Apple overtook Microsoft in stock market value on Wednesday, as the two co-founders of the PC industry, switched roles as the world's most valuable technology company, after decades of intense rivalry. 

At the close of the markets in New York, Apple was worth $222bn, just behind oil giant ExxonMobil and Microsoft was worth $219bn.

In the past year, Apple shares have risen from below $131 to an all-time high of $272.46 reached in April. Co-founder Steve Jobs returned to work after a liver transplant, more than six years after having been diagnosed with pancreatic cancer. Meanwhile, the struggling company he returned to lead in 1997, has had a string of stunning successes with the iPod music player and the iPhone smartphone, which have boosted sales of the Mac computer. The recent launch of the  iPad, a tablet computer that has already sold more than 1m units, cements Jobs' iconic status for design in tune with consumers.

Described as a high-strung workaholic, who motivates others with his enthusiasm, has a "reality distortion field," passionate about technology, a micromanager, arrogant and intolerant; on September 12, 1985 Steve Jobs stood up at an Apple board meeting and after years of internal political turmoil and power struggles, said in an unemotional voice, "I've been thinking a lot and it's time for me to get on with my life. It's obvious that I've got to do something. I'm thirty years old."

Apple was co-founded by Steve Jobs and fellow college dropout Steve Wozniak on April Fool's Day in 1976. They had both spent some time working at Hewlett-Packard in Palo Alto.

Their first product was a build-it-yourself computer kit and in the following year, the Apple II was launched. It is credited with popularising the home computer.

In the early 1980s, Bill Gates and Microsoft, which was founded in 1975, pulled off a coup with a licensing deal with IBM for the DOS operating system that it had acquired for $50,000.

Microsoft 1978 - - Top row: Steve Wood (left), Bob Wallace, Jim Lane. Middle row: Bob O'Rear, Bob Greenberg, Marc McDonald, Gordon Letwin. Bottom row: Bill Gates, Andrea Lewis, Marla Wood, Paul Allen. December 7, 1978.

Microsoft was formed soon after the introduction of the Micro Instrumentation and Telemetry Systems (MITS) Altair, the first "personal computer," a build-it-yourself kit for hobbyists. Bill Gates and Paul Allen seized the opportunity to transform this early PC into a breakthrough -- the Altair needed software, a programming language that could make it perform useful computing tasks.

Apple insisted on selling a closed system with its own software bundled with the hardware.

When Steve Jobs left Apple, it had a 16 percent share of the PC market, putting it at No. 2 behind IBM. Market researcher IDC says that it still held about 10.7 percent of the market as late as 1990.

Steve Jobs became CEO of Pixar Animation Studios, the Academy-Award -winning computer animation pioneer which he co-founded in 1986. The Northern California studio has created six of the most successful and beloved animated films of all time: Toy Story (1995); A Bug's Life (1998); Toy Story 2 (1999); Monsters, Inc. (2001); Finding Nemo (2003); and The Incredibles (2004).

The Apple Mac retained a loyal following but it wasn't enough to maintain market share. Apple's stock fell to $3.30 in July 1997, and the company reported a net loss of $708m in its second quarter that year. Those losses came as rivals like Dell and almost every other computer company except Apple were soaring during the tech boom of the late 1990s. Jobs returned to the company that year and Apple soon began getting attention with a string of new products, including the colourful iMacs. The iPod was launched in October 2001.

Microsoft even gave a $150m loan on Steve Jobs' return.

The PC Operating System

Bill Gates is certainly one of the significant businessmen of our age but it is often claimed that he and his colleagues were more brilliant at execution than invention. In a 2005 feature on Apple in the US magazine BusinessWeek, Steve Jobs said that Microsoft's Windows software had been copied  from Mac software. 

Most people in an answer to the question, who invented the light bulb, would say Thomas Edison. It would be wrong. The invention of the operating system, which ushered in the PC revolution is more recent but how many would know who was responsible for it?

In October 2004, a book titled They Made America, was published in the US. It was authored by former editor of the UK Sunday Times Harold Evans and profiled 70 American innovators.

Harold Evans gave the credit for inventing the PC operating system to Gary Kindall, (1942-1994) founder of Digital Research.

Kindall had a computer PhD and created the first personal computer operating system, CP/M. His insight was that by creating an operating system separate from the hardware, applications could run on computers that were made by different manufacturers. The CP/M operating system had sold well and IBM, the world's then biggest computer company was looking for a system for its planned Personal Computer. IBMers visited the 24 year old Bill Gates at Microsoft in August 1980 but Gates was unable to supply them with a system and he directed them to Kindall.

Kindall met with IBM but he was reluctant to agree to the onerous conditions of a non-disclosure agreement relating to confidentiality but there was apparently an oral agreement. On August 28, 1980, Microsoft signed an agreement with IBM to develop software for the PC. Gates was aware of an operating system called QDOS, which had been developed by a fellow Seattle resident named Tim Paterson. Microsoft bought QDOS for $50,000. QDOS, which was similar to CP/M was renamed DOS and improved by Microsoft.

It was a year before Kindall discovered that his longtime friend Bill Gates had signed the plum deal with IBM and to add insult to injury, it looked that parts of QDOS had been copied from CP/M. Software copyright was in its infancy and rather than take IBM to court, Kindall agreed to license CP/M to IBM. He was shocked when IBM charged $240 per copy for CP/M and only $40 for DOS. The rest is history, which is generally written by the victors.

Two years before his death in 1994, following a fall from a bar stool, Kindall was invited by the University of Washington to attend the 25th anniversary of its computer science programme. He was one of its earliest and most distinguished graduates but Harvard dropout Bill Gates was chosen as the keynote speaker.

In his unpublished memoir, Kindall wrote: "Well, it seems to me that he did have an education to get there. It happened to be mine, not his."

Gates and Jobs

In May 2007, the two co-founders of the of the PC industry, Bill Gates of Microsoft and Steve Jobs of Apple, who had been fierce rivals for thirty years, appeared jointly in a public forum for the first time in twenty years. 

"Bill built the first software company in the industry," Apple co-founder Jobs said. "Bill focused on software before anyone."

Gates, the Microsoft co-founder, hailed Jobs for taking big risks and developing products with "incredible taste and elegance".

"What Steve has done is quite phenomenal,"Gates said in reference to Jobs' eye for design that resonates with the public . "The way he does things is just different. It's magical."

Steve Jobs and Bill Gates speaking at The Wall Street Journal’s D: All Things Digital conference in Carlsbad, California in May 2007: