The CRT Controller How to modify CRTC registers to achieve a non-standard horizontal and vertical screen resolution

Video bandwidth PC systems display pixels at a rate that is determined by an internal hardware timer An oscillator generates this rate-frequency (it is known as the ‘dot clock’) Modern SVGA systems can select among several different dot clocks Faster dot clock rates are used with higher screen-resolutions

Example: VESA mode 0x101 Radeon BIOS programs CRTC hardware for 640-by-480 screen-resolution (8bpp) Screen refresh-rate is 60 Hz (frames/sec) This mode uses a 25.2 MHz Dot Clock One pixel is drawn per dot-clock oscillation But extra time is needed for the horizontal and vertical retrace operations

Horizontal timing parameters Horizontal Display-Enable End Horizontal Blanking Start Horizontal Retrace Start Horizontal Retrace End Horizontal Blanking End Horizontal Total

Vertical timing parameters Vertical Display-Enable End Vertical Blanking Start Vertical Retrace Start Vertical Retrace End Vertical Blanking End Vertical Total

Dot-clock operation Two internal counters are used: –The character-counter (horizontal) –The scanline-counter (vertical) Both counters start from zero After eight dot-clocks, the character-counter is incremented (since character-width is 8 pixels) When character-counter reaches the horizontal total, the scanline-counter is incremented (and the character-counter is reset to zero)

Dot-clock operation (2) When scanline-counter reaches vertical total, both counters are reset to zero (and a new frame begins to be displayed) As the counters reach programmed values for the horizontal and vertical parameters, the CRT enters a different ‘state’ (such as screen blanked, or a retrace operation)

The ‘border color’ After the ‘display-enabled’ state ends, and before the ‘display-blanked’ state begins, a traditional CRT displays a ‘overscan color’ The overscan-color is programmable (and normally is set to ‘black’ by default) On our classroom’s LCD monitors, we do see this border color Border width & height are programmable

Example: mode 0x101 Horiz Display-Enable End = 80 chars Horiz Blanking Start = 81 chars Horiz Retrace Start = 84 chars Horiz Retrace End = 96 chars Horiz Blanking End = 99 chars Horiz Total = 100 chars

Horizontal timings visualized Horizontal display enabled Horizontal Retrace is active Horizontal Blanking is active Overscan Color is visible

Example: mode 0x101 (cont) Vertical Display-Enable End = 480 Vertical Blanking Start = 487 Vertical Retrace Start = 489 Vertical Retrace End = 498 Vertical Blanking End = 516 Vertical Total = 525

Vertical timings visualized Vertical display enabled Vertical Retrace is active Vertical Blanking is active Overscan color is visible

Arithmetic Formulae frameRate = dot-clock / (Htotal * Vtotal) 60 Hz = 25,200,000 / (800 * 525) Horizontal freq = KHz (= 25.2 / 800) Vertical freq = 60 Hz

Reprogramming the CRTC We show how to reprogram the CRTC for a (nonstandard) 256-color display-mode We start with standard VESA mode 0x101 Then we ‘tweak’ this mode my changing the values in five of the CRTC registers Recall that CRTC registers are addressed using a pair of i/o ports: 0x3D4 and 0x3D5

Order for our modifications Our ‘mymode.cpp’ demo does these steps: 1)Lower Horizontal Display End by 8 chars 2)Lower Vertical Disply End by 48 lines 3)Reduce Horiz Retrace Start by 5 chars 4)Reduce Vertl Retrace Start by 15 lines 5)Adjust Offset for narrower screen pitch

Register details Unlock CRTC timing registers, by clearing bit 7 in Vert Retrace End reg (index 0x11) Perform ‘image clipping’ (horiz & vertl): –Horizontal Display-Enable End (index 1) outw( 0x4701, 0x3D4 );// 0x4F  0x47 –Vertical Display-Enable End (index 0x12) outw( 0xAF12, 0x3D4 ); // 0xDF  0xAF

More register details Perform image-centering (horiz & vertl): –Horizontal Retrace Start (index 0x04) outw( 0x5004, 0x3D4 );// 0x55  0x50 –Vertical Retrace Start (index 0x10) outw( 0xDA10, 0x3D4 );// 0xE9  0xDA Adjust CRT Offset register (reduced pitch) –Offset Register (index 0x13) outw( 0x4813, 0x3D4 );// 0x50  0x48

Motivation for mode-tweak Consider mode 0x101 memory-required 640x480 = 307,200 pixels 256-colors: requires one byte-per-pixel Total memory exceeds 4*64K (= 262,144) Consider tweaked memory-requirements 576x432 = 248,832 pixels Total memory does NOT exceed 4*64K

Legacy VGA addressing VGA can be programmed to access vram using 128K window: 0xA0000 – 0xBFFFF Hardware can be programmed to access vram in ‘planar’ mode (4 bytes in parallel) Thus two ‘pages’ of planar vram could be available for graphics animations, using ‘square’ pixels and 8086 real-mode 16-bit memory-addressing (cf Abrash ‘ModeX’)

In-class exercise Compile and run our ‘mymode.cpp’ demo Observe final image ‘isn’t centered right’ Can you alter the CRTC register changes that we used to produce a ‘better centered’ screen image? (horizontally & vertically)