Instruction prefetch buffer control

Abstract

An instruction prefetch buffer control (20) is provided for an instruction prefetch buffer array (10) which stores the code for a number of instructions that have already been executed as well as the code for a number of instructions yet to be executed. The instruction prefetch buffer control includes a register (201) for storing an instruction fetch pointer, this pointer being supplied to the buffer array (10) as a write pointer which points to the location in the array where a new word is to be written from main memory. A second register (205) stores an instruction execution pointer which is supplied to the buffer array (10) as a read pointer. A first adder (203) increments the first register to increment the instruction fetch pointer for sequential instructions and calculates a new instruction fetch pointer for branch instructions. A second adder (215) increments the second register to increment the instruction execution pointer for sequential instructions and calculates a new instruction execution pointer for branch instructions. Incrementing of the second register is variable depending on the length of the instruction. A third adder ( 221) is responsive to the output of the first adder and a branch target address to calculate whether the target instruction is contained in the array (10) and, if it is, causes the new instruction execution pointer calculated by the second adder (215) to be loaded into the second register (205).