TI-66 double speed mod

[pauln]

(07-08-2022 07:48 AM)Didier Lachieze Wrote:  

(07-08-2022 02:17 AM)pauln Wrote:  One problem with the linked list approach is that it needs 7 extra bits for each one of the 100 registers of the HP-65 (to point to the next step).

The program memory in the HP-65 is not a RAM (random access memory) where each element has his own address, it's just a long register containing all steps sequentially, so there is no address pointer and no individual register. You can view it as a window to a circulating ring of instructions. You view the current one, then the next one and so on but they don't have an address you can use to jump to it. Markers in this flow of instructions are inserted to identify beginning/end of memory, next step to execute/fill and return location from subroutine. All is controlled by hardware and the flow is only going forward to the next instruction.

More details from the May 1974 issue of the Hewlett-Packard Journal:

The memory itself contains no absolute addresses. Instead, it is a circulating shift register organized into six-bit words. One word is a marker that denotes the boundary between the beginning and the end of the memory. Another word is a pointer which denotes the last step executed in run mode, and the last step filled in program mode. As a program runs, this pointer is moved down through memory. Branching is accomplished by moving the pointer to the location of the destination label. User-defined function calls are implemented by leaving the main pointer at the call and activating a second pointer at the function location (see Fig. 3). When the return to the calling location occurs, the second pointer is deactivated and the first pointer reactivated.

Thanks for the additional info. I think I understand much better how it works. Very interesting and unusual.

Still, one particularity of the HP-65 is that it auto-inserts instructions meaning that if the program contains "1 Enter 2 + R/S" and the user is seeing the "2" and types "3" the program becomes "1 Enter 2 3 + R/S". (As opposed to overriding and producing "1 Enter 3 + R/S").

Even with its carousel architecture, it seems to me that the HP-65 needs to shift around 100 6-bit words (in the worst case) within the register. And it does that "instantly" (at least according to the emulator https://www.sydneysmith.com/products/gss...index.html).

In contrast, the TI-57 takes around 2 seconds to shift 50 steps (when using the "Ins" key). Not sure how the HP-65 does that so fast (or the TI-57 so slowly). Maybe it is a feature of this carousel architecture.

IMO, the sentence "Thus program steps can be inserted into the program or deleted from it at any point, without re-keying the other steps." is referring to the fact that it can do so because programs use labels instead of going to a specific step. But still, physically, bits need to be moved around.

Edited: on second thought, I see now that it is much easier to move 600 contiguous bits as in the case of the HP-65 than a similar amount of bits in the TI-57, where program steps are distributed between several smaller registers and are not necessarily contiguous (https://www.rskey.org/CMS/index.php/the-library/100).