To put it simple, a 8051 can address two external memory chips: a ROM and a RAM chip. The address bus, which you will have to demultiplex using /ALE, will provide a 16 bit address. Control signals for the memory chips are /PSEN for output control of the ROM chip, /RD (port 3 pin 7) for output control of the RAM chip and /WR (port 3 pin 6) for write enable of the RAM chip. That's right, you will HAVE to dedicate these port pins to RAM chip control. Probably, this scheme was chosen to eliminate the need for external decoding hardware.
Back in software, the RAM chip can be read and written using the MOVX instruction, and MOVC to read the ROM chip.
To make the 8051 execute from the external ROM chip, you must pull /EA down.
The A15 bit is just the A15 bit and plays NO special role other than being the sixteenth address bit.

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