Hi..

Use the 89c55, it has 20Kb, I use it, and I never had
a chance to fill it up to the end.
My largest program ever (asm51) was 18.5Kb.

the programmer should not allow you to program
more than the chip capacity, but if you skip the test,
then the CPU may jump to non existing address, which
will cause the Instruction Pointer to jmp to
unexpected address.
also, when the program will get to the last byte of the
rom, the next byte should be the first byte of the
rom...
Hope it is helpfull....

Goodluck
oren