1up.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <pthread.h>

pthread_t graphics_thread;

void graphics_thread()
{
	screen_init("GBA Screen");

	while(1)
	{
		screen_clear();
		for (int x = 0; x < 240; x++)
			for (int y = 0; y < 160; y++)
				drawpixel(x, y, *(u32*)(0x6000000 + (240 * y) + x), *(u32*)(0x6000000 + (240 * y) + x + 1), *(u32*)(0x6000000 + (240 * y) + x + 2));
		screen_flip();
	}
	
	screen_end();
}

int init()
{
	// map vitual memory for screen mode
	mmap((void*)0x4000000, 0x2, PROT_WRITE | PROT_READ, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, -1, 0);

	// map virtual memory for screen buffer
	mmap((void*)0x6000000, 0x9600*3, PROT_WRITE | PROT_READ, MAP_ANONYMOUS | MAP_SHARED | MAP_FIXED, -1, 0);
	
	pthread_create(&graphics_thread, NULL, &graphics_thread, NULL);
}

int deinit()
{
	// create a screen.ppm file
	FILE * fp = fopen("screen.ppm", "wb");
	int fd = fileno(fp);

	// header for ppm
	fprintf(fp, "P6\n%d %d\n255\n", 240, 160);
	fflush(fp);

	// write pixel data to disk
	write(fd, (void *)0x6000000, 0x9600*3);

	printf("Wrote screenshot to screen.ppm\n");

	// exit syscall to stop gracefully
	exit(0);
}

draw.c

#include "draw.h"
#include "SDL.h"

uint8_t * screen;
SDL_Surface *sdl_screen;

void screen_clear()
{
	SDL_FillRect(sdl_screen, NULL, 0xFFFFFF);
}

void screen_flip()
{
	SDL_Flip(sdl_screen);
}

void screen_init(const char * title)
{
	SDL_Init(SDL_INIT_VIDEO);
	
	sdl_screen = SDL_SetVideoMode(screen_x, screen_y, 24, SDL_HWSURFACE | SDL_FULLSCREEN);
	SDL_WM_SetCaption(title, 0);
	
	screen = (uint8_t *)sdl_screen->pixels;
	screen_clear();
	screen_flip();
}

void screen_end()
{
	SDL_QuitSubSystem(SDL_INIT_VIDEO);
	SDL_Quit();
}

void draw_pixel(int x, int y, char r, char g, char b)
{
	uint32_t v = (y * screen_x + x) * 3;
	screen[v] = b;
	screen[v+1] = g;
	screen[v+2] = r;
}

void draw_line(int x0, int y0, int x1, int y1, char r, char g, char b)
{
	int dx = abs(x1 - x0), sx = (x0 < x1) ? 1 : -1;
	int dy = abs(y1 - y0), sy = (y0 < y1) ? 1 : -1;
	int err = ((dx > dy) ? dx : -dy) / 2, e2;
 
	for(;;) {
		draw_pixel(x0, y0, r, g, b);
		if (x0==x1 && y0==y1) break;
		e2 = err;
		if (e2 >-dx) { err -= dy; x0 += sx; }
		if (e2 < dy) { err += dx; y0 += sy; }
	}
}

void draw_rect(int x1, int y1, int x2, int y2, char r, char g, char b)
{
	draw_line(x1, y1, x2, y1, r, g, b);
	draw_line(x2, y1, x2, y2, r, g, b);
	draw_line(x1, y2, x2, y2, r, g, b);
	draw_line(x1, y1, x1, y2, r, g, b);
}

void draw_fill_rect(int x1, int y1, int x2, int y2, char r, char g, char b)
{
	int X1, X2, Y1, Y2, i, j;

	if (x1 < x2){
		X1 = x1;
		X2 = x2;
	}
	else {
		X1 = x2;
		X2 = x1;
	}

	if (y1 < y2){
		Y1 = y1;
		Y2 = y2;
	}
	else {
		Y1 = y2;
		Y2 = y1;
	}
	for (i = X1; i <= X2; i++){
		for (j = Y1; j <= Y2; j++){
			draw_pixel(i, j, r, g, b);
		}
	}
}

void draw_circle(int xCen, int yCen, int radius, char r, char g, char b)
{
	int x = 0;
	int y = radius;
	int p = (5 - radius * 4) / 4;
	draw_circle_circum(xCen, yCen, x, y, r, g, b);
	while (x < y){
		x++;
		if (p < 0){
			p += 2 * x + 1;
		}
		else{
			y--;
			p += 2 * (x - y) + 1;
		}
		draw_circle_circum(xCen, yCen, x, y, r, g, b);
	}
}

void draw_fill_circle(int xCen, int yCen, int radius, char r, char g, char b)
{
	draw_circle(xCen, yCen, radius, r, g, b);
	int x, y;
	for (y = -radius; y <= radius; y++){
		for (x = -radius; x <= radius; x++)
			if (x*x + y*y <= radius*radius + radius * .8f)
				draw_pixel(xCen + x, yCen + y, r, g, b);
	}
}

void draw_circle_circum(int cx, int cy, int x, int y, char r, char g, char b)
{
	if (x == 0){
		draw_pixel(cx, cy + y, r, g, b);
		draw_pixel(cx, cy - y, r, g, b);
		draw_pixel(cx + y, cy, r, g, b);
		draw_pixel(cx - y, cy, r, g, b);
	}
	if (x == y){
		draw_pixel(cx + x, cy + y, r, g, b);
		draw_pixel(cx - x, cy + y, r, g, b);
		draw_pixel(cx + x, cy - y, r, g, b);
		draw_pixel(cx - x, cy - y, r, g, b);
	}
	if (x < y){
		draw_pixel(cx + x, cy + y, r, g, b);
		draw_pixel(cx - x, cy + y, r, g, b);
		draw_pixel(cx + x, cy - y, r, g, b);
		draw_pixel(cx - x, cy - y, r, g, b);
		draw_pixel(cx + y, cy + x, r, g, b);
		draw_pixel(cx - y, cy + x, r, g, b);
		draw_pixel(cx + y, cy - x, r, g, b);
		draw_pixel(cx - y, cy - x, r, g, b);
	}
}

draw.h

#pragma once

#include <stdint.h>

// change these to size of the screen:
#define screen_x 300
#define screen_y 200

extern uint8_t * screen; // bgr

void screen_init(const char * title);
void screen_end();
void screen_clear();
void screen_flip();

void * render_text(char * text, char r, char g, char b);
void draw_text(int x, int y, void * render);

void draw_pixel(int x, int y, char r, char g, char b);
void draw_line(int x0, int y0, int x1, int y1, char r, char g, char b);
void draw_rect(int x1, int y1, int x2, int y2, char r, char g, char b);
void draw_fill_rect(int x1, int y1, int x2, int y2, char r, char g, char b);
void draw_circle(int xCen, int yCen, int radius, char r, char g, char b);
void draw_fill_circle(int xCen, int yCen, int radius, char r, char g, char b);
void draw_circle_circum(int cx, int cy, int x, int y, char r, char g, char b);