1. M68K Assembly Language Programming R.Renner Chapters 9-10 Memory-Mapped I/O Exceptions & Interrupts Notice: Chapter slides in this series originally provided by B.Britton. This version of chapter 9-10 slides have been created by R.Renner to reflect Spring 2006 course content.

2. Memory-Mapped I/O Mechanism used to communicate with external (to processor) devices M68K move instructions w/absolute long address Address corresponds to a device register Device connected to system bus, like CPU and memory (see figure 9.1) Processor can both inquire and modify state of connected device, simply Renner slide

3. System Bus with Typical Keyboard/Display Interface Main Memory M68K CPU Ethernet Controller Disk Controller Interrupt Enable Control KEYBOARD Data (2 8-bit registers) Control DISPLAY Data (2 8-bit registers) Ready Fig.9.1 from Britton text Write-only Read-only Renner slide

4. Device Controllers Provide interface protocol for programmers Provide registers for communication Number of available device (receiver) registers grows with complexity Renner slide

5. Keyboard Controller Method 1: polling –> ready bit (lsb) & interrupt enable bit –Keypress: load data register with key value, ready bit with ‘1’ Example mapping (assume byte addresses are $E00100 and $E00101): CkKEYBD move.b$E00100,D0*Read from kbrd control register btst.b#0,D0*Test kbrd ready bit (bit 0) beqCkKEYBD*Branch if ready bit D0 equals 0 move.b$E00101,D1*Get char from kbrd data register Notice btst instruction (bit test) Renner slide

6. Display Controller Method 1: polling –> ready bit (lsb) & interrupt enable bit –Ready-to-Receive: controller sets ready bit to‘1’ Example mapping (assume byte addresses are $E00102 and $E00103): CkDisplay move.b$E00102,D0*Read from display control register btst.b#0,D0*Test display ready bit (bit 0) beqCkDisplay*Branch if ready bit D0 equals 0 move.bD1,$E00103*Send char to display data register Notice btst instruction (bit test) Renner slide

7. Exceptions An event or condition that initiates a change in the normal flow of program execution (see Easy68K Help-Sim.Op-Exceptions) examples: breakpoints, trace, pause, reset, interrupts, illegal instructions, privilege violation (supervisor bit in SR), TRAP, CHK, Div/0, address errors (word cannot bind to odd address), bus errors* *(virtual refs must be between $0-$FFFFFF) Renner slide

8. Exception Processing Exception processing begins by creating the appropriate exception stack frame for the particular exception group on the supervisor stack. Then, the supervisor mode is turned on and trace mode is turned off. After that, instruction execution resumes at the location referenced by the appropriate exception vector. When the simulator starts up the supervisor bit is set on and the supervisor stack pointer is set to the value 1000000 (hex). Note that the stack grows downward, so the stack frame for any exceptions will grow from 1000000 (hex) downward. Renner slide

9. Address (Hex) Assignment 000 Reset: Initial SSP 004 Reset: Initial PC 008 Bus error 00C Address error 010 Illegal instruction 014 Divide by zero 018 CHK instruction 01C TRAPV instruction 020 Privilege violation 024 Trace Address (Hex) Assignment 028 Line 1010 emulator 02C Line 1111 emulator 064 Level 1 Interrupt 068 Level 2 Interrupt 06C Level 3 Interrupt 070 Level 4 Interrupt 074 Level 5 Interrupt 078 Level 6 Interrupt 07C Level 7 Interrupt 080-0BC TRAP instruction vectors Exception Vector Locations – for Easy68K Renner slide

10. Interrupts Special type of Exception Signal coming from external device Asynchronous -> Hardware initiated by controller & supported by driver Syncrhonous -> Programmable timer w/polling

11. Practice INTERRUPTS RUN “BALL” program … modify (using synchronous polling of toggle switches) Your program will use the LED display to produce the effect of a red ball that constantly bounces back and forth. In addition make use of one of the seven-segment display units to dynamically report the position of the red ball, which will range from position 0 to position 7. In addition, the rate that the ball bounces back and forth will be controlled by the settings of the toggle switches. When all toggle switches are in the zero position the ball should not move. With any one toggle switch in the 1s position the ball should start moving. As more toggle switches are set to the 1s position the ball should move faster. The speed of the ball will be a function of how many toggle switches are in the 1s position. In other words, a user can specify eight different speeds for the bouncing ball, and the user can change the speed at any time while the simulation is running. Hint – Within you program there will be a wait loop. The longer the program is in the wait loop the slower will be the movement of the ball. In my case a wait loop counter in the range of 1million decimal or $100000 hexadecimal was a starting point to experiment with the wait loop counter. You will have to fine tune your wait loop counter depending upon the speed of your computer.