1. TRAP Routines Subroutines Privileged Instructions
Chapter 9
TRAP Routines
Subroutines
Privileged Instructions

2. Subroutines Used for Can be either
Frequently executed code segments: written only once.
Hides the implementation details.
Passing of parameters and return values, in registers or memory.
Is called from within a program; control is returned to the same point it was called from.
Can be either
Procedures, or
Functions

3. Jump to Sub - 1: JSR/JMP JSR/JMP: jump subroutine (direct)
JSR: IR[11] = 1, and R7  (PC) i.e. PC is saved in R7
JMP: IR[11] = 0, PC is not saved.
PC  IR[8:0]
JSR L x
0 0
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4. Jump to Sub - 2: JSRR/JMPR
JSRR/JMPR: jump subroutine (base + offset)
JSRR: IR[11] = 1, and R7  (PC) i.e. PC is saved in R7
JMPR: IR[11] = 0, PC is not saved.
PC  (BaseReg) + Zext(IR[5:0])
JSR L x
0 0
x x x
index6
BaseReg

5. Privileged Instructions
There are several instructions that are best executed by a supervisor program (OS) rather than a user program:
IO instructions
Loading of memory mapping registers
Resetting the clock
Halt
i.e. instructions where one program can affect the behavior of another.
Hardware enforces two modes of operation
User Mode
Privileged Mode (aka. supervisor, kernel, monitor mode)
Only the supervisor program can execute privileged instructions.

6. TRAP Instructions TRAP: A special instruction RET – return instruction
A subroutine call used to invoke a specific supervisor routine.
It switches the execution to privileged mode, and reverts back to user mode when the routine completes.
PC  Mem[Zext(IR[7:0])]
RET – return instruction
Return the execution to the last calling point.
PC  (R7)
trapvector8
TRAP

7. TRAP Example System Control Block TRAP & Interrupts
Or Interrupt Vector Table
In LC-2
8 bits specify one of 256 locations (x0000 to x00FF)
The location contains the address of the TRAP service routine.
TRAP & Interrupts
Exact same mechanisms
A TRAP is an instruction (event internal to a program).
An interrupt is external to a program (from an I/O device)
Both invoke a supervisor service routine.
= x04A0
x300B R7
Service routine
Address x300A

8. LC-2 TRAP Routines GETC (TRAP x20) OUT (TRAP x21) PUTS (TRAP x22)
Read a single character from KBD.
ASCII copied in R0, R0[15:8] cleared.
OUT (TRAP x21)
Write R0[7:0] to CRT.
PUTS (TRAP x22)
Write a string to CRT. String address in R0.
IN (TRAP x23)
Print a prompt on the screen and read a single character from KBD.
Character is echoed to CRT. ASCII copied to R0[7:0] and R0[15:8] cleared.
HALT (TRAP x25)
Prints message on CRT & halts execution. :
x0430
x0450
x04A0
x04E0
xFD70
x0000
x0020
x0021
x0022
x0023
x0024
x0025
x00FF
The LC-2 System
Control Block

9. Character Input Service Routine (IN)
Figure 9.4 p.177

10. IN (continued)

11. OUT
Figure 9.2 p. 178

12. HALT
Figure 9.6 p. 181

13. PUTS ;puts.asm (TRAP x22) ;R0 points to null-terminated string
;R0, R1 & R3 saved; R7 is lost.
.ORIG x0450
ST R7,SaveR7
ST R0,SaveR0
ST R1,SaveR1
ST R3,SaveR3 ;
LOOP LDR R1,R0,#0
BRz RETURN
L2 LDI R3,CRTSR
BRzp L2
STI R1,CRTDR
ADD R0,R0,#1
BR LOOP
RETURN LD R3,SaveR3
LD R1,SaveR1
LD R0,SaveR0
LD R7,SaveR7
RET ;
CRTSR .FILL xF3FC
CRTDR .FILL xF3FF
SaveR0 .FILL x0000
SaveR1 .FILL x0000
SaveR3 .FILL x0000
SaveR7 .FILL x0000
.END

14. Subroutine call examples
;Calling program
.ORIG $3000
ld r1,num1
ld r2,num2
jsr multi
st r3,prod
HALT ;
;Input Data & Result
num1 .fill x0006
num2 .fill x0003
prod .blkw 1
;Subroutine multi
;Multiply 2 positive numbers
;Parameters:
; In: R1,R2; Out: R3 ;
multi and r3,r3,#0
add r4,r1,#0
brz zero
loop add r3,r2,r3
add r1,r1,#-1
brp loop
zero ret
.end

15. Library Routines Library Library routine invocation
A set of routines for a specific domain application.
Example: math, graphics, GUI, etc.
Defined outside a program.
Library routine invocation
Labels for the routines are defined as external. In LC-2:
.External Label
Each library routine contains its own symbol table.
A linker resolves the external addresses before creating the executable image.