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Early PC Graphics

Explore the progression of IBM's Color Graphics Adapter (CGA) and Enhanced Graphics Adapter (EGA) from their introductions in the early PC era. Understand their specifications, modes, screen resolutions, and display technologies, depicting the transition to higher color depths and resolutions with EGA's advanced features.

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Early PC Graphics

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  1. Early PC Graphics Capabilities of the IBM Color Graphics Adapter (CGA) and Enhanced Graphics Adapter (EGA)

  2. IBM product introductions • MDA: introduced with IBM-PC in 1981 • CGA: introduced as an option in 1982 • EGA: introduced in 1984 (to replace CGA) • VGA: introduced in 1987 (as PS/2 option)

  3. CGA • Engineered to coexist with IBM’s Monochrome Display Adapter (MDA), used for text display • Designed to operate with Intel’s 8086/8088 CPU • MDA: max 32K VRAM: 0xB0000-0xB7FFF • CGA: max 32K VRAM: 0xB8000-0xBFFFF • Designed to operate with Motorola’s 6845 CRTC • MDA: uses cpu’s i/o ports 0x3B4-0x3B5 • CGA: uses cpu’s i/o ports 0x3D4-0x3D5

  4. CGA graphics capabilities • Two graphics modes (2-color or 4color) • Both use “packed-pixel” memory-mode • 4 pixels-per-byte, or 8 pixels-per-byte • Four 4-color palette choices: • black+cyan+red+white • black+cyan+violet+white • black+green+red+yellow • black+dark-gray+light-gray+white

  5. CGA screen resolutions • color: 320x200 (4 packed pixels-per-byte) memory: 320x200/4 = 16000 bytes • mono: 640x200 (8 packed pixels-per-byte) memory: 640x200/8 = 16000 bytes 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0

  6. “Interlaced” VRAM addressing • Even-numbered scanlines in upper bank: • scanline 0: starts at offset 0 • scanline 2: starts at offset 80 • scanline 4: starts at offset 160 • Odd-numbered scanlines in lower bank: • scanline 1: starts at offset 0x2000 • scanline 3: starts at offset 0x2000 + 80 • Scanline 5: starts at offset 0x2000 + 160

  7. Pixel-drawing Algorithm (mono) void draw_pixel_1( int x, int y, int color ) { int locn = 0x2000*(y%2) + 80*(y/2) + (x/8); int mask = (1<<7) >> (x%8); unsigned char temp = vram[ locn ]; color &= 1; color <<= 7; color >> (x%8); temp &= ~mask; temp |= color; vram[ locn ] = temp; }

  8. Pixel-drawing Algorithm (color) void draw_pixel_2( int x, int y, int color ) { int locn = 0x2000*(y%2) + 80*(y/2) + (2*x/8); int mask = (3<<6) >> (2*x%8); unsigned char temp = vram[ locn ]; color &= 3; color <<= 6; color >> (2*x%8); temp &= ~mask; temp |= color; vram[ locn ] = temp; }

  9. CGA pixels aren’t square • Physical screen has 4:3 aspect-ratio • CGA visual screen-resolutions: • color screen is 320x200 (ratio is 8:5) • b&w screen is 640x200 (ratio is 16:5) • Physical square would be: • 4-color mode: 240 wide by 200 high • 2-color mode: 480 wide by 200 high • So logical pixels are “stretched” vertically

  10. Enhanced Graphics Adapter (EGA) • Backward compatibility with the CGA • Plus four additional display modes • Higher graphics resolutions • Greater color depths (16-colors) • Faster screen refresh rates • Needed to support more video memory • Simplify video memory-byte addressing • Needed additional “controller” hardware

  11. EGA display modes • New display modes 13, 14, 15, 16 • 13: 320x200 with 16-colors • 14: 640x200 with 16-colors • 15: 640x350 2-colors (monochrome) • 16: 640x350 4-colors w/64K vram or 16-colors w/128K vram • But uses “planar” memory organization, so relies on “Graphics Controller” hardware

