Here’s what happens when you press 123.45 EEX 78 CHS. Continue reading HP-65 Number Entry Microcode →

The GTO function is pretty straight forward in the HP-65 calculator. Here’s how it works: Continue reading HP-65 GTO →

What causes the HP-65 display to flash? What triggers it, how is it done in the microcode, and how does it get out of it again? Continue reading HP-65 Error Display →

Now that the HP-65 Delayed Select Group instruction is behaving just like the real thing, it becomes possible to see what the real microcode does for the [R/S] key and as a program step. Here’s the details: Continue reading The Real HP-65 R/S Sequence →

As discussed in HP-65 R/S, the commonly available microcode for the HP-65 calculator goes into an (almost?) infinite loop if you press the [R/S] key. Here’s what it looks like: Continue reading HP65 Microcode R/S Infinite Loop →

The [R/S] key is supposed to allow you to Run and Stop a user entered program. That, by itself, is useful. However, it also allows you to stop a program at a predetermined point and allow data to be keyed in. That is also very useful. When you’ve keyed in (or calculated) the value, you just press [R/S] to resume the program.

But it doesn’t work. Continue reading HP-65 R/S →

The RTN key does a return from a subroutine. It also stops a main program and, if nothing’s running at all, it will take you to the “top of memory marker” (“00 00” in w/prgm mode).

Here’s how it does all that: Continue reading HP-65 RTN →

When you press the [A] key the HP-65 calculator starts looking for a “LBL A” in program memory and then runs it. Unlike the HP-67 which has a “(use) default functions” flag, the HP-65 fires up with its default functions entered into program memory. This simplifies the ’65 processing for [A].

Here’s what happens: Continue reading HP-65 – Pressing the [A] key →

It looks like s8 controls whether a program runs or not. Here’s why: Continue reading HP-65 – The s8 “Running” Flag →

Here’s how we get X displayed when we’re in FIXed display mode. Continue reading HP-65 Display X in FIXed mode →

Here’s how we get from a key press to the HP-65 (or emulator) actually doing something … Continue reading HP-65 Key Press Routine →

Interestingly, all of the STO and RCL instructions in program memory goto the same microcode address. Also of interest is these instructions show the clash between “merged” keystrokes and unmerged ones: STO and RCL are merged keycodes (one step for two key presses); but not all of them are merged. (I think they ran out of available codes before they ran out of key sequences and decided to keep what they could.) As a result, you can see the behaviour of both approaches in the same instruction. Continue reading HP-65 Microcode – STO and RCL →

When the HP-65 runs program steps it gets a program code (0-63) from program memory and then does something depending on the program code. Continue reading HP-65 Program Code Jump Table →

How does the HP-65 display a program step? If you run the HP-65 emulator with logging turned on and flick the switch to W/PRGM you’ll see. Here’s the details: Continue reading HP-65 Display Program Step →

The HP-65 calculator has an internal set of status flags known as the “s register”. “s3” is one of those flags but it behaves a bit strangely … Continue reading HP65 s3 – W/PRGM →