The LC-3

1
The LC-3 Chapter 5

• COMP 2620

2
Control Instructions

• Recall that there are three types of instructions
• Data movement
• Operate
• Control
• The control instructions influence the PC value
  so that execution is no longer linear in nature

3
Control Instructions

• Recall, that so far we have only considered the
  branch instruction BRxxx.
• This instructions worked by comparing the
  condition codes against the flags set in the
  instruction and jumping to a new location if one
  matched appropriately
• We could get a unconditional jump using BRnzp

4
JMP Instruction

• One problem with the BRnzp instruction is it is
  limited in the range of addresses near the PC
• It had an effective range of -255256 from the
  branch itself
• If we needed to branch to 2000 locations away, we
  could not easily do it

5
JMP Instruction

• The JMP (opcode 1100) instruction will allow us
  to do this
• The JMP instruction loads the PC with the
  contents of a specified register in bits 86
• All the remaining bits are 0.
• The change of the PC value always occurs!

6
JMP Instruction
7
TRAP Instruction

• The TRAP instructions changes the PC to a memory
  address that is part of the operating system
• The OS then performs a task on behalf of the
  program that is running
• We say the TRAP instruction invokes a SERVICE CALL

8
TRAP Instruction

• Bits 70 form the trap vector
• The trap vector is a number that identifies which
  service call that the OS needs to perform
• Hence, there are 256 possible trap vectors

9
TRAP Instruction

• When routine is done, PC is set to the
  instruction following TRAP.

vector routine
x23 input a character from the keyboard
x21 output a character to the monitor
x25 halt the program