Chapter 9TRAP Routines and

Subroutines

System Calls

System Call

LC-3 TRAP Mechanism

TRAP Instruction

TRAP

RET (JMP R7)

TRAP Mechanism Operation

TRAP Routines and their Assembler Names

Example: Using the TRAP Instruction

9-12

Example: Output Service RoutineTRAP_PUTS

ST R0,OS_SAVE_R0 ; save R0, R1, and R7ST R1,OS_SAVE_R1ST R7,OS_SAVE_R7ADD R1,R0,#0 ; move string pointer (R0) into R1

TRAP_PUTS_LOOPLDR R0,R1,#0 ; write characters in string using OUTBRz TRAP_PUTS_DONEOUTADD R1,R1,#1BRnzp TRAP_PUTS_LOOP

TRAP_PUTS_DONELD R0,OS_SAVE_R0 ; restore R0, R1, and R7LD R1,OS_SAVE_R1LD R7,OS_SAVE_R7RET

stored in table,location x22