The Int 12h service returns the number of 1K byte blocks of system RAM in register AX. The value is copied directly from the MEMSIZE RAM Variable at 40h:13h. This variable is the amount of user RAM found in the system during the power-on and initialization process. It does not include any expanded RAM that may be present.
The current HP 100LX design has 1 MB of RAM memory built in as standard equipment. This can be increased by adding a plug-in RAM card. No method is currently provided to disable a portion of the 1M RAM. The number returned by Int 12h is approximately the 1 MB minus the number of KB in the RAM portion of the drive C: Edisk and will never exceed 636 KB (027Ch).
Note that an application can use this service to determine the total amount of user RAM in the system. However, this service does not indicate how much RAM is available ("free") for running applications.