560 NOTES on Video Display DEMO Driver
//************ main.c file ******************* #include "vid.c" main() { vid_init(); // MUST initialize video driver first printf("\nWelcome to the 460 Multitasking System\n"); } 2. HOW does PC's color display work: 0. Before execute vid_init(), MUST be in mono mode, otherwise, no display at all. 1. Screen = 25 rows X 80 cols = 2000 WORDs = [attribue byte|char byte] 4000 bytes 2. Video RAM at 0xB800, size=16 to 64 KB >> screen=8000 bytes; To display a screen: write 4000 bytes (2000 WORDs) to the video RAM at the current ORG location; tell the CRT register where the ORG is at. The hardware will display the 2000 chars from ORG in the video RAM to screen. 3. Cursor shape and position can be changed by writing to CRT register 4. Software must keep track of the Cursor position (row,col). After displaying a char, must advance the Curcor by one word, which may change (row,col): When col >= 80, reset col to 0 and increment row by 1 When row >= 25, must scroll up by one row. 5. To Scroll up one row: inc org by one row (which tells CRT to display from the new ORG). write a row of blanks to ORG + one screen in video RAM. if the last row exceeds VRAM size, physically copy CURRENT screen to VRAM beginning and reset org to 0 *****************************************************************/ #define C_6845 0x3D0 /* port for 6845 color board */ #define VINDEX 4 /* 6845's index register */ #define VDATA 5 /* 6845's data register */ #define CUR_SIZE 10 /* 6845's cursor size register */ #define VID_ORG 12 /* 6845's origin register */ #define CURSOR 14 /* 6845's cursor position register */ #define LINE_WIDTH 80 /* # characters on a line */ #define SCR_LINES 25 /* # lines on the screen */ #define SCR_BYTES 4000 /* bytes of ONE screen=25*80 */ #define CURSOR_SHAPE 15 /* block cursor for MDA/HGC/CGA/EGA/VGA... */ ushort base = 0xB800; // VRAM base address ushort vid_mask = 0x3FFF; // mask=Video RAM size - 1 int vid_port = C_6845; // vid port address ushort offset; // current display position int color; // atttirbute byte int org; // current display origin int row, column; // logical row, col position // vid_init() initializes the display org=0 (row,col)=(0,0) int vid_init() { int i, w; org = row = column = 0; color = HYELLOW; set_6845(CUR_SIZE, CURSOR_SHAPE); // set cursor size set_6845(VID_ORG, 0); // set display origin to 0 set_6845(CURSOR, 0); // set cursor position to 0 //clear screen w = 0x0700; // White, blank char // attribute byte=0000 0111=0000 0RGB for (i=0; i<25*80; i++){ put_word(w, base, 0+2*i); // write 24*80 blanks to VGA memory } } /************************************************************************* scroll(): scroll UP one line by **************************************************************************/ int scroll() { int i; ushort w, bytes; // Normal scrolling using the 6845 registers. // try offset = org + ONE screen + ONE more line offset = org + SCR_BYTES + 2*LINE_WIDTH; if (offset <= vid_mask){ // offset still within vram area org += 2*LINE_WIDTH; // just advance org by ONE line } else{ // offset exceeds vram area ==> reset to vram beginning by // copy current rows 1-24 to BASE, then reset org to 0 for (i=0; i<24*80; i++){ w = get_word(base, org+160+2*i); put_word(w, base, 0+2*i); } org = 0; } // org has been set up correctly offset = org + 2*24*80; // offset = beginning of row 24 // copy a line of BLANKs to row 24 w = 0x0C00; // HRGB=1100 ==> HighLight RED, Null char for (i=0; i<80; i++) put_word(w, base, offset + 2*i); set_6845(VID_ORG, org >> 1); /// 6845 thinks in words } //NOTE: putc() is here, no longer call BIOS int putc(c) char c; { // display c to cursor position, advance cursor int pos, w, offset; if (c=='\n'){ row++; if (row>=25){ row = 24; scroll(); } pos = 