# ADR-0003: Target System Hierarchy & Modeling Scope

## Status

Accepted

## Context

We need a system-level simulator to evaluate LLM kernel performance on our AI Accelerator platform.
The platform is organized as a compute tray containing multiple identical SIPs connected via PCIe or UAL
through switching fabrics, with a host CPU issuing commands/kernels.

## Decision

We model the system hierarchy explicitly:

### D1. Tray-level

- A compute tray contains:
  - Host CPU (issues requests / coordinates runtime & data placement)
  - Multiple identical SIPs (accelerators)
  - Interconnect fabric between SIPs (PCIe and/or UAL via switches)

### D2. SIP-level

- A SIP is a multi-die package composed of:
  - Multiple CUBEs (HBM die + compute PEs + UCIe)
  - One or more IO chiplets (host/SIP interfaces)
- IO chiplets:
  - provide interfaces: PCIe-EP, IO_CPU, optionally UAL-EP
  - can be multiple per SIP
  - placement constrained to SIP shoreline (top/bottom/left/right); each shoreline may host 1–2 IO chiplets

### D3. CUBE-level

- A CUBE contains:
  - HBM + memory controller (HBM_CTRL)
  - NOC (on-die fabric): carries all intra-cube traffic including HBM data,
    inter-cube (UCIe), command (M_CPU↔PE_CPU), and shared SRAM access.
    Must provide: full-BW PE↔local HBM path, PE↔SRAM connectivity,
    PE↔UCIe connectivity, M_CPU↔PE command path.
    NOC topology is an implementation choice (e.g., 2D mesh, ring, crossbar);
    current implementation uses a 2D mesh with XY routing (see ADR-0017).
    HBM_CTRL is attached to each PE's local NOC port (local HBM = minimal hop).
  - Shared SRAM: cube-level shared memory accessible by all PEs via NOC
  - management/control CPU (M_CPU) coordinating PE command distribution and completion aggregation
  - multiple PEs
  - up to 4 UCIe endpoints (N/E/W/S) for CUBE↔CUBE and CUBE↔IO connectivity

### D4. PE-level

- A PE can execute one kernel instance
- PE contains internal control + accelerators (modeled at PE view granularity):
  - PE_CPU, command handler, PE_TCM, DMA/GEMM/MATH engines, internal queues

## Consequences

- The simulator supports abstraction by “views”:
  - SIP view hides PE internals
  - CUBE view treats each PE as a single block
  - PE view expands PE internals
- Topology remains parameterized; sizes/counts/links come from configuration.

## Links

- SPEC R3/R5
- ADR-0005 (diagram views)
- ADR-0017 (cube NOC 2D mesh architecture)