Motivated by reducing the data transfer activities in data-intensive neural network computing, SRAM-based compute-in-memory (CiM) has made significant progress. Unfortunately, SRAM has low density and limited on-chip capacity. This makes the deployment of large models inefficient due to the frequent DRAM access to update the weight in SRAM. Recently, a ROM-based CiM design, YOLoC, reveals the unique opportunity of deploying a large-scale neural network in CMOS by exploring the intriguing high density of ROM. However, even though assisting SRAM has been adopted in YOLoC for task transfer within the same domain, it is still a big challenge to overcome the read-only limitation in ROM and enable more flexibility. Therefore, it is of paramount significance to develop new ROM-based CiM architectures and provide broader task space and model expansion capability for more complex tasks. This paper presents Hidden-ROM for high flexibility of ROM-based CiM. Hidden-ROM provides several novel ideas beyond YOLoC. First, it adopts a one-SRAM-many-ROM method that "hides" ROM cells to support various datasets of different domains, including CIFAR10/100, FER2013, and ImageNet. Second, Hidden-ROM provides the model expansion capability after chip fabrication to update the model for more complex tasks when needed. Experiments show that Hidden-ROM designed for ResNet-18 pretrained on CIFAR100 (item classification) can achieve <0.5% accuracy loss in FER2013 (facial expression recognition), while YOLoC degrades by >40%. After expanding to ResNet-50/101, Hidden-ROM even achieves 68.6%/72.3% accuracy in ImageNet, close to 74.9%/76.4% by software. Such expansion costs only 7.6%/12.7% energy efficiency overhead while providing 12%/16% accuracy improvement after expansion.
