Being inspired by the HP-16C there are a lot of similarities between the Ladybug module for the HP-41 and the HP-16C. Apart from the obvious differences, such as form factor, battery life, alpha capabilities on the HP-41 and dedicated decorated keyboard on the HP-16C, there are other less obvious differences, many of which are discussed in this chapter.
.. index:: flags
Flags are arranged differently. The reason for this is the annunciators 0-4 on the HP-41 that can be used to provide additional feedback compared to the HP-16C. The arrangement was inspired by making it logical, rather than trying to be compatible with the HP-16C.
The sign and zero flags are very common on micro processors and gives additional feedback, being shown by the annunciators. It also allows for implementing compares in a way more similar to micro processors, which is discussed next.
.. index:: compares
Compares on the HP-16C is done using traditional HP calculator
operations, by providing a set of operations that compares between X
and Y, and X to 0. Ladybug works more like a micro processor. It
provides a CMP instruction that compares X to any register value
and set flags accordingly. After CMP you can interpret the result
using a flag test operation FC?, FS? or some of the built in
relation test instruction like LT? and friends.
It is also possible to use the TST instruction, or rely on the
flags set after doing an operation.
Note
CMP Y subtracts Y from X, which is the opposite order compared
to SUB.
The HP-16C keeps the floating point and integer modes separate. Changing between the modes affect stack contents and word sizes. On the HP-41 the integer mode is basically a keyboard and display shell, the functional behavior remains the same when you switch mode.
As a result, the mode separation is not as strict and stack register values are not normally pruned when changing mode, as we will discuss next.
.. index:: word size; affecting stack, sign extension
The HP-16C truncates values on the stack according to the active word size. Changing a word size to a smaller one and then back will not preserve all value bits. Ladybug only alters value bits on the stack when going to a larger word size. Reducing to a smaller word size will not affect value bits on the stack.
Increasing the word size will affect all registers on the stack with Ladybug. In signed mode, values are sign extended, and in unsigned mode values are zero extended. This is done to preserve numerical values.
It is possible to keep 56-bit floating point values on the stack, provided that the word size is not increased. This allows for mixing floating point operations with integer mode.
The reason for doing it this way, is that the HP-41 can perform floating point operations at any time. Keeping the stack properly masked would be a quite elaborate task and would get in the way with the ability to keep floating point values around. The HP-16C has an easier task here, as it has a more strict separation between floating point mode and integer mode. On the positive side, you gain the ability to have both integers and floating points on the stack at the same time.
Numbers are in general masked as needed when they are used as input to operations in Ladybug, not because they are laying around somewhere.
.. index:: postfix operands, operands
Ladybug takes advantage of the prompting instructions on the HP-41 to allow for accessing stack registers in the same ways as numeric registers. Indirection can also be done on any register, not just the single index register as on the HP-16C.
.. index:: zero fill mode, mode; zero fill, setting; zero fill
With Ladybug, the zero fill mode does not indicate available digits above the current window if they are all zero. The HP-16C will always indicate that there is more to see, even if it only filled 0 digits.
The reason for this difference is that it is believed that instantly knowing if there is anything non-zero to see outside the display is more useful than to be constantly reminded that the word size is actually larger than what can be shown in a single display.
.. index:: 1-complement mode, mode; 1-complement, signed mode
The one complement mode is not present in Ladybug.
.. index:: windows, display windows
The window display only provides for moving a full window at a time, not by single digits which is also available on the HP-16C.
The keyboard layout to do this does not require pressing a shift key, which makes it somewhat easier to work with windows with Ladybug, compared to the HP-16C.
.. index:: operations; double precision, double precision
Double divide will result in a double quotient. The HP-16C gives a
single word quotient, or an error if a double result would have been
needed. Giving the full quotient is believed to be more useful, but
changes may be needed to HP-16C programs that uses DDIV.
.. index:: status; machine, machine status
There is currently no machine status display in Ladybug. Most of the
information about the status is already visible in the display, the
rest can be queried using WSIZE? or FS? 05 for zero fill
mode.
Ladybug does not offer an integer square root function, which is present on the HP-16C.
.. index:: floating point conversions, conversions; floating point
There are no support for floating point number conversions built in to Ladybug at this point. It is something that is considered for a future extension.
.. index:: postfix operands, operands; postfix, prompting instructions, instructions; prompting
Ladybug takes full advantage of the prompting facility of the
HP-41. Instructions such as MASKL and WSIZE prompt for their
argument and are not limited to take it from the X register. To get
the same behavior as on the HP-16C, use the indirect X postfix
argument:
WSIZE IND X
However, for MASKL which takes two values on the HP-16C, such
straight translation would not work as the instruction would take the
the stack registers in opposite order.
In most cases you will probably just use a postfix numeric argument rather than a register indirection:
MASKL 4
Shift operations prompt for the shift count, which makes it unnecessary to have two instructions to implement the same shift operation, compared to the HP-16C.
Note
No savings would be made by making two instructions, as the default behavior of the semi-merged shift instructions is to shift by 1. In other words, the shift instructions do dual duty as shift by one and shift by arbitrary number of steps.
Is currently not present in Ladybug.