HP-71B-Programming ROWDRV (Display driver)


Hello all,

I’m studying the HP-71B internals, especially the display driver. To do that, I try to modify/improve existing LEX and got stuck with one particular entry. While studying the DSPLEX that changes the behaviour of the display (such as “small fonts, invert etc) I was surprised how easy the implementation of the small font feature was. Changing fonts from normal to small is achieved just by setting ROWDRV to the value “0000800360041008”.

What does the ROWDRV entry point for the HP-71B. The IDS volume 1 says : “Row lines (16 nibbles). Should be set by software as follows: 8001400220041008”. This is set accordingly in “LCDINI” (see IDS volume 3 page SB&DVR-34) and says “Bit pattern for driving display”.

Notice that the hex sequence is symmetrical from the middle (: 80014002 Vs 20041008).

Is there anybody that can give a definition of the pattern and how it affects the display behaviour ?


Alain Robillard


Is this operation possible with the bare bones machine ?
Thank you.



Is it possible with the bare bones machine? I would guess so, the operations to switch to small font is :

SMA    LCHEX  0000800360041008 * Small constant
STORE D1=(5) ROWDRV * Point to row driver
DAT1=C 16 * Write out C and return
nxtstm GOVLNG NXTSTM * return to main loop

so, technically, just a poke to ROWDRV (E2350) with the proper value is required.


Alain Robillard



wrong address for ROWDRV, it is NOT E2350 but 2E350.

ROWDRV = 2E350.

Also, a simple poke does the joke as far as SMALL is concerned. But, you must enter the reverse string :

Poke "2E350", "8001400630080000" will put the HP-71B display in small font mode.

Regards !

Alain Robillard


Hello all,

CHHU Chronicle V2N1 page 30 gives all the details about ROWDRV for the HP-71B. I tried a couple of funny examples too.


Alain Robillard


Sounds interesting! Are these chronicles anywhere available in the web?




not on the WEB, but you could have all this (and much more) with the PPC CD-ROM Set. You should never leave home without it (and your HP-71B) !

Here is the link http://www.waterw.com/~jakes/ppccdrom.htm


Alain Robillard

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