Socket 423 was the original interface socket for the Pentium 4 processor upon its introduction in November 2000. Physically, it is very similar to both Socket 370 and Socket A, but asymmetrical, with more pins on one side than on the others. It appears to be a simple modification of Socket 370 to handle 53 more pins.
Even as the Pentium 4 was introduced in Socket 423 packages, the news was circulating of a new, different socket called Socket 478 that was to be introduced for the P4 in 2001 and would replace the Socket 423 completely by the second half of 2001. Unfortunately for early Pentium 4 adopters, this did happen exactly as planned; by January 2002 Socket 478 has completely replaced Socket 423 as the interface on all Pentium 4 CPUs and motherboards.
All Willamette-based Pentium 4 chips were available in Socket 423 packaging, but for the higher speed grades they were hard to find and expensive compared to the Socket 478 versions.
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