Editing Analytical engine (section)

== Instruction set ==
[[File:Babbage Analytical Engine Plan 1840 CHM.agr.jpg|thumb|upright=3|center|Plan diagram of the analytical engine from 1840]]

Babbage is not known to have written down an explicit set of instructions for the engine in the manner of a modern processor manual.  Instead he showed his programs as lists of states during their execution, showing what operator was run at each step with little indication of how the control flow would be guided.

[[Allan G. Bromley]] has assumed that the card deck could be read in forwards and backwards directions as a function of conditional branching after testing for conditions, which would make the engine Turing-complete:

<blockquote>...the cards could be ordered to move forward and reverse (and hence to loop)...{{sfn|Bromley|1982|p=215}}</blockquote>

<blockquote>The introduction for the first time, in 1845, of user operations for a variety of service functions including, most importantly, an effective system for user control of looping in user programs.

There is no indication how the direction of turning of the operation and variable cards is specified. In the absence of other evidence I have had to adopt the minimal default assumption that both the operation and variable cards can only be turned backward as is necessary to implement the loops used in Babbage's sample programs. There would be no mechanical or microprogramming difficulty in placing the direction of motion under the control of the user.{{sfn|Bromley|2000}}</blockquote>

In their emulator of the engine, [[Fourmilab]] say:

<blockquote>The Engine's Card Reader is not constrained to simply process the cards in a chain one after another from start to finish. It can, in addition, directed by the very cards it reads and advised by whether the Mill's run-up lever is activated, either advance the card chain forward, skipping the intervening cards, or backward, causing previously-read cards to be processed once again.</blockquote>

This emulator does provide a written symbolic instruction set, though this has been constructed by its authors rather than based on Babbage's original works.  For example, a factorial program would be written as:

 N0 6
 N1 1
 N2 1
 ×
 L1
 L0
 S1
 –
 L0
 L2
 S0
 L2
 L0
 CB?11

where the CB is the conditional branch instruction or "combination card" used to make the control flow jump, in this case backward by 11 cards.