here the insystem programable means
i think 89C51 can be programed through RXD and TXD serial pins
we connect it to PC db9 port through max232ic and program it with relavent software.but in case of 89S51 it is programed using “master and slave mode”
MOSI:master out slave in
MISO:master in slave out
I think you are using an usb programer for 89S51 that containing an atmega8
microcontroller from AVR family from atmel.this mcu also contains mosi,miso pins.
so 89C51 wont support it