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Knowledge: My memory is going!
By: Larry Garfield, Thursday 24th July 2003, 22:03 GMT
Many Palm users have noticed that newer devices don't have as much RAM available as they claim. Larry Garfield explains why.
All Palm OS handhelds use a memory archicture known as eXecute-In-Place (XIP). XIP architecuture lets Palm OS devices store programs and data in RAM, and then run (execute) the program right where it is in RAM rather than copying it to another part of RAM. That saves a considerable amount of memory, as the system doesn't have to pretend that part of RAM is a pseudo-hard disk and part is "active" memory. It can use the same RAM for both functions. That's one reason why Palm devices can have so much less RAM than their Pocket PC-based counterparts, which do use the split memory architecure; The Palm OS is just much more efficent with its RAM.
So where is the RAM going on Palm OS 5 devices?
When they're running, all programs require the RAM that is taken up by the actual program code itself as well as an extra "scratch paper" space to keep track of what the program is doing. That includes the operating system itself, which also has its own working space in RAM. This scratch pad memory is known as the system heap, and is taken out of the available RAM.
On a desktop PC or Windows CE-based system, that heap is taken out of the available active RAM and is usually far smaller than the space taken up by the running program, so it usually isn't noticed. On a Palm, however, the active RAM is the same as the storage area, and the operating system can't take memory away from that because it's used by programs that the user has installed.
In order to allow programs the ability to actually run, the Palm OS reserves a small portion of RAM that can be used only by the system heap. Every version since Palm OS 1.0 has had a reserved system heap, but the size has changed. As devices have gotten more RAM, and programs have become more complex, the size of the heap has steadily increased from a mere 32 KB in Palm OS 1.0 to 256 KB in Palm OS 4.0. All temporary space needed by the operating system itself and any running program had to fit inside that small heap.
In Palm OS 5, however, PalmSource decided to let the size of the system heap be more flexible. Licensees of the Palm OS are able to fix the heap at whatever size they wish at manufacture time, depending on how they are targeting their device. Palm has generally used about a 2 MB system heap in its Palm OS 5 devices, which is ample room for most programs and gives them some elbow room to spare. Sony focuses more on multimedia, which generally takes up a lot more heap space for processing. Earlier OS 5 devices from Sony had a 5 MB system heap, while some of Sony's newest devices, which have a heavier emphasis on video, have a whopping 16 MB system heap.
When buying a newer Palm OS handheld, therefore, it is important for buyers to consider both the total RAM listed and the RAM "available to the user". The larger the difference, the less space the user has to store programs and data but the more complex are the types of programs that it will support. Currently the only programs that really need the full 5 or 16 MB heap on the latest CLIEs are Sony's pre-bundled multimedia apps, but that may change in time.
Despite the larger system heap, the Palm OS's integrated memory system is one of the most efficient available today in the handheld market. |
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