Why the Right State Matters

This chapter follows the second half of InSPECtor’s validation method: turning a generated SLEIGH instruction into a state-aware execution test and comparing the result with a hardware reference. The specific focus is why bytes alone are insufficient. The same instruction can appear correct for ordinary inputs and diverge only when a register, memory location, flag, floating-point value, or alias takes a carefully chosen value. The practical judgment is that a discrepancy is an observation whose meaning depends on both the initial state and the reference environment.

GenSYS addresses this problem by using the p-code already attached to a constructor path. Instead of choosing initial registers and memory randomly, it symbolically follows the operations and asks which inputs would make their boundary cases visible. JuxtaPlayer then runs the resulting case on the SLEIGH-derived emulator and on hardware, recording comparable architectural outcomes. The connection between those two stages is what turns a structurally reachable instruction into evidence about behavioral fidelity.

P-code can describe the state that exposes its own mistakes

An instruction encoding tells the decoder what form of instruction to select. It does not determine the complete condition under which the instruction will execute. An addition may set a flag only when a carry occurs. A comparison may take a different branch at equality. A shift may behave differently when the shift amount reaches or exceeds a width boundary. A load or store may reveal an address-alignment problem only for a particular memory location. A floating-point operation may distinguish ordinary numbers from special values such as NaNs.

Those conditions live in the architectural state: register values, memory contents, status bits, and other state visible to the instruction. InSPECtor treats the p-code sequence as a compact description of the relationships that can make such conditions observable. The state generator starts with symbolic values and processes the p-code operations in order. For an operation, it identifies its symbolic inputs and output, expresses an edge case as an assertion about those values, and asks an SMT solver for a concrete initial state that satisfies the assertion. It then updates the symbolic state with the operation’s output before considering later operations.

This sequencing matters. Consider an addition whose p-code first computes a result and later derives a zero or negative flag from that result. The edge case for the later flag is not a condition on an unrelated fresh value; it is a condition on the sum produced by the earlier operation. If the generator propagated the symbolic expression, it can solve for initial operands whose sum is zero, negative, or overflowing. A random state might happen to hit one of those conditions, but the symbolic method asks for it directly and preserves the causal link between the input registers, the arithmetic result, and the flag operation.

The generated state is therefore targeted at an operation, not merely at an instruction mnemonic. For arithmetic, useful boundaries include overflow, underflow, zero, and negative results. For comparisons, equality and values immediately on either side can matter.