pub(crate) fn eval_is_halt_syscall<AB: SP1AirBuilder>(
        &self,
        builder: &mut AB,
        local: &SyscallInstrColumns<AB::Var>,
    ) {
        // `is_halt` is checked to be correct in `eval_is_halt_syscall`.

        // The syscall code is the read-in value of op_a at the start of the instruction.
        let syscall_code = local.op_a_access.prev_value();

        let syscall_id = syscall_code[0];

        // Compute whether this ecall is HALT.
        let is_halt = {
            IsZeroOperation::<AB::F>::eval(
                builder,
                syscall_id - AB::Expr::from_canonical_u32(SyscallCode::HALT.syscall_id()),
                local.is_halt_check,
                local.is_real.into(),
            );
            local.is_halt_check.result
        };

        // Verify that the is_halt flag is correct.
        // If `is_real = 0`, then `local.is_halt = 0`.
        // If `is_real = 1`, then `is_halt_check.result` will be correct, so `local.is_halt` is
        // correct.
        builder.assert_eq(local.is_halt, is_halt * local.is_real);
    }
Returns a boolean expression indicating whether the instruction is a HALT instruction.
crates/core/machine/src/syscall/instructions/air.rs