The critical few things you need to know about it are:
- They are 1T (single clock cycle) 8051 compatible MCUs (but some instructions may take as many as 5 or 6 clock cycles to complete) and most are capable of operating at up to 35MHz. Check the markings on your part. If it says something like "28I", then it means that your part has been rated to operate at up to 28MHz. Note that rather than use nice round numbers like 12MHz, which is possible, it is more customary to use specific operating frequencies that facilitate the generation of a lower error UART baud rate. I.e., 11.0592MHz, than would otherwise be possible.
- They have a built-in bootloader that emits a magic number, then listens on UART0 (RxD, TxD) on bootup. The ISP programmer uses this.
- There are two main HEX file download tools, the manufacturer's own STCISP which is an all singing and dancing development support tool (it has pinout information, timer configuration and code generation t