Why does my kernel crash when I add more arguments to a function?-CodePudding

I am trying to build a custom kernel in C and I want to put a colored pixel on the screen.
I already switched into protected mode and set the VGA mode to 0x13, so I wrote a function in C to put a pink pixel on the right upper corner of the screen:

#include <stdint.h>

#define VGA_MODE 0x13
#define VRAM 0xA0000
#define MAX_HEIGHT 200
#define MAX_LENGTH 320

void PutPixel(uint8_t color, uint16_t x)
{
    // Return if x greater than 320
    if(x > MAX_LENGTH) return;

    unsigned char* ptr = (unsigned char*) VRAM;

    ptr  = x;

    *ptr = color;

    return;
}

void KERNEL_MAIN()
{
    PutPixel(5, 320);

    return;
}

This code runs perfectly fine, until I want to add a y-offset as parameter to my PutPixel function in order to draw a pixel on another row:

#include <stdint.h>

#define VGA_MODE 0x13
#define VRAM 0xA0000
#define MAX_HEIGHT 200
#define MAX_LENGTH 320

void PutPixel(uint8_t color, uint16_t x, uint16_t y)
{
    // Return if x greater than 320
    if(x > MAX_LENGTH) return;

    unsigned char* ptr = (unsigned char*) VRAM;

    ptr  = x;

    *ptr = color;

    return;
}

void KERNEL_MAIN()
{
    PutPixel(5, 320, 1);

    return;
}

Even when I don't actually use the parameter, when I run the code in VirtualBox, it crashes immediately and I get following error in the VirtualBox debugger:

dbgf event: DBGFSTOP (hyper)
File:     VINF_EM_TRIPLE_FAULT
Line:     0
Function: <NULL>
eax=000a013e ebx=00007eae ecx=00000005 edx=00000005 esi=00000000 edi=0000fff0
eip=00000083 esp=0002ffac ebp=490002ff iopl=0 nv up di pl nz na pe nc
cs=0008 ds=0010 es=0010 fs=0010 gs=0010 ss=0010               eflags=00200002
0008:00000083 f0 53                   Illegal opcode
VBoxDbg>

Can I get some help please?

EDIT: I found that the stack got messed up by one byte, so I disassembled the function call and it looks like this:

%0000000000007ee7 55                      push ebp
%0000000000007ee8 48                      dec eax
%0000000000007ee9 89 e5                   mov ebp, esp
%0000000000007eeb 48                      dec eax
%0000000000007eec 83 ec 10                sub esp, byte 000000010h
%0000000000007eef 89 d0                   mov eax, edx
%0000000000007ef1 44                      inc esp               ;???
%0000000000007ef2 89 c2                   mov edx, eax
%0000000000007ef4 88 4d 10                mov byte [ebp 010h], cl
%0000000000007ef7 66 89 45 18             mov word [ebp 018h], ax
%0000000000007efb 89 d0                   mov eax, edx
%0000000000007efd 66 89 45 20             mov word [ebp 020h], ax
%0000000000007f01 66 81 7d 18 40 01       cmp word [ebp 018h], word 00140h
%0000000000007f07 77 1c                   jnbe  01ch (000007f25h)
%0000000000007f09 48                      dec eax
%0000000000007f0a c7 45 f8 00 00 0a 00    mov dword [ebp-008h], 0000a0000h
%0000000000007f11 0f b7 45 18             movzx eax, [ebp 018h]
%0000000000007f15 48                      dec eax
%0000000000007f16 01 45 f8                add dword [ebp-008h], eax
%0000000000007f19 48                      dec eax
%0000000000007f1a 8b 45 f8                mov eax, dword [ebp-008h]
%0000000000007f1d 0f b6 55 10             movzx edx, byte [ebp 010h]
%0000000000007f21 88 10                   mov byte [eax], dl
%0000000000007f23 eb 01                   jmp  001h (000007f26h)
%0000000000007f25 90                      nop
%0000000000007f26 48                      dec eax
%0000000000007f27 83 c4 10                add esp, byte 000000010h
%0000000000007f2a 5d                      pop ebp
%0000000000007f2b c3                      retn

When I discard the inc esp instruction at 0x7ef1,retn pops the return address correctly, but why is this instruction even there in the first place?

CodePudding user response：

I found the solution when adding the compiler option:

gcc -mno-red-zone

It now works flawless, thanks!