“It’s simple,” Tara added. “If you AND a zero to an existing zero in memory, it leaves it alone. But if you AND a zero to an existing one in memory, it changes it to a zero. So with the simple instruction—‘AND 0’—I can change any memory location to zero.”
“And that would leave you with NOPs,” Mauchly said, nodding.
“No Operation.” Dorfman’s voice rose with excitement. “Precisely. Leaving the computer’s memory full of empty instructions.”
“It wouldn’t work,” Silver said.
“Why not?” Tara asked.
“I’ve already explained. There are a dozen virtual simulacra of this machine, running elsewhere in Liza’s consciousness. They’re compared to each other every thousand machine cycles. They’ll see the new programming and ignore the original computer.”
“That’s just the point,” Tara said with a cough. “We’re not introducing any new programming. We’re just resetting the computer’s memory. Manually.”
“Out of the question,” said Silver.
Lash was surprised by the sharpness of Silver’s answer. For what seemed a long time — since Liza had gone silent, perhaps even before — Silver had acted defeated. Resigned. But now, there was a fierceness in his voice Lash hadn’t heard since their first confrontation.
“Why?” Tara asked.
Silver turned away.
“Can you tell me for sure—for sure—that you took that specific possibility into account when you coded the security protocols?”
Silver folded his arms, refusing to answer.
“Isn’t there a chance that zeroing Liza’s original memory will abort this self-destructive behavior? Or, at the very least, cause a system crash?”
Again, the question hung in the air. And now, for the first time, Lash made out a large gout of open flame — ugly orange against the black smoke — flaring up from a rack of equipment near the far wall.
“Dr. Silver,” Mauchly said. “Isn’t it worth a try?”
Silver turned slowly. He looked surprised to hear Mauchly voice such a question.
“Hell with it,” Tara said. “If you won’t help me, I’ll do it myself.”
“Can you program this thing?” Lash asked.
“I don’t know. Legacy IBM assembler didn’t change that much from machine to machine. All I can tell you is I’m not going to stand around, waiting to die.” And she stepped up to the archaic control surface.
“No,” said Silver.
All eyes turned toward him.
He’s not going to let her do it, Lash thought. He’s not going to let her stop Liza. He watched, transfixed, as the man seemed to wage some desperate inner battle.
Ignoring him, Tara raised her hands toward the row of buttons.
“No!” Silver cried.
Lash took an instinctive step forward.
“You need to deal with the parity bit first,” Silver said.
“Sorry?” Tara asked.
Silver fetched a deep breath, coughed violently. “The 2420 has a unique addressing scheme. The instructions have nine bits instead of the usual eight. If you don’t mask out the parity bit as well, you won’t get the empty instruction you want.”
Lash’s heart leapt. Silver’s getting on board, after all. He’s going to help.
Silver walked to a nearby teletype, snapped it on, threaded the attached spool of paper tape into the plastic guide of the reader. Then he moved behind the main housing of the 2420, his step increasingly decisive.
“What are you doing?” Tara asked.
Silver knelt behind the housing. “Making sure this computer will still respond to manual input.”
“Why?”
Silver’s head reemerged above the housing. “We’re only going to get one chance at this. If we fail, she’ll adapt. So I’m going to dump the current contents of her memory to paper tape.”
Tara frowned. “I thought you said you didn’t have any back doors.”
“I don’t. But there are a few early diagnostic tools, hard-wired, no hacker could ever have any use for.” Silver ducked back behind the housing. A moment later, the teletype came to life. The faded spool of tape began moving through the machine punch. A shower of thin yellow chads rained down onto the floor beneath.
Within a minute, the process was complete. Silver pulled an extra length of bare tape through the punch, ripped it away. He ran the tape through his fingers, scanning it. Then he nodded. “It appears to be a successful memory dump.”
“Then let’s get on with it.” Behind Tara, more gouts of flame were rising, and her dark hair was backlit with angry flames.
Silver folded the tape and stuffed it in his pocket. “I’ll give you the opcodes. You enter them.”
Tara raised her hands again to the control surface.
“Press the LDA button to load the first memory location into the register.”
Tara complied. Lash saw a tiny light illuminate beneath her finger.
“Now move to that panel of nine toggle switches. Enter ‘001111000.’ That’s 120 in decimal, the first available memory location.”
Tara ran her finger down the row of toggle buttons.
“Now press the execute button.”
A small light glowed green on the panel. “Done,” she replied.
“Now press the ADD button.”
“Done.”
“On the toggle switches, enter ‘100000000.’
“Wait. That ‘one’ at the beginning will screw everything up.”
“The parity bit, remember? It has to stay set.”
“Okay.” Tara ran her hands over the buttons again. “Done.”
“Press the execute button to ‘AND’ the zeros to memory location 120.”
Another press of a button; another confirmation.
“Now press the STM button to store the new value in memory.”
Tara pressed a button at the end of the row. Nodded.
“Now press INC to increment the memory pointer.”
“Done.”
“That’s it. You’re ready for the next set. You’re going to have to press those four buttons — LDA, ADD, STM, and INC — in order, executing the sequence each time, over and over until you reach the end of memory.”
“How many memory locations in all?”
“One thousand.”
Tara’s face fell. “Jesus. We’ll never have time to erase them all.”
There was a terrible pause.
“Oh. Sorry.” It was Silver speaking again. “I meant, one thousand in octal.” The smile that followed was even more ghostly than before.
“Base eight,” Tara muttered. “What’s that in base ten?”
“Five hundred twelve.”
“Better. But it’s still a hell of a lot of button-pressing.”
“Then I suggest you get started,” Mauchly said.
They worked as a team — Dorfman keeping track of the iterations, Tara punching in the opcodes, Silver checking her entries. Gilmore, the security tech, was dispatched to the exit hatchway, instructed to alert them if he observed any stand-down from Condition Gamma. Lawson was ordered to keep a clear avenue of escape between them and the interstructural hatch — just in case they succeeded.
They closed ranks around the little computer as the heat and smoke pressed in ever more fiercely. The air thickened, until Lash could barely see the figures around him. His eyes were streaming freely, and his throat was so parched by the acrid smoke that swallowing became all but impossible. Once or twice, Sheldrake disappeared in the direction of the backup generator and its lethal payload; each time he returned, his expression was grimmer.
At last, Tara stepped away from the control surface, flexing and unflexing her fingers.
Dorfman nodded. “Check. That’s five hundred and twelve.”