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PicoWeb P-Code

Teddy Rogers

About This File

A pcode instruction interpreter was developed for the PicoWeb Server. The use of a custom pcode provides:

- program code simplification,
- the option to execute program code out of EEPROM (including external serial EEPROM), and
- in many cases a reduction in program code size as compared to native code.

Code simplification is achieved because the p-code makes no reference to native registers, and because the pcode "virtual machine" uses 16-bit wide data types for most operands. Pcode execution from EEPROM is possible because the pcode "instruction pointer" is 16 bits wide, more than is needed to address the program memory in the Atmel microcontroller. Therefore, the "extra" address bits can be used to indicate that the next pcode instruction should be fetched from EEPROM, and not from the microcontroller's internal program memory.

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