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Returns actual number of pages that can be allocated. Actual allocation happens when the sys.buffalloc method is used. The serial port is unable to RX data if its RX buffer has 0 capacity. Actual current buffer capacity can be checked through the ser.rxbuffsize which returns buffer capacity in bytes.
Relationship between the two is as follows: ser.rxbuffsize=num_pages*256-X (or =0 when num_pages=0), where "num_pages" is the number of buffer pages that was GRANTED through the ser.rxbuffrq. "-X" is because a number of bytes is needed for internal buffer variables. X=17 bytes on 16-bit platforms and 33 bytes on 32-bit platforms. Platform type is specified in your platform documentation.
Buffer allocation will not work if the serial port to which this buffer belongs is opened (ser.enabled= 1- YES) at the time when sys.buffalloc executes. You can only change buffer sizes of ports that are closed.
Older devices had smaller RAM sizes, so the numpages argument and the value returned by the method were of the byte type. The maximum possible number of pages, therefore, could not exceed 255. On newer devices with larger RAM capacities this method takes and returns word values. Inspect the method's declaration on the platform you are using to make sure you know the data type it employs.