EPAC: The Last Dance

Filippo Mantovani; Fabio Banchelli; Pablo Vizcaino; Roger Ferrer; Oscar Palomar; Francesco Minervini; Jesus Labarta; Mauro Olivieri; Sebastiano Pomata; Pedro Marcuello; Jordi Cortina; Alberto Moreno; Josep Sans; Roger Espasa; Vasileios Papaefstathiou; Nikolaos Dimou; Georgios Ieronymakis; A. Psathakis; Michalis Giaourtas; Iasonas Mastorakis; Manolis Marazakis; Eric Guthmuller; Andrea Bocco; Jérôme Fereyre; César Fuguet; Mate Kovač; Mario Kovač; Luka Mrković; Josip Ramljak; Luca Bertaccini; Tim Fischer; Frank K. Gurkaynak; Paul Scheffler; Luca Benini; Bhavishya Goel; Madhavan Manivannan; Tiago Rocha; Nuno Neves; Jens Krüger

doi:10.1145/3801488.3806242

EPAC: The Last Dance
Paper in proceeding, 2026

This paper presents EPAC, a RISC-V-based accelerator chip developed within the European Processor Initiative (EPI) as part of a multi-year, multi-partner effort to build a European HPC processor ecosystem. EPAC is implemented in GlobalFoundries 22FDX (GF22FDX) technology, covers an area of 27 mm² with approximately 0.3 billion transistors, and integrates three distinct RISC-V compute tiles targeting different workload classes: VEC, a vector processing tile for double-precision HPC workloads; STX, a many-core tile optimized for stencil and machine learning computations; and VRP, a variable-precision tile for iterative numerical solvers requiring extended floating-point formats. All tiles are connected through a Coherent Hub Interface (CHI) based network-on-chip with a distributed L2 cache system and communicate with external memory via a SerDes link. The chip was taped out in GF22FDX technology and successfully brought up, with all major IP blocks validated. This paper describes the architecture of each tile and the uncore infrastructure, the integration and physical implementation process, and the board-level bring-up activities. It also reflects on the engineering and coordination lessons learned from a full chip design effort distributed across academic and industrial partners in Europe.

Stencil

Variable Precision

EPAC

RISC-V

European Processor Initiative

Chip Tapeout

Vector Computing