Hybrid Design Blocks

A Hybrid Design Block (HDB) is Kathryn’s core control-flow abstraction. Each HDB is a hybrid RTL/behavioral, cycle-accurate description of control flow: it orchestrates Cycle-Considered Operations (CCOs — the <<= and = assignments) and nested HDBs according to its own semantics. Because CCOs are user-defined operators and HDBs are composed from them, you specify cycle-accurate control flow entirely at the user level.

Every HDB elaborates to a graph of State Nodes and their supporting nodes. The State Node is the atomic element of the HDF state machine; each HDB instantiates its own State Nodes to orchestrate the internal nodes and define their execution conditions and connections. Writing an HDB is therefore not writing an FSM by hand — it is describing intent, which the model lowers to a predictable, per-cycle state machine.

The HDB family

The blocks below make up the control-flow HDB family. Each links to its detail page.

Block	Semantics
seq	All CCOs and sub-HDBs execute sequentially.
par / parMan	All CCOs and sub-HDBs execute in parallel.
cwhile / swhile / cdowhile	Body repeats in the manner of the *while master block.
cloop	Fixed-count loop with a hardware counter and a loop variable.
cif / sif / celif / celse	The matched conditional block executes in the manner of the *if master block.
pick / pif / pickDef	Matched pif blocks execute; the block does not wait for all branches to complete.
scWait / syWait	Stall execution until a condition holds, or for a fixed number of cycles.
sbreak	Exit the enclosing loop from inside its body.
zif / zelif / zelse, ztate / zcase / zcasedef	Zero-time, combinationally-resolved structural RTL fallback.

Pipelines (pip / zync) are the most flexible members of the family and get their own chapter — see Pipelines.

flowchart TD
    HDB["Hybrid Design Block"] --> COMP["composition"]
    HDB --> CTRL["control flow"]
    HDB --> STALL["stalls"]
    HDB --> RTL["structural RTL fallback"]
    COMP --> SEQ["seq"]
    COMP --> PAR["par / parMan"]
    CTRL --> LOOP["cwhile / swhile<br/>cdowhile / cloop"]
    CTRL --> COND["cif / sif<br/>celif / celse"]
    CTRL --> PICK["pick / pif / pickDef"]
    STALL --> SCW["scWait"]
    STALL --> SYW["syWait"]
    RTL --> ZIF["zif / zelif / zelse"]
    RTL --> ZTATE["ztate / zcase / zcasedef"]

Where to go next

Start with seq and par for composition, then conditionals, loops, and waits. For the structural-RTL escape hatch see z-blocks, and for the node vocabulary every block shares see Nodes.