@Nick Gammon,
I tried using your bootloader programmer sketch for an Atmega8A device. It did not work. After some investigation I discovered that your sketch sends the Poll RDY/BSY* instruction in the pollUntilReady() function. This instruction is not supported by the Atmega8A. As a quick-and-dirty patch, I re-wrote pollUntilReady as follows:
// poll the target device until it is ready to be programmed
void pollUntilReady ()
{
if (signatures[foundSig].pageSize < 128) {
delay(12);
} else while ((program (pollReady) & 1) == 1) {
;
}
} // end of pollUntilReady
With this change I was able to successfully program the Atmega8A. Note that the delay value of 12ms is more than enough to cover the longest wait time specified in the datasheet.