Computer Science 210Computer Organization

Introduction to Subroutines

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

Subroutines• 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

• Jumps to a location (like a branch but unconditional), and saves current PC (address 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]

Data Path for JSR

• Just like JSR, except Register addressing mode.– target address is Base Register

– bit 11 specifies addressing mode

• What important feature does JSRR providethat JSR does not?

Data Path for JSRR

Returning from Subroutine

• RET (JMP R7) gets us back to the calling routine.– works just like in TRAP

;; Author: Ken Lambert

;; This program resets the value of the variable NUMBER to its absolute value, using the ABS subroutine

.ORIG x3000

;; Pseudocode design: number = abs(number)

;; Main program register usage:; R1 = number

; Main program codeLD R1, NUMBER ; Set argument for absJSR ABSST R1, NUMBER ; Use returned valueHALT

; Data for main programNUMBER .BLKW 1

;; Subroutine ABS; Converts the number in R1 to its absolute value; Input parameter R1 = the number to convert; Output parameter R1 = the number convertedABS ADD R1, R1, #0 ; if R1 < 0

BRzp ENDABS NOT R1, R1 ; R1 = -R1ADD R1, R1, #1

ENDABS RET

.END

Example: Absolute Value

Interface to Trap Service Routines

• Registers serve as input parameters and output parameters

• Examples: – OUT and PUTS use R0 as an input parameter– GETC and IN in use R0 as an output parameter

Passing Data to Subroutines

• Input parameters– The values passed in to a subroutine are called its

input parameters.– These values are needed by the subroutine to do its

job.– Examples:

• In ABS routine, R1 is the number to be converted

• In OUT service routine, R0 is the character to be printed.

• In PUTS routine, R0 is address of string to be printed.

Returning Data from a Subroutine

• Output parameters– Values passed back from a subroutine are

called output parameters.– These are the results of the subroutine’s

computation.– Examples:

• In ABS routine, converted value is returned in R1.

• In GETC service routine, character read from the keyboard is returned in R0.

;; Author: Ken Lambert

;; This program subtracts the number in the variable SECOND from FIRST and;; and stores the result in DIFF

.ORIG x3000

;; Pseudocode design: diff = first - second

;; Main program register usage:; R1 = first; R2 = second; R3 = diff

; Main program codeLD R1, FIRST ; Set the parameter for the minuendLD R2, SECOND ; Set the parameter for the subtrahendJSR SUBST R3, DIFF ; Store the returned differenceHALT

; Main program data variablesFIRST .BLKW 1SECOND .BLKW 1DIFF .BLKW 1

;; Subroutine SUB ; Subtracts R2 from R1 and stores result in R3 ; Input parameters: R1 (minuend) and R2 (subtrahend) ; Output parameter: R3 (difference)SUB NOT R3, R2

ADD R3, R3, #1 ADD R3, R1, R3RET

.END

Example: Subtraction

Note that the SUB routine does not modify its input parameters!

The Namespace• A subroutine should communicate with its caller

only via registers

• A subroutine may have its own data variables, but should not access anyone else’s data variables (those of other routines or the main program)

• A subroutine should leave its input registers unchanged (may save and restore)

Saving and Restoring Registers• Called routine -- “callee-save”

– At startup, 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

Saving and Restoring Registers

• 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 TRAP.

;; Author: Ken Lambert

;; Calls the subroutine ORDER to guarantee that FIRST <= SECOND

.ORIG x3000

;; Main program register usage:; R1 = initial value of FIRST; R2 = initial value of SECOND

; Main program codeLD R1, FIRSTLD R2, SECONDJSR ORDERST R1, FIRSTST R2, SECOND HALT

; Main program dataFIRST .BLKW 1SECOND .BLKW 1

;; Subroutine ORDER; Guarantees that R1 <= R2 ; Input parameters: R1 and R2; Output parameters: R1 and R2; R3 = temporary working storageORDER ST R3, ORDERR3 ; Save R3

JSR SUBBRnz ENDORD ; Exit if difference <= 0ADD R3, R2, #0 ; Swap values in R1 and R2ADD R2, R1, #0ADD R1, R3, #0

ENDORD LD R3, ORDERR3 ; Restore R3RET

; Data variable for subroutine ORDERORDERR3 .BLKW 1

;; Subroutine SUB . . .

Example: ORDER makes R1 <= R2

Is there a bug here?

;; Author: Ken Lambert

;; Calls the subroutine ORDER to guarantee that FIRST <= SECOND

.ORIG x3000

;; Main program register usage:; R1 = initial value of FIRST; R2 = initial value of SECOND

; Main program codeLD R1, FIRSTLD R2, SECONDJSR ORDERST R1, FIRSTST R2, SECOND HALT

; Main program dataFIRST .BLKW 1SECOND .BLKW 1

;; Subroutine ORDER; Guarantees that R1 <= R2 ; Input parameters: R1 and R2; Output parameters: R1 and R2; R3 = temporary working storageORDER ST R3, ORDERR3 ; Save R3 ST R7, ORDERR7 ; Save R7

JSR SUBBRnz ENDORD ; Exit if difference <= 0ADD R3, R2, #0 ; Swap values in R1 and R2ADD R2, R1, #0ADD R1, R3, #0

ENDORD LD R3, ORDERR3 ; Restore R3 LD R7, ORDERR7 ; Restore R7

RET

; Data variables for subroutine ORDERORDERR3 .BLKW 1ORDERR7 .BLKW 1

;; Subroutine SUB . . .

Example: ORDER makes R1 <= R2

Back up ret addressbefore another call