Graphics Bitmaps Drawing characters glyphs and multicolor patterns Limitations of BIOS routines Last time we used a ROM BIOS routine to draw a text message on a graphics screen i e the so called write string service But some limitations were apparent String location had to conform to a cell grid Background color pattern was obliterated We can overcome both these limitations if we draw the character glyphs ourselves Built in character bitmaps The ROM BIOS has full sets of bitmaps for the images of all the ASCII characters in several sizes e g 8x8 8x14 and 8x16 Example An 8 by 8 glyph for the letter H Layout of 8x16 glyph table The Glyph Table is an array of bit images Each bit image is 16 bytes in length Each ascii code serves as array index 0x40 0x41 0x42 0x43 0x44 0x45 0x46 A B C D E F etc The 8086 physical memory The VGA firmware Character Glyph tables are located in this region Boot ROM 0xF0000 Reserved for add ins Video ROM 0xC0000 VRAM EBDA 0xA0000 1 MB A built in BIOS function will report each of the glyph table start locations RAM BIOS DATA VECTORS 0x00000 Finding the ROM fonts Standard BIOS service locates ROM fonts Designed to execute in 8086 real mode Normal service call protocol is followed Parameters are placed in CPU registers Software interrupt instruction is executed ROM BIOS code performs desired service Parameters may be returned in registers Example AT T assembly language syntax mov 0x11 ah char gen services mov 0x30 al get font information mov 6 bh 8x16 font address int 0x10 request BIOS service the font address is returned in ES BP and count of scanlines should be in CX Invoking BIOS from Linux Include our int86 h header file Define struct vm86plus struct object vm Initialize necessary register fields of vm vm regs eax 0x1130 vm regs ebx 0x0600 Call our function int86 0x10 vm Retrieve the returned parameters from vm e g from vm regs es and vm regs ebp Drawing a character in mode 18 Must enable I O and 1 MB memory map Virtual address of VRAM is 0x000A0000 Use any ascii code as a glyph table index Use x y coordinates for character location Specify the character s foreground color Thus your function call could look like draw char x y color ascii Using VGA Write Mode 3 To preserve the background pattern when a character glyph is drawn you can use a Masked Write operation Write Mode 3 To position the character at an arbitrary horizontal pixel position use Data Rotate The foreground color goes in Set Reset The Bit Mask register prevents unwanted pixels from being drawn when rotation Illustration Adjacent bytes on the screen Character glyph is right rotated Afterward this rotated glyph gets masked Left hand Bit Mask Right hand Bit Mask Implementation details void draw char int x int y int fg int ascii int which y hres x int where which 8 int shift which 8 int left mask 0xFF shift int right mask left mask char glyph char fontbase 16 ascii Draw char continued outw 0x0305 GCPORT use Mode 3 outw color 8 GCPORT Set Reset first pass draws glyph s left portion outw left mask 8 8 GCPORT for int row 0 row 16 row char temp vram where row 80 vram where row 80 glyph row Draw char concluded if shift 0 return we can do an early exit else second pass draws glyph s right portion where location of the adjacent byte outw right mask 8 8 GCPORT Bit Mask for int row 0 row 16 row char temp vram where row 80 vram where row 80 glyph row Our drawtext cpp demo We used an 8 by 8 bitmap to generate the patterned background for drawing text on Background color 0001 binary Foreground color 1001 binary 0x00 0x00 0x33 0x33 0x00 0x00 0x33 0x33 How Write Mode 0 works Byte from pattern glyph ENABLE SET RESET 0 0 1 1 0 0 1 1 Data from CPU SET RESET 0 x 0 0 1 1 0 0 1 1 Latch plane 3 1 0 0 0 0 0 0 0 0 0 Latch plane 2 1 0 0 0 0 0 0 0 0 0 Latch plane 1 1 1 1 1 1 1 1 1 1 1 Latch plane 0 VRAM byte Screen Pixel planar Algorithm to tile screen area Use Write Mode 0 Direct Write Data Rotate 0 Function Select is copy Bit Mask is 0xFF e g all pixels writable Enable Set Reset is 0x7 Set Reset is 0x1 pattern glyph described by array of 8 bytes for y ymin y ymax y for x xmin x xmax x vram y 80 x glyph y 8 Designing bitmap patterns You can create interesting backgrounds Fill screen regions with a copied pattern 0xFF 0x80 0x80 0x80 0xFF 0x08 0x08 0x08 foreground color background color Algorithm for tiling mode 19 unsigned char pat 8 8x8 2 color bitmap unsigned char vram 0x000A0000 color for int y 0 y vres v for int x 0 x hres x int r y 8 k x 8 color pat r 0x80 k fg bg vram y hres x color In class exercise Take a look at our brickpat cpp demo It tiles the screen with a brick wall pattern But it executes in mode 19 256 colors Can you revise this demo so that it will work in a planar mode i e 16 colors Hint Apply the principles of Write Mode 0 in a manner similar to drawtext cpp demo Can you write text against your brick wall
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