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This is a rather unconvensional approach to a NMEA parser - completely based on an ISR to parse the incoming bytes. It uses 2.2KB of code space and 44 bytes of RAM.

#include "main.h"
 
// Interrupt driven NMEA parser for Atmel AVR
 
ISR(USART_RX_vect)			//UART interrupt on mega xx8 series
{
	static char buffer[5];
	static u08 GGA;
	static u08 RMC;
	static u08 stage;
	static u08 commacount;
	static u08 bufferindex;
	static u08 pointcount;
	char c=UDR0;
	switch(c)
	{
		case '$':			//start of a packet
		  commacount=0;
		  bufferindex=0;
		  pointcount=0;
		  stage=0;
		  memset(buffer,' ',5);
		  GGA=FALSE;
		  RMC=FALSE;		//we dont know what sort of packet it is yet
		break;
		case ',':
		  commacount++;
		  bufferindex=0;
		  pointcount=0;
		  stage=0;
		break;
		case '.':
		  pointcount++;				//we need to be able to detect number of points in the case of altitude
		//break;                      THIS is deliberate, we want to run when we get a decimal point.
		default:				//we have some of the CSV data
		  if(bufferindex<5)		//dont mess up !
		  {
		    	buffer[bufferindex]=c;		//stick the character in our buffer		
		  }
		  if(GGA)
		  {
			if(commacount==2)		//the latitude from the GGA
			{
				if(bufferindex<1)
				{
					bufferindex++;
				}
				else
				{
					if(!stage)
					{				
						gps.latitude=(float)atoi(buffer);	//integer degrees
					}
					if(stage==1)
					{
						gps.latitude+=minutes*(float)atoi(buffer);	//integer minutes
					}
					if(stage==2)
					{
						gps.latitude+=0.01*minutes*(float)atoi(buffer);	//decimal degrees
					}				
					memset(buffer,' ',5);
					stage++;
				}
			}
			if(commacount==3 && c=='S')
			{
				gps.latitude=-gps.latitude;
			}
			if(commacount==4)
			{
				if( (stage && bufferindex<1) || bufferindex<2)
				{
					bufferindex++;
				}
				else
				{
					if(!stage)
					{				
						gps.longitude=(float)atoi(buffer);	//integer degrees
					}
					if(stage==1)
					{
						gps.longitude+=minutes*(float)atoi(buffer);	//integer minutes
					}
					if(stage==2)
					{
						gps.longitude+=0.01*minutes*(float)atoi(buffer);	//decimal degrees
					}				
					memset(buffer,' ',5);
					stage++;
				}
			}
			if(commacount==5 && c=='W')
			{
				gps.longitude=-gps.longitude;
			}
			if(commacount==6)
			{
				gps.status=atoi(&c);
			}
			if(commacount==9)
			{
				if(stage)					//decimal altitude
				{
					gps.altitude+=((float)atoi(&c))*0.1;
					memset(buffer,' ',5);
				}
				else
				{
					if(!pointcount)				//wait until we get to a decimal point		
					{
						bufferindex++;
					}
					else
					{
						gps.altitude=(float)atoi(buffer);
						memset(buffer,' ',5);
						stage++;
					}
				}
			}
		  }
		  else if(RMC)
		  {
			if(commacount==7)		//speed in knots
			{		
				if(stage)
				{
					gps.speed+=((float)atoi(&c))*0.1;
					memset(buffer,' ',5);
				}
				else
				{
					if(!pointcount)
					{
						bufferindex++;
					}
					else
					{
						gps.speed=(float)atoi(buffer);
						memset(buffer,' ',5);
						stage++;
					}
				}
			}	
			if(commacount==8)				//the heading field
			{
				if(stage)
				{
					gps.heading+=((float)atoi(&c))*0.1;
					memset(buffer,' ',5);
					gps.packetflag=TRUE;		//this is usually the last interesting part of the fix info to come through
				}
				else
				{
					if(!pointcount)
					{
						bufferindex++;
					}
					else
					{
						gps.heading=(float)atoi(buffer);
						memset(buffer,' ',5);
						stage++;
					}
				}
			}
		  }
		  else
		  {		
			if(commacount==0)	//the header
			{
				if(bufferindex<4)
				{
					bufferindex++;		//increase the position in the buffer
				}
				else
				{
					if(buffer=="GPGGA")	//the last character will be a space
					{
						GGA=TRUE;
					}
					if(buffer=="GPRMC")
					{
						RMC=TRUE;
					}
					memset(buffer,' ',5);		//wipe the buffer so it can be reused
				}
			}
		  }	
	}
}