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VGA Text Mode

VGA Text Mode. An introduction to font selection and to reprogramming of the Character Generator ram. alphanumeric information. Most early PC programs weren’t graphical Screen was similar to a typewriter’s output Used only a limited set of character-images:

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VGA Text Mode

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  1. VGA Text Mode An introduction to font selection and to reprogramming of the Character Generator ram

  2. alphanumeric information • Most early PC programs weren’t graphical • Screen was similar to a typewriter’s output • Used only a limited set of character-images: • letters, numerals, and punctuation symbols • Hardware can efficiently render such glyphs • VGA can emulate the MDA/EGA text modes: • mode 1: emulates EGA 40x25 text (320x200 pixels) • mode 3: emulates EGA 80x25 text (640x400 pixels) • mode 7: emulates MDA 80x25 text (720x350 pixels)

  3. Font images in ROM • VGA has built-in firmware (ROM-BIOS) • Both code and data are provided in ROM • VGA’s ROM is normally at 0x000C0000 • Can be addressed by CPU in ‘real-mode’ • VGA ROM includes character glyph tables • 8x8 character-set is used with mode 1 • 8x16 character-set is used with mode 3 • 8x14 character-set is used with mode 7

  4. A character-glyph example • Here’s a sample 8x8 character glyph (‘A’): This glyph can be represented as an array of 8 bytes: 0x00, 0x38, 0x6C, 0xC6, 0xFE, 0xC6, 0x00, 0x00

  5. VGA Timer-Sequencer • A ‘character table’ is copied into VRAM • Glyphs organized as an array of bitmaps • Array holds 256 images (32 bytes/image) • Table-size: 256x32 = 8K bytes (= 0x2000) • The ascii-codes serve as array-indexes: • Example: ‘A’ has ascii-code 0x41 (=65) • Sequencer hardware accesses these images • Dedicated area of VRAM is used (plane 2) • Planes 0 and 1 are used as a text frame-buffer

  6. Arrangement of VRAM planes plane 3 plane 2 plane 1 plane 0 Not used (masked) Holds the character glyph table(s) Stores color attribute-bytes for displayed text Stores ASCII- codes for the currently display text CPU can rea/write only to planes 0 and 1 (using odd/even addressing mode)

  7. multiple fonts supported • Plane 2 is the Character Generator ram • Enough room for eight separate tables: • 8 x 8K = 64K • Two tables can be in use simultaneously • Tables must start at prescribed offsets: • Table 0: 0x0000 Table 4: 0x2000 • Table 1: 0x4000 Table 5: 0x6000 • Table 2: 0x8000 Table 6: 0xA000 • Table 3: 0xC000 Table 7: 0xE000

  8. Text color ‘attributes’ • In text mode, the picture-elements consist of character images, shown in two colors: • Foreground color, and Background color • Character colors individually programmed • A byte-pair in VRAM selects the bit-image (ascii code) and color-pair (attribute byte) • All colors come from a palette of 16 • But the color palette is ‘programmable’

  9. Layout for an ‘attribute’ byte Background Color Foreground Color R G B R B G Bit function is programmable Default is “EnableIntensity” Bit function is Programmable: Default is “EnableBlinking”

  10. VGA Sequencer Registers • Five registers comprise VGA Sequencer: • index 0: Reset register • index 1: Clocking Mode register • index 2: Map Mask register • index 3: Character Map Select register • index 4: Memory Mode register • All accessed via i/o ports 0x3C4-0x3C5 using “multiplexing” scheme (index/data)

  11. Access to character ram • In text modes, CPU can’t access Plane 2 • Plane 2 gets accessed by the Sequencer • Sequencer performs a ‘glyph-lookup’ for each ascii code stored by CPU in Plane 0 • Both Sequencer and Graphics Controller must be suitably reprogrammed in order for the CPU to a read or write to Plane 2 • Six VGA registers are involved in that step

