From the source:
Enhancements: */
/* Fits in 512 bytes, saving 1.5K of code space */
/* Background page erasing speeds up programming */
/* Higher baud rate speeds up programming */
/* Written almost entirely in C */
/* Customisable timeout with accurate timeconstant */
/* */
/* What you lose: */
/* Implements a skeleton STK500 protocol which is */
/* missing several features including EEPROM */
/* programming and non-page-aligned writes */
/* High baud rate breaks compatibility with standard */
/* Arduino flash settings */
/* */
/* Currently supports: */
/* ATmega168 based devices (Diecimila etc) */
/* ATmega328P based devices (Duemilanove etc) */
/* */
/* Does not support: */
/* ATmega1280 based devices (eg. Mega) */
/* */
/* Assumptions: */
/* The code makes several assumptions that reduce the */
/* code size. They are all true after a hardware reset, */
/* but may not be true if the bootloader is called by */
/* other means or on other hardware. */
/* No interrupts can occur */
/* UART and Timer 1 are set to their reset state */
/* SP points to RAMEND */