Assembly, draw an image - c

Question

I need to draw QRCode via Assembly(intel)+C(c99) in DOS. But it seems I have too little memory for it.
I tried to store image as bit array:
image
db 11111110b,
...
But anyway I had no result(Illegal read from 9f208c70, CS:IP 192:9f20734f). Now I don't know what to do. Here is code I used:
module.asm:
[bits 16]
global setpixel
global setVM
global getch
global getPixelBlock
section .text
setVM:
push bp
mov bp, sp
mov ax, [bp+6]
mov ah, 0
int 10h
pop bp
ret
setpixel:
push bp
mov bp,sp
xor bx, bx
mov cx, [bp+6]
mov dx, [bp+10]
mov al, [bp+14]
mov ah, 0ch
int 10h
pop bp
ret
getch:
push bp
mov ah,0
int 16h
mov ah,0
pop bp
ret
getPixelBlock:
push bp
mov cx, [bp+6]
mov bx, image
add bx, cx
mov ax, [bx]
pop bp
ret
section .data
image
db 11111110b,
db 10011011b,
db 11111100b,
db 00010011b,
db 00010000b,
db 01101110b,
db 10110000b,
db 10111011b,
db 01110101b,
db 01100101b,
db 11011011b,
db 10100000b,
db 00101110b,
db 11000001b,
db 01110001b,
db 00000111b,
db 11111010b,
db 10101111b,
db 11100000b,
db 00011000b,
db 00000000b,
db 11010011b,
db 01011111b,
db 01101011b,
db 11100100b,
db 11101000b,
db 00110101b,
db 11001111b,
db 01001111b,
db 11100000b,
db 00011011b,
db 11010001b,
db 00100111b,
db 00000011b,
db 10000000b,
db 00000011b,
db 10001111b,
db 11111010b,
db 00100000b,
db 01010000b,
db 01000110b,
db 01011011b,
db 10111010b,
db 01001111b,
db 01010101b,
db 11010110b,
db 10001110b,
db 00101110b,
db 10010001b,
db 01111011b,
db 00000101b,
db 01100001b,
db 10001111b,
db 11101110b,
db 11000001b
main.c:
__asm(".code16gcc\n");
int run();
int _start()
{
return run();
} // Dirty hack to code as I used to
#include "ASM.inl"
#include "Painter.inl"
int run()
{
setVM(0x10);
_brushSize = 5;
drawLogo(30,30);
uint ret = (uint)getch();
return ret>>5;
}
ASM.inl
#ifndef __ASM_H__
#define __ASM_H__
typedef unsigned short int uint;
typedef unsigned char uchar;
void setpixel(uint x, uint y, uint color);
void setVM(uint vm);
uchar getch();
uchar getPixelBlock(uchar);
#endif /* __ASM_H__ */
Painter.inl:
/**
* You can create other colors by using bitwise or
*/
enum Color {
White = 0b0111,
Black = 0b0000,
Red = 0b0100,
Green = 0b0010,
Blue = 0b0001,
Bright = 0b1000,
};
int _brushSize = 5;
void rect(uint x, uint y, uint width, uint height, uint color)
{
uint i,j;
for (i=x; i<width+x; i++) {
for (j=y; j<height+y; j++) {
setpixel(i,j,color);
}
}
}
uint getColor(uchar element, uchar offset)
{
element = element & (1 << offset) >> offset;
return element ? Black : White;
}
void drawLogo(uint x, uint y)
{
uchar current;
uchar counter = 0;
for (uint i=0; i<21; i++) {
for (uint j=0; j<21; j++) {
counter = i*21+j;
current = getPixelBlock((uchar)counter/8);
rect(x+i*_brushSize, y+j*_brushSize, _brushSize, _brushSize, getColor(current, counter%8));
}
}
}
Compilation script:
#!/bin/bash
nasm -f elf32 module.asm -o module.o
gcc -std=c99 -m32 -ffreestanding -masm=intel -c main.c -o main.o
ld -m elf_i386 -Ttext=0x100 main.o module.o -o os.com
objcopy os.com -O binary
GCC version: 4.8.3 (Gentoo 4.8.3 p1.1, pie-0.5.9)
NASM version: 2.11.05
DOSBox version: 0.74
What I am doing wrong? Maybe I should write directly into graphic memory or something like that? Or maybe I should change gcc optimizations?

Answer 1

The assembly code looks generally alright. You might want to check the interrupt calling sequences against the order of parameters on the stack by setting a breakpoint right on the int 10h and checking the register values. I haven't done that stuff for well over 20 years, and I'm rusty.
You have at least two probable operator precedence problems. I don't think these do the right thing.
element = element & (1 << offset) >> offset;
current = getPixelBlock((uchar)counter/8);
You have a hard-coded 'magic number': 21. I have no idea what that means.
After that, the question is: where did it crash? Time to get that debugger stoked up and paying for itself.
I meant to ask: why on earth write this stuff in assembly? You can easily call int 10h either directly from C, from embedded asm in C, or by a single wrapper function.

Answer 2

The way you define your data with a trailing comma introduces an extra byte with zero value. At least in my assembler!
I think you need to double the value of CURRENT in the DRAWLOGO function in order to synchronize with the data.
The function GETPIXELBLOCK recieves values from 0 To 55 which is 1 more than the data lines available!