- Bitsavers has an interesting paper on PA-RISC 2.0 (the jump to 64 bits). It's got a high-level introduction, and then all the actual details.
https://bitsavers.org/pdf/hp/pa-risc/PA_RISC_2.0_Architectur... (2MB file)
- Having briefly worked at HP on the IA64 effort IIRC the PA-RISC chips fabbed at Intel were the side-effect of the Itanium agreement. HP was owed a certain volume of chips and since Merced was very delayed they had to make those chips for HP.