I thought you say Kai has explained th topic in some prev discussions.
in this discussion
I see esacademy.com But cant get much :( on 89C668.
If you had read (sorry, ugly word) AN461 you would have seen that the ISP/IAP is the same for P89C66x and P89C51Rx2

// Philips 591 I2C Demonstration Project 3
// For the Phytec Development Board with 87C591 phyCORE module
// using the Keil 8051 C Compiler
It explains about 591 and some Phytech board and Keil compiler all the three I dont have .I dont even know 668 so how can I understand from 591 ?.
Do you expect esacademy to publish using your board?
What is 591?

1)I want to know as you have said IAP to be used for I2C programming what is this? . 2)To program 668 as I2C slave dont one write simply some ASM routine and it stars working in slave mode 3),I dont understand inserting codes in IAP layer.
1) see 3)
2) yes
3) how else are you going to get yhe code into the flash?

Erik

PS DO read what esacademy say about FlashMagic and the AN461 and then come back with questions that show that you have, at least partially, grasped what these documents are about instead of asking me to provide extracts from them.