Hardware

In the handheld computers we study, the Intel StrongARM SA-1110 system-on-chip (SOC) is used. It has an integrated LCD controller (LCDC). A framebuffer is implemented in off-chip memory (main memory) to store pixel data for a full screen. Whenever there is a screen change, the processor generates new data for the changing screen pixels and stores them into the framebuffer. This implies a higher energy consumption with increased temporal changes in the screen. Meanwhile, to maintain a screen on the LCD, the LCDC must sequentially read screen data from the framebuffer and refresh the LCD pixels even when there is no screen change. This in turn implies a higher energy consumption with increased spatial changes in the screen. The on-chip system bus and off-chip data buses also consume energy for data transfers. For other handheld computers, details may vary but the fundamental organization and operating principles are similar. The display itself consists of several parts: LCD power circuitry, a front light, and an LCD. The LCDs used in the systems we studied are color active thin film transistor (TFT) LCDs. In such LCDs, each pixel has three components: R, G and B, signifying red, green and blue, respectively. Liquid crystals for each component are independently oriented by two polarizers, which are connected to a storage capacitor. The capacitor is in turn charged and discharged through a TFT to accommodate screen changes. Moreover, the capacitor must be refreshed at a high rate to maintain an appropriate voltage across the polarizers so that the corresponding liquid crystals remain properly oriented. The hardware perspective is summarized in Figure 1.

Figure 1: A hardware perspective for GUI energy consumption.