In a text mode display, the display controller makes one access to the frame buffer for every character displayed. It also makes one access to the font buffer for every scan line of every character. Therefore, the number of accesses (per frame) is equal to the number of characters in the frame times 1 plus the number of scan lines in a character. The display controller requires 9 additional clocks per active line in the display plus 5 additional clocks per character row. Put all of this together, and the se equations can be used to determine the performance impact of a given text mode:
where DspClk_divider is 1.0, 1.5, 2.0, or 3.0 according to the DspClk bits in the DspSpd register. This table gives the performance impact of some text displays with a frame rate of 90Hz.
Performance Impact for text modes
Attribute name Attribute ========================================================================================== Physical size 80x25 64x18 40x25 40x16 ------------------------------------------------------------------------------------------ Character size 8x8 10x11 8x8 16x12 ------------------------------------------------------------------------------------------ Frame buffer accesses 2000 1152 1000 640 ------------------------------------------------------------------------------------------ Font buffer accesses 16000 12672 8000 7680 ------------------------------------------------------------------------------------------ Clocks per frame 19925 15696 10925 10125 ------------------------------------------------------------------------------------------ Performance Impact 22.7% 17.9% 12.5% 11.6% ------------------------------------------------------------------------------------------In a graphics mode, the display controller makes one access to the frame buffer for every 16 pixels (high resolution) or 8 pixels (low resolution). Therefore, there are 8000 accesses per frame for CGA graphics modes, regardless of the resolution. There are only 1920 accesses in Jaguar graphics mode. The display controller requires 9 additional clocks per active line in the display. The following equation can be used to determine the number of clocks required by a given graphics mode:
The performance impact can then be found using the same equations as for text modes. This table gives the performance impact of some graphics displays with frame rates of 60Hz or 90Hz.
Performance Impact for graphics modes
Attribute name Attribute ========================================================================================== Physical size 320x200 320x200 640x200 240x128 ------------------------------------------------------------------------------------------ Colors 4 4 2 2 ------------------------------------------------------------------------------------------ Frame rate 90 Hz 60 Hz 60 Hz 60 Hz ------------------------------------------------------------------------------------------ Frame buffer accesses 8000 8000 8000 1920 ------------------------------------------------------------------------------------------ Clocks per frame 9800 9800 9800 3072 ------------------------------------------------------------------------------------------ Performance Impact 11.2% 7.5% 7.5% 2.3% ------------------------------------------------------------------------------------------