First, the BIOS looks at PINS to determine if it should use nonstandard frequencies for VGA clocks. There are two bytes in PINS which store VGA clock information for two clock frequencies. Otherwise it is assumed that the field contains the value of pixel clock frequency coded in 250 kHz units. This gives the opprotunity to tune the card's VGA modes to the capabilities of some non-VGA monitors. The table below summarizes timing capabilities of Matrox controllers in standard VGA modes.
All frequencies are in kHz unless noted otherwise.
Standard CLK freq. Max CLK freq. Assume we have a monitor with min. To use it in standard VGA graphics modes, CLK1 field should be programmed to get pixel clock of 38400 kHz. Rounded to the nearest multiple of 250 kHz this gives 38500, and dividing this number by 250 we get PINS field value of 154.