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1 .Chapter 9 TRAP Routines and Subroutines

2 .9- 2 Subroutines A subroutine is a program fragment that: lives in user space performs a well-defined task is invoked (called) by another user program returns control to the calling program when finished Like a service routine, but not part of the OS not concerned with protecting hardware resources no special privilege required Reasons for subroutines: reuse useful (and debugged!) code without having to keep typing it in divide task among multiple programmers use vendor-supplied library of useful routines

3 .9- 3 JSR Instruction Jumps to a location (like a branch but unconditional), and saves current PC ( addr of next instruction) in R7. saving the return address is called “linking” target address is PC-relative (PC + Sext (IR[10:0 ])) bit 11 specifies addressing mode if =1, PC-relative: target address = PC + Sext (IR[10:0]) if =0, register: target address = contents of register IR[8:6 ] (called "JSRR" in assembly language)

4 .9- 4 JSR NOTE: PC has already been incremented during instruction fetch stage.

5 .9- 5 JSRR Instruction Just like JSR, except Register addressing mode. target address is Base Register bit 11 specifies addressing mode What important feature does JSRR provide that JSR does not?

6 .9- 6 JSRR NOTE: PC has already been incremented during instruction fetch stage.

7 .9- 7 RET (JMP R7) How do we transfer control back to instruction following the TRAP? We saved old PC in R7. JMP R7 gets us back to the user program at the right spot. LC-3 assembly language lets us use RET (return) in place of “JMP R7”. Must make sure that service routine does not change R7, or we won’t know where to return.

8 .9- 8 Saving and Restoring Registers Called routine -- “callee-save” Before start, save any registers that will be altered (unless altered value is desired by calling program!) Before return, restore those same registers Calling routine -- “caller-save” Save registers destroyed by own instructions or by called routines (if known), if values needed later save R7 before TRAP save R0 before TRAP x23 (input character) Or avoid using those registers altogether Values are saved by storing them in memory.

9 .9- 9 Saving and Restoring Registers Must save the value of a register if: Its value will be destroyed by service routine, and We will need to use the value after that action. Who saves? caller of service routine? knows what it needs later, but may not know what gets altered by called routine called service routine? knows what it alters, but does not know what will be needed later by calling routine

10 .9- 10 Example: Negate the value in R0 2sComp NOT R0, R0 ; flip bits ADD R0, R0, #1 ; add one RET ; return to caller To call from a program (within 1024 instructions): ; need to compute R4 = R1 - R3 ADD R0, R3, #0 ; copy R3 to R0 JSR 2sComp ; negate ADD R4, R1, R0 ; add to R1 ... Note: Caller should save R0 if we’ll need it later!

11 .9- 11 Passing Information to/from Subroutines Arguments A value passed in to a subroutine is called an argument. This is a value needed by the subroutine to do its job. Examples: In 2sComp routine, R0 is the number to be negated In OUT service routine, R0 is the character to be printed. In PUTS routine, R0 is address of string to be printed. Return Values A value passed out of a subroutine is called a return value. This is the value that you called the subroutine to compute. Examples: In 2sComp routine, negated value is returned in R0. In GETC service routine, character read from the keyboard is returned in R0.

12 .9- 12 Using Subroutines In order to use a subroutine, a programmer must know: its address (or at least a label that will be bound to its address) its function (what does it do?) NOTE: The programmer does not need to know how the subroutine works, but what changes are visible in the machine’s state after the routine has run. its arguments (where to pass data in, if any) its return values (where to get computed data, if any)

13 .9- 13 Saving and Restore Registers Since subroutines are just like service routines, we also need to save and restore registers, if needed. Generally use “ callee -save” strategy, except for return values. Save anything that the subroutine will alter internally that shouldn’t be visible when the subroutine returns. It’s good practice to restore incoming arguments to their original values (unless overwritten by return value). Remember : You MUST save R7 if you call any other subroutine or service routine (TRAP). Otherwise, you won’t be able to return to caller.

14 .9- 14 Example Write a subroutine FirstChar to: find the first occurrence of a particular character (in R0 ) in a string (pointed to by R1 ); return pointer to character or to end of string (NULL) in R2 . (2) Use FirstChar to write CountChar , which: counts the number of occurrences of a particular character (in R0 ) in a string (pointed to by R1 ); return count in R2 . Can write the second subroutine first, without knowing the implementation of FirstChar!

15 .9- 15 CountChar Algorithm (using FirstChar) save regs count <- 0 call FirstChar Look at M[R2 ] =0? count <- count + 1 R1 <- R2 + 1 R2 <- count restore regs return no yes save R7, since we’re using JSR

16 .9- 16 CountChar Implementation ; CountChar: subroutine to count occurrences of a char CountChar ST R3, CCR3 ; save registers ST R4, CCR4 ST R7, CCR7 ; JSR alters R7 ST R1, CCR1 ; save original string ptr AND R4, R4, #0 ; initialize count to zero CC1 JSR FirstChar ; find next occurrence (ptr in R2) LDR R3, R2, #0 ; see if char or null BRz CC2 ; if null, no more chars ADD R4, R4, #1 ; increment count ADD R1, R2, #1 ; point to next char in string BRnzp CC1 CC2 ADD R2, R4, #0 ; move return val (count) to R2 LD R3, CCR3 ; restore regs LD R4, CCR4 LD R1, CCR1 LD R7, CCR7 RET ; and return

17 .9- 17 FirstChar Algorithm save regs R2 <- R1 R3 <- M[R2 ] =0? R3= R0? R2 <- R2 + 1 restore regs return no no yes yes

18 .9- 18 FirstChar Implementation ; FirstChar: subroutine to find first occurrence of a char FirstChar ST R3, FCR3 ; save registers ST R4, FCR4 ; save original char NOT R4, R0 ; negate R0 for comparisons ADD R4, R4, #1 ADD R2, R1, #0 ; initialize ptr to beginning of string FC1 LDR R3, R2, #0 ; read character BRz FC2 ; if null, we’re done ADD R3, R3, R4 ; see if matches input char BRz FC2 ; if yes, we’re done ADD R2, R2, #1 ; increment pointer BRnzp FC1 FC2 LD R3, FCR3 ; restore registers LD R4, FCR4 ; RET ; and return

19 .9- 19 Library Routines Vendor may provide object files containing useful subroutines don’t want to provide source code -- intellectual property assembler/linker must support EXTERNAL symbols (or starting address of routine must be supplied to user) ... .EXTERNAL SQRT ... LD R2, SQAddr ; load SQRT addr JSRR R2 ... SQAddr .FILL SQRT Using JSRR, because we don’t know whether SQRT is within 1024 instructions.