TRAP Routines and Subroutines

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TRAP Routines and Subroutines

• Chapter 9

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Outline

• LC-3 TRAP Routines
• Intro, TRAP Mechanism, TRAP Instruction, Complete
  Mechanism, TRAP Routines for I/O, TRAP Routine
  for Halting, Saving and Restoring Registers
• Subroutines
• Call/Return Mechanism, JSR(R) Instruction, TRAP
  Routine for Character Input (revisit), PUTS
  Write string to monitor, Library Routines,

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Why TRAP?
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TRAP Mechanism

• Set of Service Routines
• Table of Starting Addresses
• TRAP Instruction
• Linkage

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Executing a TRAP Instruction

• Zero extend trapvect8 to 16 bits and load into
  MAR
• Read memory into MDR
• R7 ? PC (The Linkage)
• PC ? MDR

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HOW DO WE GET BACK?
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Compare Contrast
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Saving Restoring Registers

• Why should we save a register?
• When should we save a register?
• Who should save the register?

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In the same vein...

• Why do we have functions, methods, subroutines,
  procedures, etc.?

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Call/Return Mechanism
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What does JSR do?
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What does JSRR do?
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What does RET do?
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Assembly

• jsr sub
• sub xxx xxx xxx
• ...
• ...
• ...
• ret

• ld R2, far
• jsr sub
• far .fill sub
• ...
• ...
• sub xxx xxx xxx
• ret

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TRAP vs JSR(R)

• TRAP
• Uses trap vector table
• (Can get to from anywhere)
• Normally do system functions
• I/O
• Written very carefully!
• Typically tied into some sort of system
  protection mechanism

• JSR(R)
• Local (JSR)
• Anywhere (JSRR)
• with some work
• Routine abstraction
• Code reuse/libraries
• Written
• No protection mechanism

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Questions?
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