The timing is the same as that of changes to the stock-trading system under the bourse's information-technology master plan.

After the SET's board of governors approved Cinnober Financial Technology as the developer of the main IT system - ranging from equity securities, derivatives and information dissemination to surveillance - it is expected to begin operating in the second quarter of 2012.

SET president Charamporn Jotikasthira said the exchange was improving 18 rules governing capital securities trading, surveillance and dissemination of information for both the SET and the Market for Alternative Investment.

The amendments will affect ceiling and floor levels of initial public offering shares on the first trading day, market orders, and changes of intra-day orders. The first-day IPO ceiling will be maintained at 300 per cent, while the floor will be raised to 300 percent from 50 percent currently.

The exchange will amend three sections of rules regarding market orders. Under the first section, trading transactions sent into the system will be divided into three parts for amendment.

In the first part, currently trading transactions are sent into the system for automatic matching. To lower risks for customers, the exchange will add a regulation that if there are odd orders, their selling or buying prices will be set one spread higher from the last execution prices.

In the second part, if there are odd orders from the matching, they will be cancelled out. This is in line with the international standard, which is not currently followed by the SET. However, some brokerage members disagree with this as customers may lose opportunities. Thus the exchange will add one more part, allowing trading transactions to be sent to match at the best prices. If there are odd orders, their selling or buying prices shall be set at the last execution.

The second section applies to calculation of opening and closing prices. The highest trading volume matched and the least odd volume unmatched will determine opening and closing prices on a daily basis. This will reflect real share prices; the current does not, as opening and closing prices are calculated from several highest-volume orders. Then the system chooses a price close to the previous trading day's closing price as today's opening price.