PalmOS was originally designed to run on a Mobile Device to provide local copies of information that was managed from a host computer. This Personal information included such things as a contact list, ToDo list, Calendar, and Memos. Memos were limited to 4K making them not suitable as a storage mechanism for eBooks. However, the Palm OS allowed the installation of third party programs and was soon the platform of choice for a portable reading device.
The PalmOS is a simple but elegant system. Programs and data are stored in RAM which is also used to store temporary data needed for computation. There is no file system in RAM so no file extensions or file names are allowed. Programs are designed to "execute in place" so the storage copy is the same as the execution copy. However, as the RAM area is synced back to the host computer the data is split into various files. These files typically used a PRC or PDB file extension and are named with the database name.
With the release of Palm OS 3.0 there were devices with a couple Meg of RAM storage so it became feasible to store several novels on the device. A PalmDOC, often simply called DOC, format was designed (originally owned by Aportis) to hold eBooks. This is a simple format and there are currently thousands of books available in this format. Other eBook readers soon developed that added functionality and formatting capabilities that were not in the original Palm Doc format but most of these newer readers could also read PalmDOC files. See PDB for more information on eBook formats used on the Palm. Some hardware configurations such as those from Handera could add external expansion cards to the device allowing more eBooks and other items to be stored in external memory. Screen size was square with a resolution of 160x160 pixels.
Release 4.0 of the Palm OS introduced support for external cards and many devices appeared with this capability. Screen resolution remained at 160x160 pixels for most devices although some hardware companies upped that to 240x240. A third file type, PQA was added. It is used to identify network type files as new Palm Devices are able to deal remotely with computer systems. To save space PQA data are often compressed using a special compression scheme.
Release 5.0 offered support for a new processor (ARM compatible) which opened the door to more advanced processing power similar to that available on Windows Mobile systems although it is still not a multitasking system. The screen size remained square but the resolution was increased to 320x320. Some hardware implementations offer a rectangular screen with 320x480 pixel resolution by using a virtual grafitti area. Hardware memory extension cards such as SD cards became available on most devices and the ability to support file extensions other than .pdb and .prc became available on the cards although the main memory will not support other extensions. Many eBook applications were expanded to take advantage of the new resolutions and memory cards.
While the user experience is limited to single tasking to keep the interface simple the underlying features of release 5.0 does allow for some background tasks. This is generally used to support playing music while working with other tasks.
- Palm Pilot: The original Palm device with 160x160 screen.
- Palm TX: 320x480 touch screen, color, backlit to read in the dark, adjustable brightness, instant pageflip, bookmark, autoscroll, and the general benefits of PDA's over the e-ink devices: Wi-Fi, music, sketchbook, Word, Excell, TomTom, video and games.
- Garmin iQue: similar to the Palm TX but added a GPS.