*** LONG ANSWERS TO SHORT QUESTIONS ABOUT BANK SWITCHING *** by Joe Horn akcs.spirit@hpcvbbs.external.hp.com [Jeff Healey] writes: > I a little bit confused here. How, exactly, do the ports in the GX > differ from those in the SX (I'm not talking about bugs here, just > the hardware differences, what they are for, etc.)? Also, what do > you mean by ports > 2 ? Vital Terminology: "Card Slot": the *physical* place where you plug in expansion cards. There are always 2. The SX & GX have a diagram on the back. "Bank": A *physical* memory chip, intended to be located in the same address space as other chips, and electrically turned on and off to prevent address collisions with them. "Bank Switching": The act of electrically turning off one bank and turning on another. This wouldn't be necessary if the processor could address lots of memory. But it's restricted to two 128K plug-in RAMs, absolute maximum. By placing more than two in the same address space, and keeping all of them turned off except one, the HP48 is kept happy. By bank switching, we can then access the other banks, which makes us happy. "Port": a *logical* chunk of RAM. There can be any number of ports, from 1 to 3 in the SX, and from 1 to 34 in the GX. Port 0, which always exists, varies in size from 0K to 256K in the GX, and from 0K to 288K in the SX, depending on how much RAM is MERGEd and how much you store in port 0. It expands and contracts automatically, so that it is always exactly the size of its contents. Port 1 can only be 32K or 128K, depending on the size of the RAM card you've got FREEd in card slot 1, and regardless of its contents. This is the only port that is the same between the SX and GX. The LCD panel overhead projector display can be plugged into card slot 1 on either machine. In the GX, port 2 can be 32K if you plug a 32K card into slot 2. If you plug 128K *or larger* into slot 2, then port 2 will be 128K. All further banks of 128K in the card, regardless of contents, become mapped onto ports of their own (totally unlike the SX). The second 128K bank becomes port 3; the third 128K bank becomes port 4, and so on. The 1 Meg card from HP therefore creates ports 2 through 9. A 4 Meg card (the largest possible) would create ports 2 through 33. Think of card slot 1 as a single-CD player, and card slot 2 as a CD cartridge player that can hold many CD's, and play tracks at random from any of them in any order. Here's how it works. Card slot 2 in the GX is physically the same as card slot 1, but some of the pins have been redefined. The display information lines have been replaced by bank switching lines. That's why you can't plug the overhead projector display into slot 2 in the GX. > On another subject, what does it mean for a card to be "bank > switched"? Same subject, actually. Each card slot on the SX could only access 128K of RAM. So Tripod Data Systems made a 256K card that was actually two 128K banks. You had to do the bank switching yourself (or under your program's control). As far as the HP48 was concerned, only the active bank existed, and the other was 100% inaccessible. TDS even made a 512K card that contained 4 banks. TDS cards contained enough chip pads and logic to support a 1 Meg card, but they never marketed one. HP makes a 1 Meg cards for the GX. (They're a dark green color, like the GX itself). Like the TDS cards, they are really just a bunch of 128K banks. BUT they differ in one radical way: they can be bank switched directly by the GX operating system through the new bank-switching lines. So the GX can loop through all the banks, call each one a "port", and show all of them simultaneously in your PORTS menu. Having a 1 Meg card plugged into slot 2 is *exactly* the same as having a modified HP48 with 8 card slots and 128K of RAM plugged into each one, and ALL OF THEM ON-LINE!. (They can't be MERGEd, of course; slot 2 is permanently FREEd). Remember the chatter about the need for a "port extender" to give us more card slots? No need, if you have a GX. What's really cool is that this allows all of the libraries in all of the banks to be attached and active at the same time! You couldn't do that with the TDS cards. Bottom line: you never have to worry about "bank switching" on the GX. It takes care of it for you. -Joseph K. Horn- -Peripheral Vision, Ltd.- akcs.joehorn@hpcvbbs.cv.hp.com Disclaimer: "Nokavi Ot Sosumi" "If you don't like Wells Fargo, bank switch."