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# Introduction to RPL
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The original RPL (*Reverse Polish Lisp*) programming language was designed and
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implemented by Hewlett Packard for their calculators from the mid-1980s until
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2015 (the year the HP50g was discontinued). It is based on older calculators
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that used RPN (*Reverse Polish Notation*). Whereas RPN had a limited stack size of
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4, RPL has a stack size only limited by memory and also incorporates
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programmatic concepts from the Lisp programming language.
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2022-10-27 23:32:00 +02:00
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The first implementation of RPL accessible by the user was on the HP28C, circa
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1987, which had an HP Saturn processor. More recent implementations (e.g., HP49,
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HP50g) run through a Saturn emulation layer on an ARM based processor. These
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ARM-based HP calculators would be good targets for a long-term port of DB48X.
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DB48X is a fresh implementation of RPL on ARM, initially targetting the
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SwissMicros DM42 calculator. This has [implications on the design](#design-overview)
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of this particular implementation of RPL.
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## The RPL stack
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The RPL stack can grow arbitrarily in size.
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By convention, and following RPN usage, this document gives the names `X`, `Y`,
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`Z` and `T` to the first four levels of the stack. This is used to describe the
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operations on the stack with synthetic stack diagrams showing the state of the
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stack before and after the operation.
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For example, the addition of two objects in levels 1 and 2 with the result
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deposited in stack level 1 can be described in synthetic form using the
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following stack diagram:
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`Y` `X` ▶ `Y+X`
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The duplication
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## Algebraic mode
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Unlike earlier RPN calculators, RPL includes complete support for algebraic
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objects written using the standard precedence rules in mathematics. In RPL,
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algebraic expressions are placed between ticks. For example, `'2+3×5'` will
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evaluate as `17`: the multiplication `3×5`, giving `15`, is performed before the
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addition `2+15`, which gives `17`.
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Algebraic expressions are not evaluated automatically. The _R/S_ key (bound to
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the [Evaluate](#evaluate) function) will compute their value.
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