The Reg heard at the IDF that Intel is preparing to update its Pentium 4 Prescott core to make its 64-bit processors more compatible with AMD's 64-bit processors. The new Prescott stepping, called G-1, will replace the current E-0 core in the last quarter of this year. Full shipments will likely take place on November 14th while first samples should arrive on October 17th.
The tweak, which will require a BIOS update, adds a pair of instructions to the EM64T instruction set: LAHF and SAHF. These two codes copy content back and forth between the chip's status flags and its AH register.The update will add two new instructions to the 64-bit instruction set: LAHF and SAHF.
So far, only the P4 521, 531, 541 and 551 are mentioned in the core-upgrade notice, but it's likely the 6xx series will be updated too, along with the Pentium D and Pentium Extreme Edition.
At the time (the launch of Intel's first EM64T 64-bit processors), Microprocessor Report analysts speculated that this had happened because Intel based EM64T on a version of AMD's AMD64 documentation written down before AMD incorporated LAHF and SAHF into its own instruction set.The Xeon server chips will likely see a comparable update with the Paxville. More details at The Register