Circuit for generating frame buffer values

摘要

A floating point to fixed point converter suitable for determining values for an n-bit frame buffer of a graphics adapter is disclosed. The converter includes a floating point unit that receives a floating point input value and calculates a floating point adjusted input value from the received value. Comparator circuitry is configured to compare a fixed point portion of the adjusted input value to a fixed point comparison value and to generate a fixed point output value responsive to the result of the comparison. The floating point unit may add a floating point constant to the received input to calculate the adjusted input value. The floating point constant may include a rounding component and a range component. The range component adjusts received values into a range defined by a single floating point exponent value such as the range from 1.0 to 2.0. In one embodiment, the rounding component shifts received values that are within a range of x/(2n-1)±1/(2*2n-1) into a range bounded by x/(2n-1) and (x+1)/(2n-1) to take advantage of special properties of the adjusted range's boundary values to simplify the comparator circuitry. The comparator circuitry may include a comparator configured to compare the exponent field of the adjusted input value to 0x7F and a comparator that compares the value of a first portion of the adjusted input value mantissa to a second portion of the mantissa. The no converter may produce a frame buffer value of 0x00 if the exponent field is less than 0x7F and a frame buffer value of 0xFF if the exponent field is greater than 0x7F. The converter may generate a frame buffer equal to the value of the first portion of the adjusted input value mantissa if the first portion is greater than the second portion and a frame buffer equal to the value of the first portion decremented by one if the first portion is less than the second portion.