IEEE Std 1838(TM)-2019 standardizes mandatory and optional on-chip hardware components for 3D test access. It is intended that in the future a standard is developed for a formal, computer-readable language in which implementation choices for the three-dimensional design-for-test (3D-DfT) hardware can be specified and described. An idea of a language/data structure has been described in [B5]. The aim of IEEE Std 1838 is to define at die-level standardized and scalable 3D-DfT features based on and working with digital scan-based test access, such that when compliant dies are stacked, a stack-level 3D-DfT test access architecture emerges with a minimum functionality and many optional extensions. IEEE Std 1838 provides a modular test access architecture, in which dies and interconnect layers between adjacent stacked dies can be tested individually. The focus of the standard is testing the intra-die circuitry as well as the inter-die interconnects in pre-bond, mid-bond, and post-bond cases in pre-packaging, post-packaging, and boardlevel situations. The standard provides test access via a mandatory one-bit ('serial') input/output test port and optional multi-bit ('parallel') test ports. The standard is die-centric, i.e., compliance to the standard pertains to a die (and not to a stack of dies). Standardized die-level design-for-test (DfT) features comprise a stack-level test access architecture. In this way, the standard enables interoperability between die makers and stack maker. The standard does not address stack-level challenges and solutions. The most prominent example of this is that the standard does not address compliance of the stack to IEEE Std 1149.1(TM) boundary scan for board-level interconnect testing (although the standard certainly does not prohibit application thereof). IEEE Std 1838 does not mandate specific defect or fault models, specific test generation methods, nor specific die-internal 2D-DfT features. However, the standard leverages existing 2D-DfT wherever applicable and appropriate, including test access ports (such as specified in IEEE Std 1149.1), on-chip DfT such as internal scan chains and wrappers of embedded cores (such as specified in IEEE Std 1500(TM)), and on-chip design-for-debug and embedded instruments (such described in IEEE Std 1687). Stacking of dies requires that the vertical interconnects [e.g., micro-bumps and through-silicon vias (TSVs)] are aligned with respect to footprint (i.e., matching x,y layout locations), mechanical properties (i.e., matching materials, diameter, height, etc.), and electrical properties (i.e., matching driver/receiver pairs). As a generic DfT-only standard, IEEE Std 1838 does not govern these items. Similar to IEEE Std 1149.1 and IEEE Std 1500, it only defines a DfT architecture: - Number, name, type, and function of test I/Os - On-chip DfT hardware and corresponding description - Clock-cycle accurate test operation protocol