This is a summary of what I’ve found. It is likely to get updated from time to time Continue reading HP-21 Microcode Reference
Category Archives: hp21 internals
HP-21 DSP
This one should be remarkably straight forward; but it isn’t. There are standard ways of doing standard things. This is a standard thing. It just doesn’t follow any of the standard ways. Here’s what happens Continue reading HP-21 DSP
HP-21 Function Key
The HP-21 has a blue function key at the top right of the keyboard. I tend to think of it as the “g” key but that’s just my background with the HP-67 which has a blue “g” key. Whatever you call it, the question is, how does it work Continue reading HP-21 Function Key
HP-21 Low Power
Here’s what the HP-21 looks like with low power:
Here’s how that happens Continue reading HP-21 Low Power
HP-21 Keypress
The HP-21 keypress routine is incredibly simple. It looks like Continue reading HP-21 Keypress
HP-21 Scan Codes
Scanning the calculator keyboard to see if a key has been pressed, works pretty much the same on a HP-21 as all the other calculators. This is the process Continue reading HP-21 Scan Codes
HP-21 Wait Loop
You’ll never see a shorter wait loop than this one. Continue reading HP-21 Wait Loop
