Thread: hp-41cx / NoV-64d / Hepax XF multi-function

[brouhaha]

(09-07-2023 12:41 PM)Sylvain Cote Wrote:  The new PIC code (2020+) follow the more common way, system pages (0...7) are individually switched and port pages (8...15) are port switched.

That's not quite how system page bank switching works in actual HP-made hardware. Third party ROMs not manufactured by HP, should behave similarly, but there's no guarantee.

The Nut CPU is not involved in bank switching. It treats all bank switch instructions as no-ops. Only the ROM chips interpret bank switch instructions.

For actual HP ROMs, including the 41CX mainframe ROMs, the method is that all the ROMs in one physical integrated circuit, the three-page (120 kbit) ROM, honor a bank switch fetched as an instruction from that ROM only, and no other ROM chips pay any attention to it.

41CX pages 0, 1, and 2 are in one 120 kbit chip, and are not bank switched.

41CX page 3 (single bank) and page 5 (bank switched) are in one 120kbit chip. A bank switch fetched as an instruction from either page 3 or page 5 will switch the page 5 bank, but have no effect on any other pages, including pages 4, 6, 7, or ports. In particular, a bank-switched printer ROM such as 82242A infrared printer module in port 6 will not affect the 41CX page 5 or vice versa.

HP 12K ROM modules in ports normally work as you describe, with bank switching affecting the entire port, but this is only because the 120K chip was deliberately configured that way. In principle, an HP 120kbit chip could have 4K port addressed and 8K outside its page (perhaps page 4), or vice versa, and the bank switching would be triggered from any page in that chip.