Bytewise Operations

Name:

BDIF $X,$Y,$ZBDIF $X,$Y,Z
WDIF $X,$Y,$ZWDIF $X,$Y,Z
TDIF $X,$Y,$ZTDIF $X,$Y,Z
ODIF $X,$Y,$ZODIF $X,$Y,Z

Specification:

x ← y-˙z = max (0,y - z)
Here x,y,z are corresponding elements of the vectors $X, $Y, and $Z.

Timing:

1υ

Description:

Saturated Difference. An Octa byte is regarded as a vector of eight byte, four Wyde, or two Tetra respectively.

BDIF | WDIF | TDIF | ODIFFor each (byte | wyde | tetra | octa) position j, the jth (byte | wyde | tetra | octa) register $X is set to (byte | wyde | tetra | octa) j of register $Y minus (byte | wyde | tetra | octa) j of the other operand $Z or Z, unless that difference is negative; in the latter case, (byte | wyde | tetra | octa) j of $X is set to zero.

The BDIF and WDIF commands are useful in applications to graphics or video; TDIF and ODIF are also present for reasons of consistency. For example, if a and b are registers containing 8-byte quantities, their bytewise maxima c and bytewise minima d are computed by

BDIF x,a,b; ADDU c,x,b; SUBU d,a,x;

similarly, the individual "pixel differences" e, namely the absolute values of the differences of corresponding bytes, are computed by

BDIF x,a,b; BDIF y,b,a; OR e,x,y;

To add individual bytes of a and b while clipping all sums to 255 if they don't fit in a single byte, one can say

NOR acomp,a,0; BDIF x,acomp,b; NOR clippedsums,x,0;

in other words, complement a, apply BDIF, and complement the result. The operations can also be used to construct efficient operations on strings of bytes or wydes.