Write bandwidth is the least important kind of bandwidth because usually programs write continuously only when they prepare areas of memory. The percentage displayed is the gain in write bandwidth when data are written using 64 bit floating point writes instead of the standard 32 bit integer writes. Intel does state in the new version of its optimization manual (, that by writing using 64 bit MMX instructions an increase of 30% was measured. But a program relying on MMX writes is not compatible with all the non MMX Pentiums, M1 and K5 processors out there. That's why we use 64 bit floating point writes and achieve up to 100% higher performance.