osi_screen.h

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#ifndef _OSI_SCREEN_H
#define _OSI_SCREEN_H
 
#include <string.h>
 
template <unsigned int scr_base_uint = 0xD000, // start address of video RAM
          unsigned int video_ram_size = 0x400, // size of video RAM
          unsigned int screen_width = 0x1B,    // screen width
          unsigned int screen_height = 0x1B,   // screen height
          unsigned int screen_firstchar =
              0x85, // offset of cursor position (0, 0) from base of video RAM
          unsigned int scroll_dist =
              0x20 // memory distance for scrolling by one line
          >
class __osi_screen {
private:
  static unsigned char cursor_x;
  static unsigned char cursor_y;
 
  static constexpr unsigned int scroll_length(void) {
    return (screen_height - 1) * scroll_dist;
  }
 
  static constexpr char *scr_base(void) { return (char *)scr_base_uint; }
 
  static char *cursor_pos_mem(void) {
    return scr_base() + screen_firstchar + cursor_y * scroll_dist + cursor_x;
  }
 
  static void newline(void) {
    cursor_y += 1;
 
    if (cursor_y == screen_height) {
      // Bottom of screen reached, scroll
      cursor_y -= 1;
 
      char *const scr_firstchar_addr = scr_base() + screen_firstchar;
      char *const scroll_dest = scr_firstchar_addr;
      char *const scroll_src = scr_firstchar_addr + scroll_dist;
 
      memmove(scroll_dest, scroll_src, scroll_length());
 
      // Address of first character in last line of screen
      char *const last_line_start = scr_firstchar_addr + scroll_length();
 
      // Fill last line with blanks
      memset(last_line_start, ' ', screen_width);
    }
  }
 
public:
  static void clrscr(void) {
    memset((void *)scr_base(), ' ', video_ram_size);
    cursor_x = cursor_y = 0;
  }
 
  static void cputc(char c) {
    if (c == '\n') {
      newline();
    } else if (c == '\r') {
      cursor_x = 0;
    } else {
      char *const dest = cursor_pos_mem();
      *dest = c;
      if (cursor_x >= screen_width - 1) {
        newline();
        cursor_x = 0;
      } else {
        cursor_x += 1;
      }
    }
  }
};
 
extern "C" void __putchar(char c);
 
#endif // _OSI_SCREEN_H