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HP-71B Math RAM

Posted by Marcus von Cube, Germany on 16 Feb 2009, 8:06 a.m.

This is not a typo! ;)

I managed to get the HP-71B Math Pac image into a free RAM port of my real HP-71B. Here is what I did.

Neccessary Gear

1. A physical HP-71B with 32kB RAM to spare,
2. An ISA PC with HP-IL card or a PIL-BOX by J. F. Garnier,
3. HP-IL enabled EMU71 with MATHROM.BIN configured in some port,
4. A HEX editor on your desktop machine.

Modification to MATHROM.BIN

The ROM module contains a protected file which will not be copied by the HP-71B's COPY command unless you change the flag beforehand. This is quite easy: Open MATHROM.BIN with a HEX editor and change the byte 03 at address 000E to 00. Voilà, the protection is gone!

In order to save precious RAM you should set the file size in the image to a value that does not include the many 0 nibbles at the end of the ROM: Put the values 50 D8 at address 0014 in the file (original value was D6 FF.)

Copy the LEX file to a virtual drive

In EMU71 (with physical HP-71B turned off):

COPY MATHROM:PORT(2) TO :HDRIVE1
CONTROL OFF

Install the LEX on physical HP-71B

On physical HP-71B:

[ON]
COPY MATHROM:HDRIVE1 TO :PORT(5.01)
SECURE MATHROM
[OFF]
[ON]

And now to test:

DIM A(5)
MAT INPUT A
MAT PRINT A

Explanations

My EMU71 configuration has MATHROM.BIN configured in port 2. I'm pretty sure that you can even leave out the source port specification in the COPY command and EMU71 will still find the file.

I've a CMT card reader port RAM module which installs ports 5.01 to 5.04. 5.01 was just free for the purpose. That's all. If you have claimed all port RAM as main memory, you should be able to copy MATHROM in main RAM but I haven't tested it.

Good Math!

Edited: 5 May 2012, 4:58 a.m.

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