1. Apparatus for processing data, said apparatus comprising: (i) a register bank of N-bit data processing registers; and (ii) a multiplier-accumulator coupled to said register bank for performing multiply-accumulate operations upon operands held in said N-bit data processing registers; wherein (iii) said multiplier-accumulator is responsive to a first class of multiply-accumulate instructions to multiply an N-bit operand held in a first data processing register by an N-bit operand held in a second data processing register and add an N-bit operand held in a third data processing register to yield an N-bit result that is stored in a fourth data processing register, said first, second, third and fourth data processing registers within said register bank being independently specified as fields within said first class of multiply-accumulate instructions; and (iv) said multiplier-accumulator is responsive to a second class of multiply-accumulate instructions to multiply an N-bit operand held in a first data processing register by an N-bit operand held in a second data processing register and add a 2N-bit operand held in a third and a fourth data processing registers to yield a 2N-bit result that is stored in said third and said fourth data processing registers, said first, second, third and fourth data processing registers within said register bank being independently specified as fields within said second class of multiply-accumulate instructions.