DM41 Save/Restore Statistics

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====== DM41X Save/Restore Stats ======

** Using Extended Memory to save Statistics Registers **

===== Background =====

Statistics use 6 regular registers to keep track of data for calculating Mean and Standard Deviations.

Keeping a long-term record of a set of variables means not being able to use the Statistics functions for anything else unless steps are taken to save and restore the registers.

It is possible to save the 6 registers in a data file in Extended Memory and then use the statistics for other things, and restore the saved Statistics back to Main Memory when required.


|R11|∑x|Summation of x-values|
|R12|∑x<sup>2</sup>|Summation of squares of x-values.|
|R13|∑y|Summation of y-values.|
|R14|∑y<sup>2</sup>|Summation of squares of y-values.|
|R15|∑xy|Summation of products of x- and y-values.|
|R16|n|Number of data points accumulated.|
===== Extended Memory Data File =====

The statistics registers use 6 Main Memory registers, usually R11 -> R16

We need to create a Data File in Extended Memory to hold these.

  * Put the name in ''ALPHA'' e.g. ''REGS''
  * Put ''6'' in ''x'' register
  * excecute ''CRFLD'' to create the Data File ''REGS'' in Extended Memory

Once it's created it can be used to hold the contents of the 6 Main Memory Statistics Registers

===== Save Registers to Extended Memory =====

  * We need to make sure our Data File is the current file and that the pointer is at the beginning of the file
    * Put ''REGS'' in ''ALPHA''
    * Put ''0'' in ''X'' so that we put the pointer at the first register record in the data file.
    * Execute ''SEEKPTA''
  * Now we need to copy the 6 Registers R11 -> R16 to the Data File ''REGS''
    * ensure ''REGS'' is still in ''ALPHA''
    * put a number in ''X'' to indicate which range of Registers to save, this is ''bbb.eee'' where ''b'' is the start register and ''e'' is the end
      * put ''11.016'' in ''X''
    * execute ''SAVERX''

===== Restore (get) Registers from Extended Memory to Main Memory ====

  * We need to make sure our Data File is the current file and that the pointer is at the beginning of the file
    * Put ''REGS'' in ''ALPHA''
    * Put ''0'' in ''X'' 
    * Execute ''SEEKPTA''
  * We can copy the 6 registers held in the Data File ''REGS'' back to R11 -> R16
    * ensure ''REGS'' is still in ''ALPHA''
    * put a number in ''X'' to indicate where the recovered registers will be put in Main Memory 
      * put ''11.016'' in ''X''
    * execute ''GETRX''

===== A program to make it easier =====

It is possible to change the base register of the 6 Statistics Registers, away from the default of R11. 

We need to take this into account.

This uses the query ''∑REG?'' to find the base register, then create ''bbb.eee'' using simple arithmetic.

When used in ''SAVES'' we ensure we copy the current Statistics Registers data.

When used in ''GETS'' we ensure we restore our data to the Registers currently set to be the active Statistics Registers (they may have changed since first saving them)

<code>

LBL ¬SAVES
¬REGS
0
SEEKPTA
∑REG?
1.001
*
.005
+
SAVERX
¬STATS SAVED
AVIEW
PSE
CLD
RTN
LBL ¬GETS
¬REGS
0
SEEKPTA
∑REG?
1.001
*
.005
+
GETRX
¬STATS OK
AVIEW
PSE
CLD
RTN

</code>





===== Further Information =====


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