Here’s how to use the hardware multiplier to make a multiply-and-accumulate operation (MAC), an app note from Atmel (PDF!):
• 8- and 16-bit Implementations
• Signed and Unsigned Routines
• Fractional Signed and Unsigned Multiply
• Executable Example Programs
The megaAVR is a series of new devices in the AVR RISC Microcontroller family that includes, among other new enhancements, a hardware multiplier. This multiplier is capable of multiplying two 8-bit numbers, giving a 16-bit result using only two clock cycles. The multiplier can handle both signed and unsigned integer and fractional numbers without speed or code size penalty. The first section of this document will give some examples of using the multiplier for 8-bit arithmetic.