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Late last year PlanetWeb was asked by Zilog Inc. (Santa Clara, Calif.) to help create a low-cost Internet appliance. Using the TVgraphics display chip and systems reference design acquired from MSU Ltd. (Milton Keynes, England), Zilog developed an enhanced version, the Z90602, that would maintain the low cost of the original design while meeting the performance demands of this emerging consumer market.
The aim was an appliance that depended not on sophisticated high-end processors for performance and features, but on lowcost 32-bit processors such as the 386SX, which is widely available from a number of suppliers. Zilog hoped to provide many of the features necessary for a browserbased appliance through the use of low-cost dedicated components.
To understand how we went about porting our software to Zilog's hardware, it is necessary to know something about the design of the PlanetWeb browser itself. The original browser code is written to be largely platform and operating-system independent. The portions that are hardware dependent are isolated and handled separately from those that are not, easing the process of moving from one platform to another.
The core of the network-interface software is based on published standards, meaning it doesn't change from one hardware platform to the next. (The process of laying out HTML is a good example.) Only the user interface and display elements of the code need to be customized for each platform. This helps make an extremely efficient design-so much so that the entire browser application-program size is less than 500 kbytes of code.
The PlanetWeb browser operates between the user and the network. On the network side, the low-level hardware-interface modem code and/or Ethernet code is platform specific and thus needs to be customized. The next layer of network code, including support for TCP/IP and HTTP, is standardized and remains constant across all platforms. Parental control and e-mail application layers reside above the HTML engine code, which reads Web pages and decides what to do with each of them. These layers also remain standard on the browser. Three application layers extend from the HTML engine: audio, graphics/fonts and the file-management system. The audio can be customized for a specific sound system. The file-management system is also platform specific and includes organization code for any ROM/flash or...