Ardupilot, atmega1280 and barometer
Assumption: The question refers to the now discontinued ArduPilot Mega APM-1, which uses the atmega1280 microcontroller. 

The information from the barometric sensor is not explicitly stored anywhere in the code AFAIK, unless you are logging location parameters, in which case your code defines where it will be stored. 

Instantaneous barometric absolute value is obtained by barometer.read(). This returns absolute pressure sensed by the barometer, which may be of limited value, since temperature and calibration values need to be taken into account.

Altitude transform of barometric reading is obtained by read_barometer() which returns an int32_t in centimeters. This value further depends on the baseline calibrated from barometer.read() at reboot or start-up, and incorporates temperature related computation.

It is simple to add code to log the barometer value to a volatile variable or array, using the Arduino IDE.



Speculating that the barometer value requirement stems from some random barometric reading glitches observed in use, this is a known issue in older releases, and still apparently remains in release 2.5, but is much reduced, or has been solved: User reports differ on this.

Recommendations if this is the problem:


Upgrade to the 2.5 release of the software
After start-up, wait about 5 minutes for the electronics to stabilize in temperature, then press reset to reboot and re-calibrate all sensors.