IBM has increased its dominance in the market for supercomputers used to solve the toughest research problems - claiming half of the world's Top 500 supercomputers, while the share of rival Hewlett- Packard fell sharply.

HP fell to 26 per cent, or 131 of the most powerful supercomputers ranked in the semi-annual Top 500 List. That is down from 170, or 34 per cent, just six months ago.

IBM scored broad gains, with 259 machines or 51.8 per cent of the Top 500, up from 43 per cent of the world's most powerful supercomputers last November.

"IBM is on the cusp of something similar to the dominance it had in mainframes 40 years ago," said analyst Stacey Quandt, an industry analyst with Quandt Analytics in Santa Clara, California. "There is a parallel to developments taking place in its dominance of supercomputers today," she said.

For the first time since the Top 500 List - www. top500. org - was issued in 1993, all 500 computers listed could handle a teraflop, or a trillion calculations per second. Number 500 on the list can crunch 1.17 teraflops.

The rapid increase in computer firepower among the most powerful computers means that 201 of the 500 computers listed on the last Top 500 list in November have fallen out of the rankings, largely explaining the dramatic market shifts.

IBM supercomputers rank number one and two on the list and account for six of the top ten IBM business systems - as opposed to more traditional models used by government or academic laboratory customers - make up 161 of the 500.

IBM also dominates the low-end, with 193 systems running on low-cost Linux software and composed of hundreds or thousands of mass-market computer chips.

Silicon Graphics increased its share slightly to 24 of the Top 500 supercomputers. Dell had 21 supercomputers on the list, while Cray had 16.

Turning to the underlying components used to build the world's most powerful supercomputers, the big winner is Intel, whose low- cost, massproduced chips are now used in 333 of the top 500 supercomputers.

IBM's Power processors run 77 systems, while HP chips power 36 and Advanced Micro Devices's microprocessors control 25 of the machines.

As a measure of China's growing technology prowess, the world's most populous country now has 19 supercomputers on the Top 500 list, just behind Japan with 23 machines. China entered the Top 500 list only three years ago with its first machine.