how to write complete programs for PIC16F84A to perform the CPU will execute two tasks in parallel. The first task will be a normal program Execution loop which continuously tests an active-low switch connected to RB3. When the switch is pressed, an active-high LED connected to RB4 will turn on for 1 second (using software delay). Any switch presses while the LED is on will not be detected as the CPU is busy in the 1 second software delay. After the delay and the LED is turned off, CPU will return to testing the active-low switch. The second task will be to toggle an active-high LED connected to RA1 every 0.25 seconds. Timer0 will be configured to generate an interrupt every 0.25 seconds, and the code to toggle the LED will be placed in the ISR. Timing accuracy requirements is ±1% using 800 kHz RC oscillator.

To write a complete program for the PIC16F84A microcontroller that performs the given tasks, follow these steps:

1. Set up the microcontroller's configuration bits to ensure that the processor is running at the desired frequency (800 kHz RC oscillator) and enable the required peripherals (timers, interrupts, etc.). This can be done in the configuration register.

2. Define and initialize the necessary variables. In this case, you'll need a flag variable to indicate if the LED is currently on, a counter variable to count the number of 0.25-second intervals, and any other necessary variables for timing.

3. Set up the necessary input/output pins for the switch and the LEDs. Configure the RB3 pin as an input and the RB4 and RA1 pins as outputs.

4. Configure Timer0 to generate an interrupt every 0.25 seconds by setting up its control registers (T0CON and its associated registers) appropriately. Choose the prescaler and timer pre-load values based on the desired timing accuracy.

5. Write an interrupt service routine (ISR) for Timer0 that will toggle the RA1 LED every time the interrupt occurs. Inside the ISR, check if the counter variable has reached the desired number of intervals (1 second / 0.25 seconds). If so, turn off the RB4 LED, reset the counter, and clear the flag variable.

6. Write the main program execution loop. Inside the loop, continuously check the status of the RB3 switch using a polling mechanism. If the switch is pressed and the flag variable is not set (indicating the LED is currently off), turn on the RB4 LED, set the flag variable, and start the timer.

7. Implement a software delay of 1 second within the main program loop. This can be achieved by using a loop with a sufficient number of iterations to achieve the desired delay. Adjust the delay loop based on the actual clock frequency.

8. Compile and upload the program to the PIC16F84A microcontroller using an appropriate development environment or programmer.

It's important to note that writing complete programs for microcontrollers requires understanding the specific architecture, instruction set, and addressing modes of the microcontroller used. The steps provided here give a general outline of the program structure, but the actual implementation details may vary based on the specific microcontroller and development tools being used. Be sure to consult the microcontroller's datasheet and the documentation for your development environment for further guidance.