Overlay Technique | Pebble Developer Retreat 2014

Overlays Tec hniqueRon & Grégoire

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What's an overlay ?

http://www.wikiwand.com/en/Overlay

In a general computing sense, overlaying means "replacement of a block of stored instructions or data with another."

Overlaying is a programming method that allows programs to be larger than the computer's main memory.

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Overlay ma pping

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Our Build Proc es s1.Generate an initial LD script2.First build of the app3.Calculate the size of the overlay sections4.Generate final LD script5.Second build of the app6.Extract overlays sections from the elf to

external resource file7.Trim elf file to remove overlays section8.Very final build

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Generate the initia l L D s c ript

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Overlay configurationSelecting files (from sources or libraries) for every overlay section

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Generate the initia l L D s c ript

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Generated LD script

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F irs t build of the a pp

pebble build

The overlays sections are mapped at a default hard-coded position

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C a lc ulate the overlay s ec tion s izearm-none-ea bi-reade lf -S build/pebble-app.elf

Maximum overlay section size = 0x1D00

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Generate fina l L D s c ript

Maximum overlay section sizeSize of main app

= 0x1D00 + 0x2600

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Generate fina l L D s c ript

Overlay Table

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S ec ond build of the a pp

pebble build

Every section will be mapped in its correct position thanks to the final LD script

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E xtrac t overlays from e lf

Extract the compiled code in the overlay sections to one unique resource binary file.

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T hird and fina l build of the a pp

pebble build

Pack the previously generated binary resource in the app

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Trim the e lf file

Remove the extracted overlay compiled code from the elf file to reduce its size (optional)

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T he a pp is ready !

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How to load an overlay ?

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How to load an overlay ?

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Pos s ible a pplic ations

If your app can be divided in independant parts,more parts can be added.

Example:Heap Usage for App <Pebblets>: Total Size <3632B> Used <2928B> Still allocated <40B>

Only 700 bytes remaining...

Any application that not all of its parts are active simultaneously

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Adjus ting c ode for overlay● Change any string array from form of c ons t c har* days [] to

c ons t c har days [][12] (one-two bytes bigger than length of your longest string.)

● If you use GPath, move your GPath data to external file with setup function

● menu_layer_set_callbacks. Instead of defining 'MenuLayerCallbacks menu_callbacks' and giving it to that function as parameter, put it directly inside the 3rd parameter of menu_layer_set_callbacks like: (MenuL ayerC a llbac ks ){ .get_num_row s = get_num_row s , .draw _row = draw _row, .s e lec t_c lic k = s e lec t_c lic k });

● vibes_enqueue_custom_pattern. Move your custom pattern to external non-overlay file.

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C ha llenges

● Overlay slot● Debugging information

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Try it

Some apps in the appstore :

- F ina lly (12 watchfaces in one)- PPOS Demo (concept for a point of sale demo) with source code, & detailed readme- Pebble Demos (all the SDK demos + compass in one app) with source code* check the links to source code on GitHub