What is the ACE Protocol?

The **ACE protocol** is a **protocol for bidirectional data coherency**. It is designed to facilitate efficient data sharing and synchronization between different components, particularly between a CPU and a GPU, by managing how data is kept consistent across their respective caches.

## Core Functionality and Cache Coherency

Bidirectional data coherency ensures that data written by one processing unit (like a CPU) is visible and up-to-date when read by another processing unit (like a GPU), and vice versa. This is crucial in systems where CPU and GPU share memory and need to access the same data without encountering stale versions.

A key characteristic of the ACE protocol, as highlighted in the reference, relates specifically to cache management between the CPU and GPU:

*   **Before the CPU reads and writes data, the GPU driver does not flush cache data.**
*   **Before the GPU reads and writes data, the CPU does not flush cache data.**

This means that unlike protocols that might require explicit cache flushing (invalidating or writing back cache lines to main memory) by one unit's driver or controller *before* the other unit can safely access potentially shared data, ACE handles this coherency differently. The protocol itself manages the cache state, reducing the need for these manual, often time-consuming, flushing operations.

## Implications

This approach to cache management within the ACE protocol allows for more streamlined and potentially higher-performance data exchange between components like the CPU and GPU. By minimizing or eliminating the need for explicit flushing commands initiated by one unit before the other accesses data, the protocol simplifies software development and improves system efficiency when handling shared data workloads.