To program external flash in system, the easist way is to convert to von Neuman, that way you can write to both program and data memory. Another way is to have external logic for !WR and !PSEN to allow write to the flash. A very safe method is to have a small ROM (e. g. 4k internal) that hold the bootcode and logic to switch that ROM as code (e.g. !EA) and the flash as data. Been there done that. After the P89C51Rx2 came out, I do not find it worth the hassle to use external flash I program the P89C51 in circuit with no switches, just serial commands. Another advantage of using the 89C51Rx2 is that you get all ISP code for free.

Have fun in the park flashing

Erik

PS if selecting the P89C51Rx2 use FlashMagic instead of WinIsp.

Have fun,

Erik