IBM AIX

You can create one watchpoint on AIX 4.3.3.0-2 (AIX 4.3R) or later systems running 64-bit chips. These are Power3 and Power4 systems. (AIX 4.3R is available as APAR IY06844.) A watchpoint cannot be longer than 8 bytes, and you must align it within an 8-byte boundary.

Linux x86,

You can create up to four watchpoints and each must be 1, 2, or 4 bytes in length, and a memory address must be aligned for the byte length. That is, you must align a 4-byte watchpoint on a 4-byte address boundary, and you must align 2-byte watchpoint on a 2-byte boundary, and so on.

Linux x-86-64 (AMD and Intel)

You can create up to four watchpoints and each must be 1, 2, 4, or 8 bytes in length, and a memory address must be aligned for the byte length. For example, you must align a 4-byte watchpoint on a 4-byte address boundary.

HP-UX IA-64
Linux IA-64,

You can create up to four watchpoints. The length of the memory being watched must be a power of 2 and the address must be aligned to that power of 2; that is, (address % length) == 0.

Solaris SPARC

TotalView supports watchpoints on Solaris 2.6 or later operating systems. These operating system let you create hundreds of watchpoints, and there are no alignment or size constraints. However, watchpoints can’t overlap.