spent "lots of time" playing Modern Warfare, a first-person shooting game, and also took an entire year off to play World of Warcraft, a multi-player role-playing game with more than 10 million subscribers.
"I don't really like those games but it is good if you want to simulate for training purposes," Breivik said as he discussed Modern Warfare, smiling when asked about the aiming system.
Breivik killed eight people with a car bomb in Oslo on July 22 and then killed 69, mostly teenagers, at a Labour Party summer youth camp on Utoeya island, in a gun massacre.
Although he pleaded not guilty, he admitted the killings, saying his victims were traitors who supported immigration and multiculturalism, threatening Norwegian ethnic purity.
Breivik, who once played Modern Warfare 17-hours straight on New Year's Eve 2010/2011, said he used such games to simulate the police response and the best escape strategy.
"I calculated the likelihood of surviving unharmed at less than 5 percent," he told the court in his third day of testimony, referring to the bomb attack on government headquarters, when he expected to be swarmed by police officers.
"I trained myself to get out of such a situation. That is what I was simulating."
When he acquired the weapons for the actual attacks, he turned to Norse mythology in naming them.
"The rifle I called Gungnir, which is the name of the magical spear of Odin, which returns after you have thrown it. And the Glock I called Mjoelnir...It is the warrior god Thor's Hammer," he said, adding that he marked the weapons with their names in runes.
While playing computer games, Breivik said, he withdrew from his friends, saying personal relationships were not a priority.
In 2006, he moved in with his mother to save money and rarely interrupted his game of World of Warcraft, even though his mother became anxious.
"Of course I couldn't tell her I was going to take a sabbatical because I am going to blow myself up in five years' time."
"During that year I played perhaps 16 hours a day. It was a lot. Only playing for an entire year -- playing and sleeping, playing and sleeping....It was a dream I had, and I wanted to do this."
Thomas Hylland Eriksen, a professor of social anthropology at the University of Oslo, said such computer games could put Breivik in a state of delusion.
"When he went out on Utoeya, possibly at some level still believing he was still paying a computer game and shooting people in real life," Eriksen told Reuters away from the court proceedings.
"He does not seem to be very successful at distinguishing between the virtual reality of world of Warcraft and other computer games and reality," he said.
Breivik's trial, set to last 10 weeks, turns on the question of his sanity and thus whether he can be jailed. He has said that an insanity ruling would be "worse than death".
One court-appointed team of psychiatrists concluded he was psychotic, while a second team found him to be of sound mind.
On Wednesday he said he should either be executed or acquitted, calling the prospect of a prison sentence "pathetic".
Breivik has insisted he is a commander in a resistance movement but has acknowledged some of his claims were an exaggeration. He spent much of Wednesday defending the claim that it existed at all.
In court, he has Breivik struggled to defend his claim of being ordained into a militant-nationalist group called the Knights Templar in London in 2002 after preliminary contact in 2001, refusing to answer over 100 questions on the topic.