(Quelle: PPC Calculator Journal V11 N9 Nov/Dec 1984)

Has the HP-41’s lack of an „=“ key confused your friends who are more familiar with the competition’s calculators? Have they chided you for having no „parentheses“ on the keyboard? Do they snort in disgust when they try to add 1+2 and get 2?

Fear no more! HP has taken a giant leap backwards with the new HP-41 AOS EMULATOR ROM that puts the HP-41 and TI-59 on „equal“ footing!

The HP-41’s RPN operating system is 12K, occupying ROM blocks 0 through 2. Block 3 is only used by the HP-41CX, which has a 24K operating system. The new AOS ROM uses block 4, the take-over ROM block, and replaces the RPN operating system. When turned on with the AOS ROM in any port, the HP-41 uses the 1½K AOS ROM as its new operating system.

The level of TI-59 emulation is wondrous to behold. Not only are the functions on the keyboard redefined to work in the hierarchical, semi-algebraic system that the TI-59 uses, but EVERYTHING is like the TI-59! Just a few examples:

Press the PRGM key, and you enter „LRN“ mode. The display shows nothing but numeric keycodes and op codes and other oddities that require decyphering. Best of all, when you key in a program step, the LRN mode doesn’t show you the new step, it shows you the next step, so you have to BST to see what you just keyed in. Clever, isn’t it! Also, there is no automatic insertion of code into a program; keyed-in steps overwrite old steps. To insert a step, you have to execute „INS“ which pushes everything up one byte and inserts one null for you to write over. To delete a step, you execute „DEL“ once per step deleted. Needless to say, the „TI-59“ is a royal pain to program – or should I say „teach“ – in LRN mode.

Since the TI-59 has no alpha mode, but its printer (PC-100) does support alphanumerics, the ALPHA key is disabled and the user enjoys the special challenge of keying in every letter in numeric code. If you thought running the HP-41 printer was hard, wait till you see how printing on the PC-100 gobbles up program memory!

Odd to tell, the HP-41’s memory is reallocated in such a way that TI’s famous „hidden digits“ feature is emulated. Numbers are stored to 13 lucky digits of accuracy, but cannot be displayed as such. The heavy math functions are of normal TI inaccuracy, but the user is unaware of round-off errors because they are lurking in the hidden digits. So 2^3 looks like 8, even in FIX 9, but if you subtract 8 you get a fraction: the hidden error.

The HP-41 cannot be forced to accept TI-59 magnetic cards, because they are physically larger than HP-41 cards, but HP did the next best thing with the AOS ROM. First, the module is wired to the port in such a way that every turn-on causes a master clear, just like the TI-59 which has volatile memory. Next, the card reader’s built-in functions are replaced with fewer and less powerful functions. Although not a documented feature, the AOS ROM user will notice many read errors and faulty card writes; this is undoubtedly HP’s effort to avoid disconcerting TI-59 users who would be puzzled by faultless card operations. Also, different AOS ROMs have different parameters for card operations, thus fully imitating the TI-59’s typical inability to read other TI-59 cards.

Since the HP-41 has ten fewer keys than the TI-59, and since the TI-59 does not support key assignments, HP made clever use of the „INV“ keys. The TI-59 has no RTN command, for example; to return, you press INV SBR (inverse of subroutine). So HP extended that concept to access the missing keys. „(“ opens a parenthesis, and INV „(“ closes it. „+“ adds; INV „+“ subtracts. X^2 squares X; INV X^2 gives 2^X. |X| yields the absolute value of X; INV |X| returns it to its former value. 1/X inverts X, and INV 1/X provides the inverse function of that. CLR clears the stack, and INV CLR fills it with garbage. NOP is a TI-59 function that performs no operation; INV NOP in the AOS ROM provides a random operation (useful for statistical studies not of random data but of random operations on data). We all know what „=“ does; INV „=“ yields a number which is NOT the answer. GTO searches for a label and jumps to it; INV GTO searches for a label and branches from it back to the GTO. And so on.

Perhaps the most appreciated feature of the AOS ROM is the coupon you get with it for a free retrofit of a TI-59 style package for your HP-41. If you take them up on the offer, your HP-41 will return from Corvallis in record time (the operation is suspiciously rapid) dressed up just like a real-live TI-59. For example, the key will be hard to press at first, but after some months will revert to a mushy squish. The LCD is replaced with a battery-eating LED display. The tough HP-41 PVC case is replaced with TI plastic (a blend of bakelite and Tupperware) for that „delicate“ feeling. The battery pack is now more secure, and cannot be accidentally (or purposely) removed without damaging the case. The AOS ROM is installed inside, and there is only one port now available, conveniently underneath, behind a special door, making access an engaging occupation. Unfortunately, it only accepts TI modules, not HP ones, not even the PPC ROM. In fact, the revised HP-41 cannot be told apart from a TI-59! Even the funny little map of Texas is on there.

Please note: once the retrofit has been performed, HP will not undo it. In fact, if you want your HP-41 to look like it did before, there is only one way. Go to your local HP dealer and buy another HP-41. If you are lucky, you just might find your old HP-41 again, cleaned and repackaged and good as new. Then you can use the TI-59 as a doorstop, which it does very well.

Joe Horn (1537)
St. Michaels Priory
1042 Star Route
Orange, CA 92667 USA