You have

if(seg_count==0)				 
{	  							  //remove switch case;to if else latency
     address(0x01);
	 write(LCD_digits[0]);
	 seg_count=1; 
}
if(seg_count==1)
{
     address(0x02);
	 write(LCD_digits[1]);
	 seg_count=2; 
}

if(seg_count==2)
{
      address(0x04);
	  write(LCD_digits[2]);
	  seg_count=3; 
}
...

instead of

address(1 << seg_count);
write(LCD_digits[seg_count]);
seg_count = (seg_count+1) & 0x07;

And you have:

if(asc=='~'){a=0x20;return;}
if(asc=='!'){a=0x10;return;}
if(asc=='@'){a=0x08;return;}
if(asc=='#'){a=0x04;return;}
if(asc=='$'){a=0x02;return;}
if(asc=='%'){a=0x01;return;}
if(asc=='^'){a=0x80;return;}
if(asc=='&'){a=0x40;return;}
...

instead of

if (asc >= ' ' && asc < 128) {
    a = lookup[asc - ' '];
} else {
    // Replace unsupported character with '?'
    a = lookup['?' - ' '];
}

And why do you use global variables when not needed? You let convert take an input parameter, but assign (sometimes) to the global variable a. What happens if convert() gets a character that isn't supported? Your code doesn't give a new value for a, so the previous character will repeat.

Another thing: Some parts of your code is indented. Some are not. Always indent your code properly. It doesn't take long time, but you can read the code very much faster when your eyes can jump directly from a block-start to a block-end without a need to scan every single character in-between...

You are defining a huge number of bit variables for your LCD - what are you using them for?