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HP-41CL update ROM using HP-IL
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HP-41CL Update module using HP-IL

This module for the HP-41CL contains some instructions that allow reading and writing 16-bit images to and from HP-IL mass storage devices.

In addition there is an RPN program that is used to control the process which is better documented in the hp-41cl_update project.


Traditionally, a ROM page in the HP-41 is 4K large. As it uses 10-bit memory, there is a choice of compressing it when storing it to a byte oriented file system. However, the .rom file format is an uncompressed memory page, and uses 2 bytes for each memory location. Thus, it will normally have the 6 upper bits set to 0.

In the HP-41CL, the physical memory system is actually 16-bit wide, though it only exposes 10-bit to the Nut CPU (logical memory). These previosuly unused bits now have a meaning as they are used to control the speed of the faster NEWT CPU to pace it in timing loops. Additionally, some special HP-41CL modules make use of the extra bits for data purposes.

As a result, there is now a need to pass 16-bit ROM images to the HP-41CL. The read and write instructions provided in this module makes it possible to transfer such (uncompressed) 16-bit ROM images between the physical memory of the HP-41CL and HP-IL mass storage.

Read and write instructions

The RDROM16 instruction reads a physical memory page from HP-IL mass storage to HP-41CL RAM memory.

The WRROM16 instruction writes a physical memory page to HP-IL mass storage.

Both instructions expects a filename in the Alpha register and the physical address page in X.

The physical address page is a 3-digit binary number, it can be specified either as a BCD number or as a binary number (non normalized number, NNN).

To specify page 801 (hex), you can enter it as an ordinary number 801.

If the address page contains hex digits above 9, for example 80E, you need to specify it as an NNN. This can be done in two ways, either enter the address in Alpha register and use the traditional CODE instruction, or load the Ladybug module, enable hex mode and type in the hex number in X.

Some examples:

To save page 810 (hex), enter filename in Alpha register and type:

810 in X

To load page 800 (hex), enter filename in alpha register and type:

800 in X

Details on the page numbers

In case you wonder about the X register input, recall how a floating point number is represented in the HP-41


where S is the sign, MMMMMMMMMM is the mantissa and SXX is the exponent with its sign.

If the mantissa is non-zero, the address is obtained by taking the first 3 digits of MMMMMMMMMM and treating that as a binary page address. As these are BCD encoded digits, it means that if you enter 800, it will get treated the hex number 800, in fact you can actually just type 8 as it will appear at the first position and the rest of MMMMMMMMMM will be 0. If you enter 800003, it will still take the first 3 digits and treat that as 800.

If the mantissa is 0, RDROM16 and WRROM16 expects that SXX contains a binary page address (12 bits).

To enter a binary page address you need to use CODE, which is a common instruction from the good old days of the HP-41. It allows converting a hex number in the Alpha register to a binary (non-normalized) number in X.

If you are using the PPC ROM, you can do as follows:

ALPHA 8AF ALPHA       ; type in the page number
XEQ HN                ; hex to NNN (works the same as `CODE`)
ALPHA filename ALPHA
XEQ RDROM16           ; read from file

An alternative way is to use the Ladybug module which uses binary integers as its native numbers. This means that you can just enter the 3 digit hex number in X from the keyboard:

WSIZE 56              ; set word size to 56 bits
                      ; (any word size 12 or wider will work)
ALPHA filename ALPHA
8AF_           H      ; type the page number as `8AF`
XEQ RDROM16           ; read from file

Error messages

There is a wide range of error messages that can result from using these instructions, some are explained below.

  • NONEXISTENT is given if there is no HP-IL module attached

  • DATA ERROR is given if the address is outside allowed range (the allowed range is 807 to 87F inclusive)

  • SIZE ERR is given by RDROM16 if the file size is not 8K bytes.

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