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Example - 11hwint.asm - Hardware Interrupts |
; -------------------------------------------------------------- ; An example of using hardware interrupts. ; This program spins the stepper motor continuously and ; steps the traffic lights on each hardware interrupt. ; Uncheck the "Show only one peripheral at a time" box ; to enable both displays to appear simultaneously. ; -------------------------------------------------------------- JMP Start ; Jump past table of interrupt vectors DB 50 ; Vector at 02 pointing to address 50 Start: STI ; Set I flag. Enable hardware interrupts MOV AL,11 ; Rep: OUT 05 ; Stepper motor ROR AL ; Rotate bits in AL right JMP Rep JMP Start ; -------------------------------------------------------------- ORG 50 PUSH al ; Save AL onto the stack. PUSH bl ; Save BL onto the stack. PUSHF ; Save flags onto the stack. JMP PastData DB 84 ; Red Green DB c8 ; Red+Amber Amber DB 30 ; Green Red DB 58 ; Amber Red+Amber DB 57 ; Used to track progress through table PastData: MOV BL,[5B] ; BL now points to the data table MOV AL,[BL] ; Data from table goes into AL OUT 01 ; Send AL data to traffic lights CMP AL,58 ; Last entry in the table JZ Reset ; If last entry then reset pointer INC BL ; BL points to next table entry MOV [5B],BL ; Save pointer in RAM JMP Stop Reset: MOV BL,57 ; Pointer to data table start address MOV [5B],BL ; Save pointer into RAM location 54 Stop: POPF ; Restore flags to their previous value POP bl ; Restore BL to its previous value POP al ; Restore AL to its previous value IRET ; -------------------------------------------------------------- END ; -------------------------------------------------------------- TASK 28) Write a program that controls the heater and thermostat whilst at the same time counting from 0 to 9 repeatedly, displaying the result on one of the seven segment displays. If you want a harder challenge, count from 0 to 99 repeatedly using both displays. Use the simulated hardware interrupt to control the heater and thermostat. 29) A fiendish problem. Solve the Tower of Hanoi problem whilst steering the snake through the maze. Use the text characters A, B, C Etc. to represent the disks. Use three of the four rows on the simulated screen to represent the pillars. 30) Use the keyboard on Port 07. Write an interrupt handler (INT 03) to process the key presses. You must also process INT 02 (the hardware timer) but it need not perform any task. For a more advanced task, implement a 16 byte circular buffer. Write code to place the buffered text on the VDU screen when you press Enter. For an even harder task, implement code to process the Backspace key to delete text characters in the buffer. |
You can copy this example program from the help page and paste it into the source code editor.
Hardware Interrupts are short code fragments that provide useful services that can be triggered by items of hardware. When a printer runs out of paper, it sends a signal to the CPU. The CPU interrupts normal processing and processes the interrupt. In this case code would run to display a "Paper Out" message on the screen. When this processing is complete, normal processing resumes.
This simulator has a timer that triggers INT 02 at regular time intervals that you can pre-set in the Configuration Tab. You must put an interrupt vector at address 02 that points to your interrupt code. Look at the example.
Hardware interrupts are ignored unless the 'I' flag in the status register is set. To set the 'I' flag, use the set 'I' command, STI. To clear the 'I' flag, use the clear 'I' command CLI.
Hardware interrupts can be trapped in the same way that software interrupts can.
Hardware interrupts are triggered, as needed by disk drives, printers, key presses, mouse movements and other hardware events.
This scheme makes processing more efficient. Without interrupts, the CPU would have to poll the hardware devices at regular time intervals to see if any processing was needed. This would happen whether or not processing was necessary. Interrupts can be assigned priorities such that a disk drive might take priority over a printer. It is up to the programmer to optimise all this for efficient processing. In the IBM compatible PC, low number interrupts have a higher priority than the higher numbers.
This is quite complex. The command INT 02 whether triggered by hardware or software, causes the CPU to retrieve the contents of RAM location 02. After saving the return address onto the stack, the instruction pointer IP is set to the address that came from RAM.
The interrupt code is then executed. When complete the IRET command causes the return from the interrupt. The CPU instruction pointer IP is set to the address that was saved onto the stack earlier.
Hardware interrupts differ slightly from software interrupts. A software interrupt is called with a command like INT 02 and the return address is the next instruction after this. IP + 2 is pushed onto the stack. Hardware interrupts are not triggered by an instruction in a program so the return address does not have to be set past the calling instruction. IP is pushed onto the stack.
This is the same as trapping software interrupts described on the previous page.
© C Neil Bauers 2003