closed 3 years ago.
Improve this question
I am writing a toy kernel for learning purposes, and I am having a bit of trouble with it. I have made a simple bootloader that loads a segment from a floppy disk (which is written in 32 bit code), then the bootloader enables the A20 gate and turns on protected mode. I can jump to the 32 bit code fine if I write it in assembler, but if I write it in C, I get a triple fault. When I disassemble the C code I can see that the first two instructions involve setting up a new stack frame. This is the key difference between the working ASM code and the failing C code.
I am using NASM v2.10.05 for the ASM code, and GCC from the DJGPP 4.72 collection for the C code.
This is the bootloader code:
org 7c00h
BITS 16
entry:
mov [drive], dl ;Save the current drive
cli
mov ax,cs ; Setup segment registers
mov ds,ax ; Make DS correct
mov ss,ax ; Make SS correct
mov bp,0fffeh
mov sp,0fffeh ;Setup a temporary stack
sti
;Set video mode to text
;===================
mov ah, 0
mov al, 3
int 10h
;===================
;Set current page to 0
;==================
mov ah, 5
mov al, 0
int 10h
;==================
;Load the sector
;=============
call load_image
;=============
;Clear interrupts
;=============
cli
;=============
;Disable NMIs
;============
in ax, 70h
and ax, 80h ;Set the high bit to 1
out 70h, ax
;============
;Enable A20:
;===========
mov ax, 02401h
int 15h
;===========
;Load the GDT
;===========
lgdt [gdt_pointer]
;===========
;Clear interrupts
;=============
cli
;=============
;Enter protected mode
;==================
mov eax, cr0
or eax, 1 ;Set the low bit to 1
mov cr0, eax
;==================
jmp 08h:clear_pipe ;Far jump to clear the instruction queue
;======================================================
load_image:
reset_drive:
mov ah, 00h
; DL contains *this* drive, given to us by the BIOS
int 13h
jc reset_drive
read_sectors:
mov ah, 02h
mov al, 01h
mov ch, 00h
mov cl, 02h
mov dh, 00h
; DL contains *this* drive, given to us by the BIOS
mov bx, 7E0h
mov es, bx
mov bx, 0
int 13h
jc read_sectors
ret
;======================================================
BITS 32 ;Protected mode now!
clear_pipe:
mov ax, 10h ; Save data segment identifier
mov ds, ax ; Move a valid data segment into the data segment register
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax ; Move a valid data segment into the stack segment register
mov esp, 90000h ; Move the stack pointer to 90000h
mov ebp, esp
jmp 08h:7E00h ;Jump to the kernel proper
;===============================================
;========== GLOBAL DESCRIPTOR TABLE ==========
;===============================================
gdt: ; Address for the GDT
gdt_null: ; Null Segment
dd 0
dd 0
gdt_code: ; Code segment, read/execute, nonconforming
dw 0FFFFh ; LIMIT, low 16 bits
dw 0 ; BASE, low 16 bits
db 0 ; BASE, middle 8 bits
db 10011010b ; ACCESS byte
db 11001111b ; GRANULARITY byte
db 0 ; BASE, low 8 bits
gdt_data: ; Data segment, read/write, expand down
dw 0FFFFh
dw 0
db 0
db 10010010b
db 11001111b
db 0
gdt_end: ; Used to calculate the size of the GDT
gdt_pointer: ; The GDT descriptor
dw gdt_end - gdt - 1 ; Limit (size)
dd gdt ; Address of the GDT
;===============================================
;===============================================
drive: db 00 ;A byte to store the current drive in
times 510-($-$$) db 00
db 055h
db 0AAh
And this is the kernel code:
void main()
{
asm("mov byte ptr [0x8000], 'T'");
asm("mov byte ptr [0x8001], 'e'");
asm("mov byte ptr [0x8002], 's'");
asm("mov byte ptr [0x8003], 't'");
}
The kernel simply inserts those four bytes into memory, which I can check as I am running the code in a VMPlayer virtual machine. If the bytes appear, then I know the code is working. If I write code in ASM that looks like this, then the program works:
org 7E00h
BITS 32
main:
mov byte [8000h], 'T'
mov byte [8001h], 'e'
mov byte [8002h], 's'
mov byte [8003h], 't'
hang:
jmp hang
The only differences are therefore the two stack operations I found in the disassembled C code, which are these:
push ebp
mov ebp, esp
Any help on this matter would be greatly appreciated. I figure I am missing something relatively minor, but crucial, here, as I know this sort of thing is possible to do.
Try using the technique here:
Is there a way to get gcc to output raw binary?
to produce a flat binary of the .text section from your object file.
I'd try moving the address of your protected mode stack to something else - say 0x80000 - which (according to this: http://wiki.osdev.org/Memory_Map_(x86)) is RAM which is guaranteed free for use (as opposed to the address you currently use - 0x90000 - which apparently may not be present - depending a bit on what you're using as a testing environment).