Hi Mr. J and Mr Erik. I was going through the ATMEL's parallel in-circuit programming document "doc0287.pdf".

Two small paragraphs I have quoted here. It clearly says that the programmer microcontroller CAN interface directly with the to be programmed microcontroller.

PARAGRAPH1:
"
Programming requires programmer access to all of the four AT89C51 I/O ports, as documented in the data sheet. The programmer is connected directly to those controller pins which are unused by the application, while access to pins used by the application requires special treatment, as explained in the following paragraphs.
"

PARAGRAPH2:
"
Many of the signals generated by the programmer are connected directly, without buffering to the AT89C51 in the application. These signals, when inactive, are not three-stated, but are pulled high. The AT89C51 has internal pull-ups of approximately 3 Kohms on ports 1, 2 and 3. Because port 0 does not have internal pull-ups, external pull-ups of 10 Kohms have been added to permit proper operation of program verification mode. The sample application operates correctly in this environment. If required for compatibility with an application, programmer
signals may be buffered with three-state buffers similar to the 74xx125.
"

Regards
Aman