hp 41c flying ghost and hp 67 display..

[teenix]

Internally, the A and B registers inside the ACT IC are used to control the display and to serve as general purpose registers. The ACT IC decodes the A/B information and sends the data to the ROM0 IC which further decodes the information and sends it to the Anode/Cathode drivers which light the display LEDs.

This is a continuous process unless the display has been turned off by a [Display Off] instruction during the HP-67 internal program execution.

The A and B registers are also used for temporary data registers for internal operations.

Because some programs can run for a long time, the HP programmers decided to turn the display on and off for periods of time while the program is running and so give the user some feedback. The data in A/B is now just processing information and is not properly formatted for the display so when the display is turned on, it shows gibberish.

This idea was useful because the HP-67 hardware does not allow alphanumeric characters to be displayed other than "Error" or "Crd" so it cannot show something like "PrG runninG".

cheers

Tony