Overview
In the cited RISC-V self-testing framework, the focus register is the register that carries the value under test through a reversible instruction sequence. In REVERSI, a modification operation applies the test instruction to a focus value together with random data, and a later restoring operation reconstructs the original focus value so the framework can check whether execution was error-free.
Role in REVERSI
REVERSI defines a test pattern with three main register roles:
- focus register: holds the original value being tested
- random register: supplies additional operand data
- target register: stores the result of the modification and restoring operations
The modification operation uses the focus register and random register as source operands and writes to the target register. The restoring operation then uses the target register and random register to reconstruct the focus value, saving the restored result in the target register. Finally, the framework compares the focus and target registers to determine whether execution was correct.
For some instructions, restoration is not naturally lossless. The cited source notes that extra control code can be added so the value of the focus register remains fully restorable.
Role in PATARA pipeline-hazard testing
PATARA extends this model for pipeline hazard detection. In a regular PATARA test case, the focus register is always assigned to the first source operand, while the second source operand carries random data and may be either a register or an immediate. This default arrangement leaves forwarding to the second source operand insufficiently tested.
To cover both forwarding paths, PATARA can swap the positions of the focus value and random value during test-case generation. This allows hazard detection to be exercised for both source operands. The probability of performing this operand swap is configurable in the PATARA framework.
Practical significance
Within the provided evidence, the focus register is not a general architectural register class but a test-generation role used to:
- define the value whose transformation and restoration are checked in REVERSI, and
- control operand placement in PATARA so forwarding and hazard-detection logic can be tested more completely.