I'm trying to debug the following line of code and I'd like to know what the prototype of ?C?CLDOPTR looks like and get a brief description of this library function. Does anyone know what CLDOPTR stands for and what parameters it takes? I'm using the Keil C51 compiler v5.50e -dean 1964 1 byStoredChksum = *((BYTE *)&psDest->status + 1); ; SOURCE LINE # 1964 005C 900000 R MOV DPTR,#psDest 005F E0 MOVX A,@DPTR 0060 FB MOV R3,A 0061 A3 INC DPTR 0062 E0 MOVX A,@DPTR 0063 FA MOV R2,A 0064 A3 INC DPTR 0065 E0 MOVX A,@DPTR 0066 247E ADD A,#07EH 0068 F9 MOV R1,A 0069 E4 CLR A 006A 3A ADDC A,R2 006B FA MOV R2,A 006C 900001 MOV DPTR,#01H 006F 120000 E LCALL ?C?CLDOPTR 0072 FF MOV R7,A ;---- Variable 'byStoredChksum' assigned to Register 'R7' ----