2*(row*80 + column); offset = (org + pos) & vid_mask; set_6845(CURSOR, offset >> 1); return; } if (c=='\r'){ column=0; pos = 2*(row*80 + column); offset = (org + pos) & vid_mask; set_6845(CURSOR, offset >> 1); return; } if (c=='\b'){ if (column > 0){ column--; pos = 2*(row*80 + column); offset = (org + pos) & vid_mask; put_word(0x0700, base, offset); set_6845(CURSOR, offset >> 1); } return; } // above are for \n,\r,\b keys. Here we really display a char pos = 2*(row*80 + column); offset = (org + pos) & vid_mask; w = color; w = (w << 8) + c; put_word(w, base, offset); // write the word=[color| c ] to VRAM column++; // then advance CURSOR properly if (column >= 80){ column = 0; row++; if (row>=25){ row = 24; scroll(); } } pos = 2*(row*80 + column); offset = (org + pos) & vid_mask; set_6845(CURSOR, offset >> 1); // tell CRT to set the CURSOR } /*===========================================================================* * set_6845 * *===========================================================================*/ int set_6845(reg, val) int reg; /* which register pair to set */ int val; /* 16-bit value to set it to */ { /* Set a register pair inside the 6845. * Registers 10-11 control the format of the cursor (how high it is, etc). * Registers 12-13 tell the 6845 where in video ram to start (in WORDS) * Registers 14-15 tell the 6845 where to put the cursor (in WORDS) * * Note that registers 12-15 work in words, i.e. 0x0000 is the top left * character, but 0x0001 (not 0x0002) is the next character. This addressing * is different from the way the CPU addresses the video ram, where 0x0002 * is the address of the next character. */ lock(); /* try to stop h/w loading in-between value */ out_byte(vid_port + VINDEX, reg); /* set the index register */ out_byte(vid_port + VDATA, (val>>8) & 0xFF); /* output high byte */ out_byte(vid_port + VINDEX, reg + 1); /* again */ out_byte(vid_port + VDATA, val&0xFF); /* output low byte */ unlock(); } // change color syscall from Umode, just for fun // attribute byte: 0x0HRGB, H=highLight; RGB determine color #define HGREEN 0x0A #define HCYAN 0x0B #define HRED 0x0C #define HPURPLE 0x0D #define HYELLOW 0x0E int chcolor(y) ushort y; { y &= 0x7F; switch(y){ case 'r' : color=HRED; break; case 'y' : color=HYELLOW; break; case 'c' : color=HCYAN; break; case 'g' : color=HGREEN; break; } } /********************* Main Source of Infomation ************************ www.osdever.net/FreeVGA : home of FreeVGA programming information www.osdever.net/FreeVGA/vga/crtcreg.htm : CRT Controller Registers Index 00h -- Horizontal Total Register Index 01h -- End Horizontal Display Register Index 02h -- Start Horizontal Blanking Register Index 03h -- End Horizontal Blanking Register Index 04h -- Start Horizontal Retrace Register Index 05h -- End Horizontal Retrace Register Index 06h -- Vertical Total Register Index 07h -- Overflow Register Index 08h -- Preset Row Scan Register Index 09h -- Maximum Scan Line Register Index 0Ah -- Cursor Start Register Index 0Bh -- Cursor End Register Index 0Ch -- Start Address High Register Index 0Dh -- Start Address Low Register Index 0Eh -- Cursor Location High Register Index 0Fh -- Cursor Location Low Register Index 10h -- Vertical Retrace Start Register Index 11h -- Vertical Retrace End Register Index 12h -- Vertical Display End Register Index 13h -- Offset Register Index 14h -- Underline Location Register Index 15h -- Start Vertical Blanking Register Index 16h -- End Vertical Blanking Index 17h -- CRTC Mode Control Register Index 18h -- Line Compare Register Among these, only Registers 10-11 control CURSOR height (# of scand lines) 12-13 display origin address 14-15 CURSOR position are of interests here: **************************************************************************/