Hi 8051 experts please help me understanding following:- 1. which was the first microcontroller 2. what does legacy port means 3. what is pulse width modulator 4. what is phase locked loops 5. what is minimum time for level triggering in 8051 6. what are usual applications of 8051 7. what does DPTR contains 8. why bit addressable SFRs are at addresses ending with 0 or 8 9. when we use timer in mode 0 it is 5 bit prescaler. what exactly it means 10. how PD and IDL can be tested in PCON 11. how to use "user definable" flag in PSW 12. I wrote a simple program to do serial transmission. the code is hereunder: #include <REG51.h> void main() { TMOD=0x10; TR1=1; SBUF='C'; } when I m testing this program in simulator it is again and again jumping to main itself. means transmitting C again and again. what could be the reason. 13. where does interrupt destination like IE0=0003 lies RAM or ROM, 14. when we make an ISR interrupt value can be from 0 - 31, why 32 values when we have got just 5+1 interrupts 15. what r different addessing modes in 8051 16. can uC access Data memory and Code memory at same time.if yes, is it also possible for extenal RAM and ROM 17. when we use timer0 in mode 3 , it get divided into two 8bit timers, lower timer is started by setting 1 in TR0 sfr. and upper is started by setting 1 in TR1 sfr. but basically TR1 is used for Timer1. so will timer 1 also run at that time. 18. when we create a new project in Keil IDE, there remains an option as "use extended linker(LX51)instead of BL51". kindly throw some light on this. 19. how parrallel communication happens with external devices. why there is no need for baud rate. does bits in one wire of parallel port go one after another like serial or there is some ACK mechanism. 20. in case of keil 8051, what goes in Stack. where global variables remain. 21. what are different uses of 4 banks in 8051 22. what is version of keil compiler and what is the way to check it.