Reentrant Functions

1
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

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3
System Bus with Typical Keyboard/Display
Interface
M68K CPU
Main Memory
Read-only

Control KEYBOARD Data (2 8-bit
registers)
Interrupt Enable
Ready
Read-only

Read-only

Control DISPLAY Data (2 8-bit registers)
Interrupt Enable
Ready
Write-only

Fig.9.1 from Britton text
Disk Controller
Renner slide
Ethernet Controller
4
Device Controllers

• Provide interface protocol for programmers
• Provide registers for communication
• Number of available device (receiver) registers
  grows with complexity

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5
Keyboard Controller

• Method 1 polling
• gt 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)
• beq CkKEYBD Branch if ready bit D0 equals 0
• move.b E00101,D1 Get char from kbrd data
  register
• Notice btst instruction (bit test)

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6
Display Controller

• Method 1 polling
• gt ready bit (lsb) interrupt enable bit
• Ready-to-Receive controller sets ready bit
  to1
• 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)
• beq CkDisplay Branch if ready bit D0 equals 0
• move.b D1,E00103 Send char to display data
  register
• Notice btst instruction (bit test)

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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)

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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.
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9
Exception Vector Locations for Easy68K
 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
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10
Interrupts

• Special type of Exception
• Signal coming from external device
• Asynchronous -gt Hardware initiated by controller
  supported by driver
• Syncrhonous -gt 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.