a) Consider the following ISR (Interrupt Service Routine). If you run this ISR in your 8086 kits, what would you observe?
MyISR:
MOV AL, 0x81
OUT 0, AL
IRET
b) If I want to run the above code as a subroutine, what would be the change I must make in it?
a) If you run this ISR in your 8086 kits, you would observe that the ISR moves the value 0x81 into the AL register, then outputs the value of AL to port 0, and finally performs a return from the interrupt (IRET) instruction.
b) If you want to run the above code as a subroutine, you would need to remove the IRET instruction. The IRET instruction is used specifically for returning from an interrupt service routine. In a subroutine, you can simply end the code with a RET instruction, which will return to the calling code. So the updated code would be:
MySubroutine:
MOV AL, 0x81
OUT 0, AL
RET
a) When you run the above ISR in your 8086 kit, you would observe the following:
1. The ISR first moves the value 0x81 into the AL register using the MOV instruction.
2. The OUT instruction then sends the value of AL (0x81) to the specified port (port 0).
3. Finally, the IRET instruction is executed, which returns from the interrupt and resumes normal program execution.
The exact behavior you would observe depends on the specific hardware configuration and how the interrupt is triggered. However, in general, this ISR appears to be sending a specific value to a particular port and then returning from the interrupt.
b) If you want to run the above code as a subroutine, you would need to make the following change:
1. Remove the IRET instruction from the end of the code.
By removing the IRET instruction, the code will not return from the interrupt and will behave like a subroutine instead. This way, you can call the subroutine using a CALL instruction from another part of your program.