Question { 34898 }

in 8051 microcontroller port0 has no internal pullup why?

Answer

When used as outputs, all port pins will drive the state to which
the associated SFR latch bit has been set. Except for port 0,
which will only drive low (not high). When a 0 is written to a
bit in port 0, the pin is pulled low (0). But, when a 1 is
written to a bit in port 0, the pin goes into a high impedance
state - or in other words, "disconnected", no value. To be able
to get a 1 as output, you need an external pullup resistor to
pull up the port (to 1) when the port is in its high impedence
state. Typical values for pullups might be 470 ohm to drive a
LED, and 4.7K or higher to drive logic circuits.

Question { NTCP, 2038 }

what is difference between if and else if

Answer

The if portion is the only block that is absolutely mandatory. else if allows you to say "ok, if the previous condition was not true, then if this condition is true...".