Computer advertising in 270 high-tech publications increased by 11% to 104,504 pages in the first half of 1995, according to AdScope, a Eugene, Ore.-based ad tracking service. Ad spending in the period increased 13% to $1.1 billion. Most of the increase was attributable to smaller advertisers running in special interest titles; among the top 12 advertisers, ad pages actually dropped 0.8%. IBM Corp., which cut its ad pages by 33.3% in the six-month period, fell to second place with 1,437 ad pages. Microsoft Corp., which increased its ad page tally by 27.7%, was the No. 1 advertiser in the period with 1,451 ad pages. No. 3 Digital Equipment Corp.'s ad page count skidded by 41.5% to 995.7. Big gainers among the top 12 included No. 4 Hewlett Packard, up 79.2% to 963.2 pages and No. 7 Computer Associates International, up 81.6% to 796.2 pages.