Program memory Lock in microcontroller memories

Can any body tell me what is program memory lock.

AT89C51 has 3 levels of program memory lock, what does it means ?

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MODE 1

lock bits : LB 1=U(unprogrammed),LB 2=U(unprogrammed),LB 3= U.

protection type:no programmed lock features

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MODE 2:

lock bits: LB 1 = P (programmed), LB 2:U(unprogrammed) LB 3: U(unprogrammed)

protection type: MOVC instruction executed from external program memory are disabled from fetching code bytes from internal memory. EA bar sampled and latching on RESET and further programming of the FLASH is disabled.
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MODE 3:

lock bits : LB 1= P ( programmed), LB 2= P (programmed ), LB 3 = U ( unprogrammed )

protection type : same as MODE 2 also verify is disabled.
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MODE 4 :

lock bits : LB 1 = P (programmed ), LB 2 = P ( programmed), LB 3= P ( programmed)

protection type : same as MODE 3 , also external execution is disabled.

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ONCE MODE ( 5 th MODE )n chip emulation mode facilitate the testing and debugging of system using the line device without the device being removed from the circuit.

the "ones"mode invoked by

step 1 : pull ALE low while the device is in RESET and PSEN is high.

step 2 : hold the ALE low as the RESET is deactivated.

while the device is " ones" mode the PORT 0 pin go into A float state and the other PORT pins and PSEN are weakly pulled high the oscillator remain active while the device is this mode . test CPU can be used to drive the CKT . normal operation is restored after a normal RESET is applied.

Thank you friend ,but what do you mean by Lock bit and how to set or reset it.
i have no idea.would you please tell me.

the settings of the selection of the lock bit is done at the time of programming the controller, when you use the software to flash the hex file. where you select the uart, source of hex file, you have an option of what level of security you need to give memory. if you select highest level then no one can read the hex file, not modify it. if he has to change then he has to erase the complete flash and reload the hex file....