Overview
Speculative Instruction Issue is a mechanism supported by the Open Vector Interface (OVI) for vector instruction flow between a scalar core and a Vector Processing Unit (VPU). In the cited OVI description, the scalar core uses the ISSUE sub-interface to send a request together with the instruction, configuration values, and scalar input. A separate DISPATCH sub-interface then confirms or kills issued instructions, which is what enables speculative issue of vector instructions.
Role in OVI
OVI divides scalar-core/VPU communication into multiple sub-interfaces. For speculative instruction issue, the relevant interfaces are:
- ISSUE: carries the request from the core, including the instruction, configuration values, and scalar input.
- DISPATCH: confirms or kills all issued instructions.
Because issued instructions can later be either confirmed or killed, the scalar core can issue vector instructions speculatively rather than waiting until every instruction is known to be non-speculative.
Handshaking
The evidence also states that the ISSUE interface uses a credit system for handshaking between the VPU and the scalar core. This indicates that speculative issue is integrated into OVI’s flow-control scheme rather than being an independent side channel.