Virtual address capability is held, CPU and a pipeline instruction unit and effective address calculation unit

摘要

A prefetch unit includes a Branch history table for providing an indication of an occurrence of a Branch instruction having a Target Address that was previously taken. A plurality of Branch mark bits are stored in an instruction queue, on a half word basis, in conjunction with a double word of instruction data that is prefetched from an instruction cache. The Branch Target Address is employed to redirect instruction prefetching. The Branch Target Address is also pipelined and follows the associated Branch instruction through an instruction pipeline. The prefetch unit includes circuitry for automatically self-filling the instruction pipeline. During a Fetch stage a previously generated Virtual Effective Address is applied to a translation buffer to generate a physical address which is used to access a data cache. The translation buffer includes a first and a second translation buffer, with the first translation buffer being a reduced subset of the second. The first translation buffer is probed, during a Generate stage, to prefetch, if possible, the required operand. The prefetch unit further provides 24-bit or 31-bit effective address generation on an instruction by instruction basis.