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TG/CX/CHARTER-mini.md

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+# Composable Extensions (CX) Task Group Charter
+
+RISC-V reserves the custom-\* opcode space, enabling anyone to create new
+custom extensions and their software libraries.  However, composition of
+multiple extensions is unobtainable because RISC-V custom extensions are
+unmanaged, lacking standards or uniformity.  This impairs extension reuse due
+to conflicting opcodes or incompatible means of extension discovery,
+versioning, state management, etc, ultimately leading to fragmented ecosystems
+with disjoint solution silos.
+
+By expanding the custom instruction opcode space to a nearly inexhaustible
+supply of 32b opcodes, this opcode space can also be shared by future RVI ISA
+extensions. This will solve the problem of limited capacity remaining within
+the 32b opcode space.
+
+The Composable Extensions (CX) Task Group(s) will specify ISA extensions
+(within the Custom Opcode space), software (API, ABI) and hardware interfaces.
+This will enable harmonious, robust and conflict-free composition of multiple
+independently authored composable custom extensions, alongside legacy custom
+instructions or extensions, in one RISC-V system. Multiple harts will be able
+to share the same physical hardware module that implements one or more CX
+instances.
+
+To do this, CX will multiplex the custom opcode space and adopt common software
+and hardware interop interfaces.  The CX specification will be guided by and
+consider the impact of these design tenets: composability, conflict-freedom,
+decentralization, stable binaries, composable and reusable hardware modules,
+uniformity (of scope, naming, discovery, versioning, error signaling, state
+context management, access control), frugality, security, and longevity.  In
+particular, this should be done without a central management authority.
+
+### Deliverables
+
+The output of the TG is expected to include:
+1. CX Multiplexing
+1. CX State Contexts, including Supervisor/User modes
+1. CX C-API, C++-API, and Runtime System
+1. (optional) CX Hardware Interface
+
+### Acceptance criteria
+
+A proof-of-concept implementing each deliverable will involve multiple hart(s) x
+extension(s) x extension librar(y/ies) x OS combinations.  
+
+## Exclusions
+
+Not every arbitrary custom extension can be a composable extension.  In
+particular, the first release of CX will likely support only unprivileged
+computational instructions. In particular, control-flow instructions will
+likely be excluded.
+
+Load/store instructions that operate on main memory, while important, add a
+level of complexity that may require it to be deferred until a successive
+release.  Similarly, dynamic reconfiguration of the attached CX logic, an
+important capability of FPGAs that implement SoftCPUs, may also be deferred.
+
+The CX TG is focused on the minimum viable standards *enabling*
+practical composition of extensions and software. Further standards
+for infrastructure and tooling e.g. for CX packages, debug, profile,
+formal specification of CX interface contracts, CX library metadata,
+and tools, are _out of scope_.
+
+## Collaborations
+
+CX multiplexing reduces the opcode and CSR impact of such extensions to zero,
+extending the life of the 32b encodings.  The CX framework will enable many
+unpriv computational extension TGs, e.g. the Matrix extensions.  
+
+Interaction with a unified Discovery mechanism is important for forwards
+compatability and versioning.
+
+### Overlaps (probably many, more TBD)
+
+* CX discovery API may overlap uniform discovery TG
+
+## History
+
+In 2019, the RISC-V Foundation FPGA soft processor SIG members, determined
+to advance RISC-V as the preeminent ecosystem for FPGA SoCs, committed to
+"Propose extensions ... to enable interoperable RISC-V FPGA platforms
+and applications". SIG members set out to define standards by which
+FPGAs' extensible RISC-V cores might enable a marketplace of reusable
+and composable custom extensions and libraries. Through 2019-2022,
+members met to define the *minimum viable set* of interop interfaces
+culminating in the
+[Draft Proposed RISC-V Composable Custom Extensions Specification](https://raw.githubusercontent.com/grayresearch/CX/main/spec/spec.pdf),
+now proposed as a starting point for RVI CX TG work.