a simple calculator using at89c51 problem !!!

Since you're using a 20MHz crystal make sure your uc part number is a 20PC or 24PC and not 12PC.
Also try doubling all the delay values throughout your program.
Try and report.

i think you need to review your keypad code. One more thing, check your lcd with code you wrote. Make small program to write some text on lcd. If you need any help check tutorial.

i checked my uc, it is written 24PI instead of PC , wat does it mean... ?? i checked my program on proteus simulator along with the exact hardware that i built, it wkrs fine..
also my intel hex file 9kb and my rom is 4kb... can dat be a problem .. when i compile using keil compiler, it shows data=90 code=2981 but the output hexfile is 9kb...does my hex file needs to be under 4kb?

there is no problem with hex file.. coz 9kb is size of hex file not the code

and please do what i said.. write a program to display text on both lines of lcd and check it on hardware..

heeyyyyy my project worked !!! !dance
but alas for a short time period, the wire from power supply broke and i applied it manually to vcc of project ,the lcd blinked on off rapidly as my hand mite not hav been stable, so i soldered the wire, but then when i switched on the project, the lcd glew with a dim light than before..
using dmm i found dat without using lcd, i got proper 5.7 V dc input but as soon as i attach the lcd it becomes 4.8V .. i am pretty sure that when lcd used to glew fine, it had more than 5V..
is my lcd gone bad???
but atleast i am sure that code runs fine, the problem jst lies with the hardware!!!

i got proper 5.7 V dc input

You mean 5.7V Vcc?
Hope you're using a 5v regulator.Make sure you don't supply more than 5v to the uc.

i am using 5v regulator , lm7805 , the voltage is little above 5 v ... is my lcd corrupt?

connect a 5V zener diode across the o/p your VCC will be 5V

your power supply should be less than 5.5V. If LCD glow that doesnt mean LCD working, coz its just the LED backlight that glows and it takes some extra current that makes your power supply drops to 4.8V.

you should test your board after connecting everything to board. Please take precautions with power supply it should remain under 5.5V otherwise your controller may get burn.