PLT CONFUSED ABOUT TAXI INSTRUCTIONS; ENTERS ACTIVE RWY.

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ON ENG START; I CONTACTED PDK GND FOR TAXI INSTRUCTIONS. GND ANSWERED; 'MOONEY XXX?; I REPEATED THE CALL AND ADDED 'MOONEY XXXX AT RAMP TO TAXI FOR A NE VFR DEP.' I BELIEVED AT THE TIME THAT GND ANSWERED; 'MOONEY XXXX TAXI TO RWY 2R;' PERHAPS BECAUSE I HAD OBSERVED THE TFC USING RWY 2R AND RWY 2L PREVIOUSLY AND A N/E DEP WOULD FAVOR RWY 2. FULL TAXI (TAXI TO RWY XX VIA TXWY C; L TO TXWY A; R ON TXWY G) INSTRUCTIONS WERE NOT GIVEN NOR ASKED FOR. AS I SLOWLY APCHED THE INTXN OF TXWY C AND RWY 20R/2L; I OBSERVED AN ACFT ON THE FAR END OF RWY 20R; MOVING IN A DIRECTION AS TO CROSS THE RWY. I ASSUMED THAT THIS ACFT WAS MOVING FROM RWY 2R AFTER LNDG VIA TXWY G. UPON XING THE RWY CLR ONTO RWY 20R/2L; GND CALLED URGENTLY; 'MOONEY XXXX STOP IMMEDIATELY.' FOLLOWED BY; 'MOONEY XXXX EXPEDITE IMMEDIATE R TURN AND TAXI VIA RWY 27 AND EXECUTE ANOTHER R TURN TO RAMP.' THIS WAS ACCOMPLISHED WITHOUT DELAY. THE ACFT PREVIOUSLY OBSERVED WAS STILL XING AT THE FAR END OF RWY 20R. I OBSERVED NO OTHER TFC. THE ONLY OTHER XMISSION FROM GND WAS TO ASK; 'DO YOU NEED FURTHER PROGRESSIVES?' WHILE I HAD PAUSED ON TXWY A AT THE INTXN OF RWY 16/34. I ACKNOWLEDGED; 'NO; NOT REQUIRED.' I HAD SINCE HEARD OTHER ACFT BEING DIRECTED TO RWY 20R OR RWY 20L; SO I THEN KNEW THAT THE TFC PATTERN HAD TURNED AROUND' SINCE I HAD LAST SEEN A DEP.