Skip to content
Kathryn Kathryn

Decentralized update and priority

Decentralized Update is the second of Kathryn’s three abstractions. Its promise is simple: any block, anywhere, may update a resource. An assignment does not mutate a register directly and it does not have to be funneled into a single hand-written driver. Instead each <<= (Edge Assignment) or = (Level Assignment) records an update event on the target resource, and the framework resolves competing writes by priority.

Decentralized update

How competing writes resolve

Section titled “How competing writes resolve”
  • Ordinary user writes all carry the same default priority (DEFAULT_UE_PRI_USER).
  • When several writes hit the same resource in the same cycle, the highest priority wins.
  • A reset event outranks every user write — a held reset pins the register to its reset value no matter who else is writing.

Minimal example

Section titled “Minimal example”

To rank one write above another, bracket it with SET_ASM_PRI_TO_MANUAL(p) / SET_ASM_PRI_TO_AUTO(). From autoSim simAutoTest63:

SET_ASM_PRI_TO_MANUAL(DEFAULT_UE_PRI_USER + 1);  // rank the next write higher
b <<= 48;
SET_ASM_PRI_TO_AUTO();                           // back to the default
b <<= 24;

Both writes target b in the same cycle; the bracketed one has the higher priority, so b becomes 48.

The two writers do not have to sit next to each other — that is the point of the abstraction. Any two blocks can write the same resource from wherever they are, each with its own priority, and no central FSM ever owns the resource’s writes.

flowchart LR
    W1["writer 1<br/>SET_ASM_PRI_TO_MANUAL(USER + 1)<br/>b &lt;&lt;= 48"] -->|"event, higher priority"| POOL["update events of b"]
    W2["writer 2, default priority<br/>b &lt;&lt;= 24"] -->|"event"| POOL
    POOL -->|"highest priority wins"| REG["b = 48"]

Where to go next

Section titled “Where to go next”