APPARATUS AND METHOD IN A DIGITAL COMPUTER FOR ALLOWING IMPROVED PROGRAM BRANCHING WITH BRANCH ANTICIPATION REDUCTION OF THE NUMBER OF BRANCHES, AND REDUCTION OF BRANCH DELAYS

摘要

1282341 Data processing INTERNATIONAL BUSINESS MACHINES CORP 12 Jan 1970 [15 Jan 1969] 1358/70 Heading G4A In stored-program data processing apparatus, switching to another instruction stream as a result of a successful conditional branch instruction occurs after execution of a marker instruction. Instructions are accessed from memory and inserted into the top of a multi-instruction pushdown buffer together with bits indicating, for each instruction, whether or not it is a target, branch or exit instruction. The target bit is provided at the memory (see below) and the branch and exit bits are derived by a predecoder. The instruction at the bottom of the buffer is decoded. If it is a branch instruction, two fields in it select two bits from a condition register and 1 of 8 logical functions of these specified by part of the op code is evaluated in a function generator to determine whether the branch should be taken, the branching being to an address obtained by adding a field from the instruction and the contents of one of a plurality of registers selected by another field of the instruction. However, the sequence of instruction fetching branches to this address only when the next exit instruction enters the buffer. The branched-to instruction has its target bit set to 1 at the memory. Decoding and therefore execution of each instruction in the buffer between the exit instruction and the target instruction is inhibited, the inhibition terminating when the target instruction is shifted into the bottom position of the buffer, in response to the target bit of 1. Decoding of any branch instruction after a successful branch instruction and before the next exit instruction is also inhibited. If both an exit and a branch instruction are present in the buffer but a successful branch has not been determined, instruction fetching is suspended until not all these conditions exits, thus awaiting the result of the branch test. Instruction fetching is from the next sequential address if there is no exit instruction in the buffer, and if there is but there is neither a branch instruction in the buffer nor an indication of a successful branch determined.