  12. Four memory “planes” • Each CPU byte-address controls 8 pixels • CPU addresses bytes in 4 parallel planes 7 6 5 4 3 2 1 0

  13. Graphics Controller registers • 0: Set/Reset register • 1: Enable Set/Reset register • 2: Color Compare register • 3: Data-Rotate/Function-Select • 4: Read Map Select register • 5: Mode register • 6: Miscellaneous register • 7: Color Don’t Care register • 8: Bit Mask register

  14. Addressing device-registers • Nine Graphics Controller registers (8-bits) • Multiplexed i/o addressing scheme: - register index is written to i/o port 0x3CE - register value is accessed via port 0x3CF • Two read modes, and four write modes

  15. Reading a byte from VRAM • Select which memory-plane • Perform CPU read-byte instruction movb vram(%esi), %al • Bytes from all four planes are copied into Graphics Controller’s Latches (32-bits) • But only selected plane’s byte goes to AL

  16. Read operation illustrated plane 0 plane 3 plane 2 plane 1 Controller’s Latch register 2 Controller’s Read Map Select register CPU register AL

  17. Writing a byte to VRAM • Four distinct write modes (must choose) • We illustrate Write Mode 0 (“Direct Write”) • Four graphics controller registers involved: index 0: Set/Reset register index 1: Enable Set/Reset register index 3: Data-Rotate/Function-Select index 8: Bit Mask register

  18. Steps for Write Mode 0 • The new “fill color” goes into Set/Reset • Set Enable Set/Reset to enable all planes • Zero goes in Data-Rotate/Function-Select • Setup Bit Mask for the pixel(s) to modify • After these setup steps: • CPU reads from VRAM (to load the latches) • CPU writes to VRAM (to modify the pixel(s))

  19. Set/Reset (index 0) 7 6 5 4 3 2 1 0 The new fill-color Value (range is 0..15) outb( 0, 0x3CE ); // select Set/Reset register outb( color, 0x3CF ); // output the color-value Alternative programming (in one-step) outw( (color<<8)|0, 0x3CE );

  20. Enable Set/Reset 7 6 5 4 3 2 1 0 0 = plane is write-protected 1 = plane can be modified outb( 1, 0x3CE ); // select Enable Set/Reset outb( 0x0F, 0x3CF ); // output selection bits Alternative programming (in one-step) outw( 0x0F01, 0x3CE );

  21. Data-Rotate (index 3) 7 6 5 4 3 2 1 0 Data-Rotation Count 0 to 7 bits (to right) Function Select Functions: 00=copy, 01=AND, 10=OR, 11=XOR (with Latch contents) outb( 3, 0x3CE ); // select Data-Rotate register outb( 0x00, 0x3CF ); // output the register value Alternative programming (in one-step) outw( 0x0003, 0x3CE );

  22. Bit Mask (index 8) 7 6 5 4 3 2 1 0 The corresponding pixel will be modified (=1) or unmodified (=0) outb( 8, 0x3CE ); // select the Bit Mask register outb( mask, 0x3CF ); // output the register value Alternative programming (in one-step) outw( (mask<<8)|3, 0x3CE );

  23. Write Mode 0 illustrated VRAM: Latch Register 00000111 Bit Mask Fill-Color Set/Reset VRAM:

  24. Video Graphics Array (VGA) • Offers both CGA and EGA emulation • And supports three new display modes: mode 17: improved monochrome graphics mode 18: 16-colors using “square” pixels mode 19: supports 256 colors (8 bits/pixel) • Provides faster display-refresh rates • Supports analog multisync monitors

  25. Class Demos • ‘cgademo.cpp’ (4-color and b&w modes) • ‘egademo.cpp’ (shows 16-color palette) • ‘vgademo.cpp’ (square-pixels/256 colors)

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