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.
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.
The update will add two new instructions to the 64-bit instruction set: LAHF and SAHF.
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