In a pipelined computer architecture in which instructions may be removed from the instruction queue out of sequence, instruction queue status at a cycle K is determined by adding together the number of invalid instructions or free rows in the queue during cycle K-2, the number of instructions issued for cycle K-1 and the number of instructions speculatively issued in cycle K-1 that have produced a cache hit, and subtracting from the sum the number of instructions enqueued for cycle K-1. The result indicates the number of invalid instructions in the queue cycle K. The number of invalid entries instructions, the number of issued instructions, and the number of enqueued instructions are preferably represented as flat vectors, so that adding is performed by shifting in one direction, while subtracting is performed by shifting in the opposite direction. The result is compared with either the number of instructions to be enqueued in the present cycle, which number is encoded, or with a predetermined value. A stall signal is generated if the indicative value is less than the encoded number or the predetermined value.