  12. Reset (index 0) 7 6 5 4 3 2 1 0 Synchronous Reset bit (bit 1) 1 = normal sequencer operation 0 = initiate a synchronous reset A synchronous reset is used in advance of reprogramming the Clocking Mode register (or the Clock Select field in the VGA’s Miscellaneous Output register) Asynchronous Reset bit (bit 0) 1 = normal sequencer operation 0 = initiate an asynchronous reset Halts VRAM refresh cycles and clears VRAM contents

  13. Map Mask (index 2) 7 6 5 4 3 2 1 0 Plane 3 Plane 2 Plane 1 Plane 0 Enables or disables CPU’s ability to access specific memory planes 1 = write enable, 0 = write disable

  14. Memory Mode (index 4) 7 6 5 4 3 2 1 0 “Chain-4” Addressing 1 = enabled 0 = disabled i.e., each plane holds every fourth byte (For EGA only) 1= text 0 = graphics “Odd/Even” Addressing 1 = disabled 0 = enabled It’s used for text modes Extended Memory (>64K) 1 = present, 0 = absent

  15. GC: Miscellaneous (index 6) 7 6 5 4 3 2 1 0 Memory Map Odd/ Even enable G/A Memory Map options: 00 = 0xA0000 (128K) 01 = 0xA0000 (64K) 10 = 0xB0000 (64K) 11 = 0xB8000 (32K) 1 = use Odd/Even Addressing 0 = use Sequential Addressing 1 = Disable the character generator( graphics mode) 0 = Enable the character generator (use “text mode”) Graphics Controller registers are accessed via i/o ports 0x3CE/0x3CF

  16. GC: Mode (index 5) 7 6 5 4 3 2 1 0 256 colors SHIFT ODD/ EVEN Read Mode Write Mode (0, 1, 2, 3) Should be 0 for text mode 1 = cpu data at odd addresses is mapped to odd-numbered planes, cpu data at even addresses gets mapped to even-numbered planes 0 = cpu addressing is sequential NOT: This affects only the Graphics Controller. The Sequencer needs to be programmed separately to match This affects the VGA Attribute Controller’s operation (text color: foreground color and background color) Should be 1 for text

  17. How to modify character ram • Algorithm: • Reset the VGA for accessing Plane 2 • Then CPU reads or modifies Plane 2 • Reset the VGA for accessing Planes 0, 1 • Acknowledgement: • Author Richard Wilton described this process in his classic book “Programmer’s Guide to PC Video Systems (2nd Edition)”

  18. Details for this ‘prolog’ outw( 0x0100, 0x3C4 ); // do a synch. reset outw( 0x0402, 0x3C4); // write Plane 2 only outw( 0x0704, 0x3C4 ); // sequential access outw( 0x0300, 0x3C4 ); // end the reset outw( 0x0204, 0x3CE ); // read Plane 2 only outw( 0x0005, 0x3CE ); // disable odd/even outw( 0x0006, 0x3CE ); // VRAM at 0xA0000

  19. Details for the ‘epilog’ outw( 0x0100, 0x3C4 ); // do a synch. reset outw( 0x0302, 0x3C4); // write Planes 0 & 1 outw( 0x0304, 0x3C4 ); // odd/even access outw( 0x0300, 0x3C4 ); // end the reset outw( 0x0004, 0x3CE ); // restore to ‘default’ outw( 0x1005, 0x3CE ); // resume odd/even outw( 0x0E06, 0x3CE );// VRAM at 0xB8000

  20. Some Class Demos • ‘newzero.cpp’: installs new glyph for ‘0’ • ‘romfonts.cpp’: finds ROM glyph-tables • ‘backward.cpp’: flips character images! • ‘vm86blue.cpp’: changes the text’s color

  21. Algorithm for ‘backward.cpp’ for (int i = 0; i < 8192; i++) { int orig = vram[ i ], revs = 0; for (int j = 0; j < 8; j++) if ( orig & (1<<j) ) revs |= (1 << (7-j) ); vram[ i ] = revs; }

  22. In-Class Exercises • Exercise #1: Design a new image for ‘A’ • Exercise #2: Draw all text upside-down • Exercise #3: Draw ‘yellow-on-green’ text

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