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8. Hello World! Example

The following short example demonstrates programming in C using the cc65 toolset with a custom run-time environment. In this example, a Xilinx FPGA contains a UART which is connected to a 65c02 processor with FIFO (First-In, First-Out) storage to buffer the data. The C program will wait for an interrupt generated by the receive UART and then respond by transmitting the string "Hello World! " every time a question mark character is received via a call to the hardware driver rs232_tx (). The driver prototype uses the __fastcall__ extension to indicate that the driver does not use the stack. The FIFO data interface is at address $1000 and is defined as the symbolic constant FIFO_DATA. Writing to FIFO_DATA transfers a byte of data into the transmit FIFO for subsequent transmission over the serial interface. Reading from FIFO_DATA transfers a byte of previously received data out of the receive FIFO. The FIFO status data is at address $1001 and is defined as the symbolic constant FIFO_STATUS. For convenience, the symbolic constants TX_FIFO_FULL (which isolates bit 0 of the register) and RX_FIFO_EMPTY (which isolates bit 1 of the register) have been defined to read the FIFO status.

The following C code is saved in the file "main.c". As this example demonstrates, the run-time environment has been set up such that all of the behind-the-scene work is transparent to the user.

#define FIFO_DATA     (*(unsigned char *) 0x1000)
#define FIFO_STATUS   (*(unsigned char *) 0x1001)

#define TX_FIFO_FULL  (FIFO_STATUS & 0x01)
#define RX_FIFO_EMPTY (FIFO_STATUS & 0x02)

extern void wait ();
extern void __fastcall__ rs232_tx (char *str);

int main () {
  while (1) {                                     //  Run forever
    wait ();                                      //  Wait for an RX FIFO interrupt

    while (RX_FIFO_EMPTY == 0) {                  //  While the RX FIFO is not empty
      if (FIFO_DATA == '?') {                     //  Does the RX character = '?'
        rs232_tx ("Hello World!");                //  Transmit "Hello World!"
      }                                           //  Discard any other RX characters

  return (0);                                     //  We should never get here!

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